General embryology and organogenesis

List the key components of the female reproductive system:

(1) Vagina ● the lower part of the female reproductive tract: a muscular tube, lined with mucous membrane, connecting the cervix of the uterus to the exterior. It receives semen that is ejaculated into the upper part of the vagina and from there the sperms must pass through the cervix and uterus to fertilize an ovum in the Fallopian tube. (2) Cervix ● the lower third of the Uterus, being a small neck shaped connective canal which links the cavity of the uterus with the vagina. The canal is lined with mucous membrane and normally contains mucus, the viscosity of which changes throughout the menstrual cycle. The cells are stratified columnar epithelium. (3) Uterus ● (womb) a pear-shaped organ that is about 3 inches (7.5cm). Suspended in the pelvic cavity by peritoneal folds (ligaments) and fibrous bands. The upper two thirds are connected to the fallopian tubes, whilst the lower third being the Cervix projects into the vagina. (4) Fallopian tubes ● a pair of tubes that conduct ova from the ovaries to the uterus. The ovarian end opens into the abdominal cavity with a funnel shaped structure with fine finger-like projections called Fimbriae.

Explain the formation of the corpus luteum and its role:

1. As soon as ovulation takes place, the theca and the granulosa collapse because there isn't anymore the fluid pressure inside and there is a haemorrhage, with the formation of a corpus hemorragicum, but very soon, cells undergo to a process called Luteinization: they increase in size, accumulate lipid droplets, steroid-producing, form a rich vascular network and they became steroid producing cells. They are divided into granulosa lutein cells and theca lutein cells. After the transformation, they start producing a lot of progesterone and some estrogens. The corpus luteum is important for the preparation of the endometrium (inner lining of the uterus) for possible future implantation of the embryo. 2. The corpus luteum is functional for approximately 2 weeks after ovulation. If there is not a fertilization, the corpus luteum starts to regress and after 10 days it stops to produce hormones and it becomes the menstrual corpus luteum. Finally, it becomes the corpus albicans (scar tissue). 3. On the other hand, if fertilization takes place, the corpus luteum is maintained functional for about 5-6 months and it is called the gravidic corpus luteum. It is very important for maintaining pregnancy because it produces hormones (stimulated by hCG, produced by syncytiotrophoblast of the blastocyst). After 6 months, the corpus luteum starts to regress because the placenta starts to become the place where hormones are produced.

Lateral folds

1. As the amniotic sac starts to grow, there is a slide of it on the side of the embryo (image A-B) and because it slides, it starts to bend ventrally the territory of the lateral plate mesoderm. We have then the formation of the gut tube: part of the vitelline sac is incorporated in the body of the embryo (the future intestine), forming the primitive intestinal tube, lined by the endoderm. In the meanwhile, there is the formation of the somatic and splanchnic mesoderm. The splanchnic mesoderm remains on the side of the intestinal tube that is forming, while the somatic mesoderm remains on the side of the body wall. In between them we have the intraembryonic coelomatic cavity, which is lined by the mesothelium, a derivative of the splanchnic and somatic mesoderm. 2. **A serosal membrane is made by a mesothelium and a connective tissue that sustains the mesothelium.** 3. The intraembryonic coelomatic cavity is lined by the 2 mesoderm, which will give rise to a serosal layer. Part of this serosa layer remains on the side of the intestine (visceral serosa, from the splanchnic mesoderm), while the other layer remains on the side of the body wall (parietal serosa, from the somatic mesoderm). 4. At some point, the amniotic sac envelops completely the embryo, and the coelomatic cavity is completely closed, except for the caudal region, where there is still a communication with the extraembryonic coelom. 5. Organs are suspended in the coelomatic cavity by structures called mesenteries. At the beginning of the development there is a dorsal and ventral development that keeps the intestinal tube attached to the posterior and anterior body wall. The mesenteries are formed by the splanchnic mesoderm. The ventral mesentery will disappear very quickly, it only remains in a region of the intestinal tract, while the dorsal mesentery remains in different organs (it will contain vessels and nerves directed to the organs).

Describe the steps that lead to the formation of the neural plate and neural tube including secondary neurulation: Neurolation

1. At some point, the prechordal plate and the notochord start to induce the ectoderm to change its developmental destiny and it start to transform into a neuroepithelium. At day 18, begins the formation of the neural plate. 2. This process of induction, stimulated by the notochord and by the prechordal plate, is divided into induction, commitment, and regionalization. 3. The notochord starts to produce signalling molecules, such as Noggin, Chordin, Shh (Sonic HedgeHog), which start to act upon the ectoderm above the notochord. These signals make sure that the ectoderm is freed from the inhibition of another signalling molecule, like BMP-4 (Bone Morphogenetic Protein 4), a protein that keeps the cells in the ectodermal state. But with the inhibition of BMP-4, the ectodermal cells can change to become the neural plate. 4. At the same time, the prechordal plate starts to produce Shh and Cerberus-related 1, proteins that convince the most rostral region of the ectoderm to become the brain. 5. While the notochord is elongating, there is the formation of the neural plate, a structure that is larger rostrally and narrower caudally. This happens because in the rostral part there will be the formation of the brain. 6. At some point (probably day 20?), the neural groove starts to form in the middle line of the neural plate and the 2 sides of the plate start to elevate to form the neural folds. Then the folds will come together dorsally and they will start to close both in the rostral direction and in the caudal one, as to form a neural tube. 7. This process of closing does not take place all together, but starts at the level of the 4th somite and then proceeds in both directions, up to its closure, with the formation of a tube with 2 openings: the cranial neuropore and the caudal neuropore. After that, it is very important that the 2 pores get closed, the cranial will be closed around the 24th day of gestation, while the caudal around the 26th day. 8. If the cranial neuropore does not close it can give rise to serious problems, such as anencephalia, which cause the lack of development of the brain of the embryo, who will die before or very soon after birth; instead, the not closure of the caudal neuropore will cause spina bifida.

Outline the formation of the blastocyst and explain the role of its different components: inner cell mass, trophoblast, zona pellucida, blastocele

1. At the beginning of cleavage have an increase in the number of cells, which are now called blastomeres. After 3 days of cleavage, we have the formation of 16 blastomeres and the whole complex is now called morula. During this process the zona pellucida is still intact. 2. At some point, when we have 8-9 cells, we have a process of compaction of the cell, to maximize cell-to cell contact. This mechanism of compaction is mediated by the appearance on the surface of the blastomeres of adhesion molecules, like E-cadherin-catenin complex. 3. While these process are going on, 2 populations of cells start to distinguish inside the zygote: the outer blastomeres (connected by tight junction), which will remain at the periphery of the zygote, that will form the trophoblast and the inner blastomeres (connected by gap junction), which will remain more at the centre, forming the inner cell mass. 4. The trophoblast cells are flatter and stays on the side of the zona pellucida, while the inner cell mass is at the core of the embryo. The space that is formed in the embryo is a liquid filled cavity, called blastocoele. 5. In the complex there is a water transport, driven by a gradient of sodium, inside the blastocyst from the fluid of the uterine tube. When there is the formation of the cavity, we call the whole thing the blastocyst. 6. While the process of cleavage is taking place, the cells of the trophoblast start producing an early pregnancy factor (immunosuppressant protein). This factor is useful because the embryo could be rejected because of the recognition of the father genetic material as foreign. In fact, lots of the early abortion can be cause by a non-proper working of the pregnancy factor. 7. At the beginning of cleavage, the embryo takes charge of the processes going on. It is important to sustain the process of division in the early stages; but after the 2-cells stage, most of the maternal transcription material is degraded. At that point the zygotic genetic material starts to get expressed. 8. One of the first sign of the embryo taking charge is the formation of 2 separate cell lines, the inner cell mass and the trophoblast; another sign is the formation of a specialization of the plasma membrane of the blastomeres, which start to express a baso-lateral domain and an apical domain. At the beginning all cells have both of them, but afterwards, the inner most cell will lose the apical domain. 9. Transcription factors such as Oct4, Cdx2, Nanog and Gata6 are expressed in a different way by the cells of the blastomere, for example Oct4 is expressed in all the cells of the blastomere; Cdx2, Nanog and Gata6 have, instead, a stochastic expression. When the mechanism of compaction takes place, the situation starts to change a bit: for example, the Cdx2 is limited to the outer cells...(look at the picture). These transcription factors are used to transcribe the DNA, and according to them, the DNA is transcribed in different cells in different way. This is an example of the embryo taking charge.

Folding of the median plane: head fold

1. Besides the growth of the amniotic sac, folding of the rostral region is caused by the huge developing and growth of the most rostral region of the neural tube, called the forebrain (will then form the brain), which grows, overcomes the oropharyngeal membrane, and bends that region of the embryo. This growth contributes to the rostral bending of the embryonic disc and because of this fold, the relationship between oropharyngeal membrane, the heart, and the septum transversum change: they move on the ventral surface of the embryo and their sequence is inverted of 180 degrees. 2. The folding causes also the incorporation of part of the vitelline sac in the rostral region of the embryo, as to form the most rostral part of the intestine tube, called the primitive foregut or anterior endodermal pocket. The foregut lies between the brain and the heart and it is separated by the oropharyngeal membrane from the stomodeum (The stomodeum is a depression between the brain and the pericardium in an embryo, and is the precursor of the mouth and the anterior lobe of the pituitary gland). The septum transversum now lies caudally to the heart: it will develop in the tendinous centre of the diaphragm.

List the derivatives of the components of the somite: Sclerotome

1. Cells from the ventral portion of the sclerotome will migrate around the notochord and there they are going to form the vertebrae. 2. Each sclerotome contributes to the formation of 2 adjoining vertebrae, while the notochord forms the nucleus polposus of intervertebral disks. The process of formation of the vertebrae is guided by a set of genes, among which we can find the Hox gene. 3. Origin of the bones: the axial bones (vertebral column) originates from the somites, from the sclerotome. The limb bones originate from the lateral plate mesoderm, while the craniofacial bones originate from the paraxial mesoderm of the head and from the neural crest cell.

Describe the steps that lead to the formation of the neural plate and neural tube including secondary neurulation: Sonic HedgeHog

1. During the development of the human body, a lot of signalling systems are activated. One of these rely on the Sonic HedgeHog (Shh) signalling protein. Shh is frequently used for intercellular communication during development, it is important for organogenesis as well as in regeneration and in homeostasis. Some mechanism which are normally functionally during development can be re-activated not in a correct way in case of cancer and in that case, it will only cause problems. The Shh signalling pathway is linked to a specialize organelle that we have in most of the cells of human body, especially during the development, but, partly, also in an adult body, which is called primary cilium (PC). 2. There are at least 3 HedgeHog proteins, one of them is called Sonic, another is called Indian and another Desert. The second one is important in skeletal development, the third one is important in the gonads. 3. Mechanism of embryogenesis are shared somehow with tumorigenesis because both depend on coordinated mechanism of proliferation, differentiation, and migration.

Describe the steps that lead to the formation of the neural plate and neural tube including secondary neurulation: Primary cilia

1. Primary cilia are a sort of antenna that most of the cells possess. They are fundamentally important for normal cell signalling during development and homeostasis. The vast majority of signalling pathways in vertebrates function through the primary cilium. The axoneme of the primary cilium is 9+0 (there is not the central couple). 2. Another signalling system, called WNT acts upon the primary cilium. 3. When the primary cilium does not work as it should, it gives rise to some forms of ciliopathy, which cause an underlying signalling defect.

Explain the transformation of the inner cell mass into the embryonic shield (disc): becoming bilaminar

1. During the second week, we have the developmento of the bilaminar embryo and the complete implantation. The 2 layer of the embryo are called epiblast and hypoblast or primitive endoderm. The cells of the epiblast are those conteining the Nanog and the ones for the hypoblast contains Gata6. 2. With the formation of the epiblast and hypoblast, a primitive dorso-ventral axis is established: we can say that th epiblast is dorsal and the hypoblast is ventral. 3. Very soon we see that, at the level of the epiblast, a cavity called primordial amniotic cavity is formed, which will be opened for a while and then it will close again, with the formation of the amniotic membrane. In the meanwhile, we have the formation of the parietal endoderm (Heuser's membrane), which origins from the hypoblast and that lines the cytotrophoblast. The cavity enclosed by it is called primary yolk sac. At the same time, some cells of the parietal endoderm forms the extraembryonic mesoderm, important because it supports the epithelium of the amnium, the yolk sac, the chorionic villi and the blood vessels. 4. The extraembryonic mesoderm is going to be formed (10-11 days), by the proliferation of mesodermal cells, between the Heuer's membrane and the cytotrophoblast. One day later (11-12 days), we can appreciate 2 things: inside the extraembrionic mesoderm some cavities are formed and the extraembrionic mesoderm is also surrounding the amniotic cavity. All the cavities formed will come together (12-13 days) to form a larger cavity that surrounds the vitelline sac and will take the name of chorionic cavity or extraembrionic coelom. The wall of this last cavity is delimited by a membrane called chorion, made by extraembryonic mesoderm, cytotrophoblast and syncytiorophoblast. 5. In the 13th day there is a constriction of the yolk sac, which make it so that part of the yolk salc is eliminated and the definitive yolk sac is formed. In the meanwhile (14-15 days) the chorionic cavity is growing very much, and the extra emrbrionic mesoderm becomes the body or connective stalk, which suspend the embryo into the chorionic cavity. The connective stalk is the first place where blood vessels will be formed. 6. By the end of the 2nd week of development there is the development of the uteroplacental circulation. The cytotrophoblast starts to grow and forms the primary villi, covered by syncytiotrophoblast and then, at the core of them, the extraembryinoic mesoderm enters to form the secondary villi and then, from the extraembryonic mesoderm, we have the formation of blood vessels that form the fetal placental microcrculation. The villi are bathed by the blood present in the lacunae and the wall of the villi will become very thin, thus to allow an exchange of nutrients, gases, wastes between blood from the fetus with things that are in the maternal blood present in the lacunae.

List the derivatives of the components of the somite

1. Each somite is going to be subdivided into 3 main territories: the sclerotome, the dermatome, and the myotome. Each of these 3 compartments will give rise to different compartments of our body. 2. The sclerotome has a subdivision, which is called the arthrotome. The syndetome, instead, is a subdivision of the myotome.

Describe the steps that lead to the formation of the neural plate and neural tube including secondary neurulation: Secondary neurulation

1. Gastrulation ends with the formation of the tail bud. It originates from what remains of the primitive streak. By day 20, what remains of the primitive streak swell to form the tail bud or caudal eminence and then it disappears. 2. This last part of the primitive streak undergoes to a process called secondary neurulation, which means that the last part of the neural tube, rather than forming as the rest of the tube did, will be formed by a proliferation of the cells of the primitive streak that then undergo a process of cavitation and then the cavity formed will join the neural canal. In humans it appears that this process is not so prominent. 3. When we move from the 3rd week to the 4th week, there is progressive restriction of the pluripotency of cells.

Define the role of primitive streak and primitive node and the formation of the body axes

1. Gastrulation is the process that establishes the three definitive germ layers of the embryo (ectoderm, intraembryonic mesoderm, and endoderm), forming a trilaminar embryonic disk by day 21 of development. These three germ layers give rise to all the tissues and organs of the adult. 2. Gastrulation is heralded by the formation of the primitive streak and is caused by a proliferation of epiblast cells. The primitive streak consists of the primitive groove, primitive node, and primitive pit. 3. In day 15, we start to see on the dorsal surface of the embryo, on the epiblast, the appearance on the midline of an invagination in the tissue, which will be called primitive streak. The appearance of the primitive streak heralds the beginning of gastrulation. The primitive streak is characterized by the primitive node and an invagination at the centre of it, called primitive pit and at the caudal part of the streak, there is a primitive groove. 4. The appearance of the primitive streak allows us to define the major body axis. First, we had already dorsal and ventral, defined by the differentiation of the inner mass cell in epiblast and hypoblast. Now we can also define a caudal region, where the primitive streak appears and a rostral or cranial region where there is not the primitive streak. And because the streak appears on the middle line, we can define a right and a left side of the embryo. We also can define medial, close to the primitive streak, and lateral, far from it.

Describe the first molecular and morphological events indicating that the embryo is "taking charge"

1. One of the first sign of the embryo taking charge is the formation of 2 separate cell lines, the inner cell mass and the trophoblast; another sign is the formation of a specialization of the plasma membrane of the blastomeres, which start to express a baso-lateral domain and an apical domain. At the beginning all cells have both of them, but afterwards, the inner most cell will lose the apical domain. 2. Transcription factors such as Oct4, Cdx2, Nanog and Gata6 are expressed in a different way by the cells of the blastomere, for example Oct4 is expressed in all the cells of the blastomere; Cdx2, Nanog and Gata6 have, instead, a stochastic expression. When the mechanism of compaction takes place, the situation starts to change a bit: for example, the Cdx2 is limited to the outer cells...(look at the picture). These transcription factors are used to transcribe the DNA, and according to them, the DNA is transcribed in different cells in different way. This is an example of the embryo taking charge.

Summarize the key steps of the process of segmentation (somitogenesis): Segmentation

1. Segmentation in the human body involves the paraxial mesoderm, which undergoes to segmentation first and then to at least 2 somite formations. After gradients of gene productions have been established in the body embryo, further development along the rostro-caudal axis is mediated by segmentation genes, which are genes that are mostly activated by the genome of the embryo and function to subdivide the embryo into smaller and smaller regions along the axis. 2. The embryo, dorsally, is very segmented. But in adult body, the only region where we still have segmentation is the vertebral column. 3. Segmentation is one of the earliest morphological manifestation of a complex set up of gene expression that are to determine the basic body plans. Among those genes there are the Hox genes. 4. Segmentation takes place in the paraxial mesoderm (which can be find both in the trunk and in the region of the head) of the trunk region. In the first part, the paraxial mesoderm is divided into segments, in the second part, it is characterized only by a molecular segmentation: different region of the head mesoderm express different molecular patterns, but this does not ensure a physical subdivision into segment.

Clinical drop: What can cause male infertility?

1. Sperm number and motility: Infertile males produce less than 10 million sperm/mL of semen. Fertile males produce from 20 to more than 100 million sperm/mL of semen. Normally, up to 10% of sperm in an ejaculate may be grossly deformed (two heads or two tails), but these sperm probably do not fertilize an oocyte because of their lack of motility. 2. Hypogonadotropic hypogonadism is a condition where the hypothalamus produces reduced levels of gonadotropin-releasing factor (GnRF), leading to reduced levels of follicle-stimulating hormone (FSH) and LH, and finally, reduced levels of testosterone. Kallmann syndrome is a genetic disorder characterized by hypogonadotropic hypogonadism and anosmia (loss of smell). 3. Drugs: cancer chemotherapy, anabolic steroids, cimetidine (histamine H2-receptor antagonist that inhibits stomach HCl production), spironolactone (a K+-sparing diuretic), phenytoin (an antiepileptic drug), sulfasalazine (a sulfa drug used to treat ulcerative colitis, Crohn disease, rheumatoid arthritis, and psoriatic arthritis), and nitrofurantoin (an antibiotic used to treat urinary tract infections). 4. Other factors: Klinefelter syndrome (XXY), seminoma, cryptochordism, varicocele, hydrocele, mumps, prostatitis, epididymitis, hypospadias, ductus deferens obstruction, and impotence.

Explain the process of cleavage of the zygote and its role

1. The early development of the zygote involves a process that is called cleavage. Cleavage begins around 30 hours after fertilization and it takes place in the fallopian tube mostly. 2. Through this process there is an increase in the number of the cells of the embryo, without increasing the size of the embryo. With each successive subdivision, the nuclear/cytoplasmic ration increases. 3. The cytoplasm of the large oocyte is progressively divided among the cells formed by mitotic division. Mammalian cleavage is a leisure process, it takes a few days.

Explain the mechanisms that prevent polyspermy:

1. The first barrier: the pH of the vagina is quite low, around 4.3. The spermatozoa can't swim well and survive into that environment, but since they are immersed in the seminal fluid, which is basic and has a buffer capacity that increases the pH of the vagina as much as 7.2 in few seconds. Even though buffering lasts only few minutes, this time is enough for the sperm to approach the cervix of the uterus with a pH optimal for motility (6 - 6.5) 2. The second barrier is the cervical canal and the cervical mucus (changes during the cycle). The cervix of the uterus produces a mucus secretion, usually quite thick and viscous, but close to the period of ovulation, the mucous became more fluid to favour the passage of spermatozoa. The spermatozoa are characterized by a fast phase passage and a slow phase. In the fast phase passage, some spermatozoa can manage to cross quickly the cervix and the body of the uterus and to reach the uterine tube in 5-20 minutes. Even though they reach the uterine tube before the others, they are the less probable to fertilize the egg because they do not remain enough in the female genital tract to mature. The only reason why they are able to reach the uterine tube that quickly is due to the contraction of the female reproductive tract. The slow phase is the one done by the majority of the spermatozoa, which will reach the uterine tube because of their swimming movement through the cervical mucus (2-3 mm per hour) first and then along the wall of the uterus. They are still helped a bit by the contraction of the uterine wall. Some spermatozoa can remain stored in the cervical crypts, crypts that can be found mucosa of the neck of the uterus, and they may pass through the cervical canal even 2 - 4 days later. This spermatozoa that reach the fallopian tube later, are the ones that stayed more in the female genital tract and therefore are the ones that are more likely to be able to fertilize the egg. 3. Passage through the uterus: the spermatozoa pass through the uterus and some of them get stuck in the mucosa of the uterus and get attacked by the immune cells of the female, which recognize them as foreign, so they get destroyed. Following the passage of the uterus, only several hundreds reach the tube, of which most of them will be on the side of ovulation. Once they reach the uterine tube, they bind to the isthmus (the initial part of the uterine tube, the narrow one) for 24 hours and the capacitation reaction takes place. 4. With the capacitation reaction we have the removal of glycoproteins and some cholesterol that were added at the sperm head when it was maturing in the epididymis. Only sperm which are capacitated have exposed molecules that are able to bind to the zona pellucida of the oocyte. 5. After capacitation the spermatozoa have to detach from the mucosa of the isthmus and to do that, they have to use their flagellum very rapidly. They have a period of hyperactivity: breaking free of the bond with the tubal epithelium, but only a small amount of the spermatozoa can do that, all the others die. Combination of smooth muscle contraction and swimming movements bring them to the ampullary portion, where fertilization takes place.

Describe the formation of the intraembryonic coelom

1. The intraembryonic cavity will be smoothly lined by a mesothelium and filled with a fluid. Most triploblastic animals have this fluid-filled cavity somewhere between the body wall and the gut. 2. In humans the splitting takes place in the lateral place mesoderm, which splits forming a dorsal and a ventral layer of mesoderm. The first is called somatic mesoderm (it will form the somatopleura) and it continues outside the body of the embryo with the extraembryonic mesoderm, which surround the amniotic sac. The ventral layer is called the splanchnic mesoderm (it will form the splanchnopleura) and it is in continuity with the extraembryonic mesoderm that surrounds the vitelline sac. In the central region of the embryo, the intraembryonic coelom communicates with the extraembryonic coelom for a few weeks and then the communication will be terminated.

Outline the steps that characterize gastrulation.

1. The invagination of the primitive streak is formed because gastrulation is started. This means that some cells of the epiblast starts to migrate toward the middle line and invaginate below the epiblast, causing a depression called primitive streak. There are several factors that guide the migration of cells towards the primitive streak, one of these is a grow factor called FGF 8(fibroblast grow factor 8). The cells which are undergoing migration are epithelial cells, so in order to be able to migrate, it is necessary that those cells undergo an epithelio-mesenchymal transition (EMT), which permits a change from cell-to-cell to cell-to substrate adhesion, the size of the cells also changes and then there is a cytoskeletal reorganization, with the formation of lamellipodia and filopodia. Due to all these necessary changes, this process is quite difficult and complex. When the cells reach the right position, they undergo an opposite transition, a mesenchymal-epithelio transition. 2. There are many factors involved in these procedures, one of them is Snail, a zinc finger transcription factor involved in EMT. It has some zinc molecules and it is able to bind and to regulate the transcription function of DNA. Snail is one of those transcription factors able to repress the epithelial features. 3. The first wave of migration (14-15 days), is used to form the definitive endoderm: they substitute the endoderm of the hypoblast (or primitive endoderm) with the definitive endoderm. Then at 16 days, there is a second wave of migration that leads to the formation of the mesoderm, which will be in between the definitive endoderm and what remains of the epiblast, that will be called ectoderm.

List the different compartment of the mesodermal layer and their derivatives

1. The mesoderm will after be divided into intermediate, lateral, and paraxial mesoderm. 2. While the mesodermal migration, there are 2 places where the mesodermal cells don't go: the oropharyngeal membrane and the cloacal membrane. At the level of these 2 membranes, the body of the embryo is thinner, there is only the ectoderm attached to the endoderm. The membranes will then transform in the 2 main openings of our body: the oral cavity, and the opening of the digestive and urogenital tract. 3. The migration of mesoderm goes on according to a specific scheme. The migration proceeds also rostral to the oropharyngeal membrane. 4. Mesenchyme is a type of undifferentiated connective tissue mostly derived from mesoderm. It can differentiate in connective tissue proper, bones, cartilage, cells of the circulatory system... 5. In the embryo, most of the mesoderm remains for quite a long time undifferentiated, that's why we call it also mesenchyme. 6. The migration pattern gives rise to different subdivision of the mesoderm.

List the derivatives of the components of the somite: Myotome

1. The myotome gives rise to most of the skeletal muscle of our body. The myotome very soon gets divided into a dorsal division and a ventral division, called epimere and hypomere. From the epimere originates the epaxial muscles (intrinsic muscles of the back), while the hypomere is going to give rise to the hypaxial muscles (lateral body wall and migration of myoblasts to form muscles of the limb). 2. Muscles of the limbs derive from progenitor myogenic cells that migrate into the limb bud from the ventral portion of the myotome. As the limb buds form, the muscle progenitors form 2 major muscle masses, the ventral and the dorsal muscle mass. The ventral muscle mass give rise mainly to the flexors, pronators, and adductors, whereas the dorsal muscle mass gives rise mainly to extensors, supinators, and abductors.

Outline the stages of formation of the notochord.

1. The notochord is a tubular structure elongating rostral to the primitive streak. 2. The prechordal plate is a group of mesenchymal cells which associate with the anterior visceral endoderm. 3. They play very important inductive and patterning roles: the notochord will be helpful for the formation of the axial skeleton and for the neural plate; the prechordal plate is important, instead, for the formation of the territory of the head, including the most rostral portion of the neural plate that is going to give rise to the brain. 4. In the 17th day, the notochordal process elongates rostral to the notochord, up to reach the territory of the prechordal plate. 5. The formation of the notochord takes place in 2 stages. At the beginning, the notochord is a hollow tube, then this tube attaches to the underlying endoderm and it fuses with it. Very soon, the region fused with the endoderm is pinched off, to form again a tube, which is almost filled and soon after it won't be a tube anymore, but it becomes a rode. All this process is carried on while the notochordal process is elongated (16-22 days). 6. While the notochord elongates, the primitive streak regresses. That happens because the role of the primitive streak is lost: once all the migration process has taken place, there is no more need of a primitive streak. 7. While the primitive streak regresses, the epiblast is transformed in a special region called neural plate.

Summarize the key steps of the process of segmentation (somitogenesis): Somitogenesis

1. The process of somitogenesis proceeds from rostral to caudal, it starts about day 20, at the end of the 3rd week, at the head-trunk border. It ends around the day 30 of gestation. 2. The somitogenetic process is guided by opposite gradients of proliferating and differentiating factors and gene expression. 3. At the beginning we have the formation of 42-44 pairs of somitomers (pairs because there is one on each side). The somitomers 1 to 7 do not give rise to somites because they will be formed in the head region, moreover the most caudal ones will disappear. At the end the effective numbers of somites that will be formed is 35 to 37. 4. The somites will be formed through a very complex process: first, there is a gradient of molecules that goes from rostral to caudal and vice versa and these molecules are proliferating and differentiating factors. For example, Retinoic Acid (RA) is a differentiating factor and it is more concentrated rostrally, while Fibroblast Growth Factor (FGF) is a pro-proliferating factor which is more concentrated caudally. When these 2 opposite gradients meet at a certain level and they reach a sort of balance between each other, then a somite is formed. This is called the wavefront model, which represent a balance between the concentration between the RA and FGF. This wavefront activates some genes which belongs to the notch family, which oscillates continuously from a state of permissive to a non-permissive state. For a somite to be formed, the wavefront must reach the genes of the paraxial mesoderm when there are in a permissive state. 5. **All the process of formation of somites depends on gradient of differentiating factors that goes from rostral to caudal and vice versa and on the presence of genes which oscillates between a permissive and a non-permissive state. This leads to the progression of somites from rostral to caudal.** 6. During somitogenesis, mesenchymal cells of the paraxial mesoderm undergo a mesenchymal to epithelial transition: the paraxial mesoderm is formed basically by undifferentiated cells, which have to undergo differentiation to form the somites.

All primary oocytes are formed by (A) week 4 of embryonic life (B) month 5 of fetal life (C) birth (D) month 5 of infancy (E) puberty

B. During early fetal life, oogonia undergo mitotic divisions to populate the developing ovary. All the oogonia subsequently give rise to primary oocytes by month 5 of fetal life; at birth, no oogonia are present in the ovary. At birth, a female has her entire supply of primary oocytes to carry her through reproductive life.

Which of the following is a major character- istic of meiosis I? (A) Splitting of the centromere (B) Pairing of homologous chromosomes (C) Reducing the amount of DNA to 1N (D) Achieving the diploid number of chromosomes (E) Producing primordial germ cells

B. Pairing of homologous chromosomes (synapsis) is a unique event that occurs only during meiosis I in the production of gametes. Synapsis is necessary so that crossing over can occur.

Describe the formation of the diaphragm.

1. The septum transversum, while it gives rise to the diaphragm, it also separates the coelomatic cavity into a thoracic and abdominal portion. 2. Before folding, the pericardium and peritoneal cavity are in continuity, but it is necessary to separate them. This happens by the reposition of the septum transversum ventrally, which together with other events will form the diaphragm. 3. Once the septum transversum has repositioned caudal to the heart, it starts progressively to grow towards the vertical column, and it keeps on growing towards the dorsal region of the embryo, but for a while there is still a communication between the 2 cavities, but when the diaphragm will be completed, it will attach posteriorly to the vertebral column, obliterating completely the communication between the coelomatic cavity of the abdomen (peritoneal cavity) with the one of the thorax (pleuro-pericardial cavity). And during this formation, the diaphragm starts to be innervated by the phrenic nerve, a nerve that originates by the cervical plexus and this communication will be maintained even when the diaphragm will be elongated up to the caudal part.

Explain the different steps of fertilization

1. The spermatozoa and the oocyte usually meet in the ampullary portion of the uterine tube. In the ampullary portion, the folds of the uterine tube are very high, so that they form a sort of labyrinth, which slow down the oocyte and, at the same time, create a good environment to allow the sperm and the egg to meet each other. 2. During fertilization the sperm has to cross the different layers of the oocyte. The first one is the corona radiata, then we have the zona pellucida, a specialized extracellular matrix between the corona radiata and the cytoplasm of the oocyte; after that there is the perivitelline space and then the membrane of the oocyte. Just after the zona pellucida, there can be found some granules, produced by the oocyte during the maturation of the follicle. They are important to prevent polyspermy. Fertilization phase 1: Penetration of the corona radiata - The first step for the spermatozoa is to cross the corona radiata. This step takes place by the release of the hyaluronidase enzyme, which come from the spermatozoa acrosome. It is used to digest hyaluronic acid of the sticky matrix. Besides that, there is also the swimming movement of the spermatozoa that helps to get through the corona radiata. At this point there might be also some enzymes (not sure), released by the uterine tubes that help the spermatozoa in passing through the corona radiata. Fertilization phase 2: attachment to and penetration of the zona pellucida - After the cross of the zona radiata, the spermatozoa attach to the zona pellucida. The first spermatozoa that will attach to the zona pellucida will be the one to fertilize the egg. Binding to zona pellucida is a species-specific mechanism. It depends on the presence of the ZP (zona pellucida proteins), the most important protein of this type is the ZP3. The binding of the spermatozoa triggers a reaction in the acrosome: entrance of calcium inside the head of the spermatozoa, an increase of intracellular pH, which allows the sperm plasma membrane of the head to fuse with the anterior region of the acrosome membrane. This leads to the release of specific enzymes (like acrosin), which start to digest the zona pellucida, so to let the spermatozoa to enter in the perivitelline space (can happen that more than 1 spermatozoa are able to enter). Fertilization phase 3: binding to the oocyte membrane and fusion of spermatozoon and egg - Some spermatozoa manage to cross the zona pellucida and to enter the perivitelline space. One of them, though, is faster than the others and it binds to the microvilli that characterize the plasma membrane of the oocyte. At that moment, the head of the spermatozoa fuses with the plasma membrane of the oocyte and head, midpiece and tail of that sperm sink inside the egg. As soon as this contact takes place, there are mechanism that prevent all the others to enter in the cytoplasm. Juno and Izumo are 2 proteins that allows the attaching of the spermatozoa to the oocyte membrane. Juno is a plasma membrane receptor present on the oocyte; Izumo I is a membrane protein present on the spermatozoa. The 2 proteins are very important for the attaching of the spermatozoa to the egg membrane. As soon as Juno recognize Izumo, all the Juno receptors are ejected, thus not to allow other spermatozoa to attach to the plasma member of the oocyte. When the spermatozoon enters the oocyte, it is necessary to eliminate the paternal mitochondria. In our body we only have maternal mitochondria, so the paternal one has to be eliminated. There are 2 models explaining how they are eliminated, an active elimination model: the mitochondria are destroyed progressively by phagocytosis; or there is a dilution model: progressively the mitochondria, as the zygote divide, they disappear. In mammals the most common process of getting rid of paternal mitochondria is by degradation. Fertilization phase 4: prevention of polyspermy - The release of Juno receptors is not enough to prevent polyspermy. Additionally to that, there are other 2 mechanism to prevent it: the fast and the permanent block. The fast block happens 2-3 seconds after the penetration of the single spermatozoon, the membrane of the oocyte undergoes to a depolarization from -70 to +10. This depolarization lasts few minutes and it prevents adhesion of other spermatozoa. The permanent block consists in a change in the concentration of Ca in the oocyte, that causes the fusion of the granules below the plasma membrane, called cortical granules, releasing their content in the perivitelline space, which swells and becomes harder, engulfing the other spermatozoa. Moreover we have a zona pellucida reaction, which involves the hydrolyzation of ZP receptor proteins. At this point no other sperms can penetrate the egg. Fertilization phase 5: formation of the male and female pronuclei - The entrance of the spermatozoon in the oocyte, causes the completion of meiosis II and the formation of the second polar body, but, besides this, there is a metabolic activation of the egg, which results in recruitment of maternal mRNA and in an intensification of egg respiration and metabolism. At the same time there is a decondensation of the sperm nucleus, caused by an increased permeability of nuclear membrane, loss of the protoamines, spreading of chromatin that leads to the formation of the male pronucleus; and at the same time there is a demethylation of the paternal DNA. At this point, the oocyte contains 2 haploid pronuclei, the one just formed by the spermatozoon and the one already presents from the oocyte. In this phase, the oocyte is called ootid. If also the first polar body manages to undergo meiosis, there might be 3 polar bodies instead of 2. The fusion of the 2 pronuclei leads to the formation of the zygote. The zygote, 1 diploid cell, is ready for the process used to increase the number of cells, the cleavage. When the 2 pronuclei fuse together there is a mixing and mingling of the chromosomes from the father and from the mother.

Clinical drop: What is angiogenesis and vasculogenic mimicry in cancer?

1. Unfortunately, angiogenesis takes place also during the formation of tumours. Tumours requires blood supply to grow: when the tumour mass is small, it can grow getting nutrients by diffusion from normal vessels, but arrived at a certain point, it needs to attract blood vessels. At some point, the tumour environment will start to have very low blood tension and it will trigger angiogenesis, with the producing of some molecules, like VEGF-A (vascular endothelial growth factor A), which will bind to the endothelium of adjacent blood vessels and trigger the angiogenesis of those vessels. The new vessels are leaky and facilitate intravasation and metastasis of tumour because the cells of the tumour can now enter into these blood vessels and migrate into other parts of the body. 2. Also other cells (neutrophils, lymphocytes, macrophages) produce angiogenic factors. 3. Another mechanism with which tumours try to attract blood cells is vasculogenic mimicry. It refers to the ability of cancer cells to organize themselves into vascular like structure for the obtention of nutrients and oxygen independently of normal blood vessels or angiogenesis.

Explain the mechanism/s of breaking down of the symmetry of the embryo.

1. Up to this moment, looking at the embryo, we can notice that it is quite symmetrical: morphologically speaking there is no difference between the right and the left part. But if we look at an adult body, especially the internal organs are not symmetrically at all, so at some point there must be a breaking of the symmetry of the embryo. 2. During the 3rd week of development there is this breaking down. If we look exteriorly at the embryo, we will notice no differences between the right and left part, but if we consider the blue print of the individual cells and the molecular events that are taking place in the layers of the embryo, we would notice the breaking down of the symmetry. 3. The first sign of this breaking down can be recognized in the primitive node: node cells contain a monocilium, the cells at the centre of the node have motile cilia, while the cells in the lateral portion of the node have a not motile monocilium. So, the first manifestation of asymmetry in the embryo, involves the central cilia of the primitive node, which start to beat all in the same direction (left), and because of that this can be defined as the first sign of breaking down of the symmetry of the embryo. This is important because the beating of the cilia sweeps some molecules on the left side of the embryo, side where these molecules will be more concentrated. After that, the molecules brought there will activate some homeoboxes such as Pitx2 (homeobox that contains transcription factor responsible for establishing left sidedness. Its expression is repeated on the left side of the heart, stomach and gut primordia as these organs assumes their normal asymmetrical position in the body). 4. The sweeping of those substances also activates another gene called Lefty-1, expressed on the left side of the primitive streak, which will create a barrier not allowing those substances to go back. 5. One of the substances more concentrated on the left side of the embryo is 5HT (serotonin), while the enzyme MAO (monoamine oxidase) is more concentrated on the right side. That is why is necessary not to use anti depression drugs while you are pregnant: among the classical anti depression drugs, there are the MAO inhibitors, which will cause an altering in the balance of the embryo. 6. There is more than one model to explain concentration on one side of the embryo. One model is called the mechanosensory model of nodal flow, which tells that the movement of morphogens from one side of the embryo to the other is due to the fluid currents which then degenerate by the sweeping movement of the cilia, which are due to the presence inside the cilia of dynein molecule (Lrd, left-right dynein), molecular motors important to move cargos towards the minus end of microtubules but also in bending cilia and flagella creating a sliding forces between microtubules. The bending of the cilia creates the nodal flow, triggering a higher concentration of calcium on one side, compared to the other. 7. The second model is the Nodal Vesicular Parcels Model (NVP), which tells that the morphogens that are carried are packaged inside membrane vesicle and the beating of the cilia in one direction is able to move the vesicles towards the left.

Explain the difference between vasculogenesis and angiogenesis.

1. Vasculogenesis is ex novo (created by something else) vessel formation from precursors. They originate from the mesenchyme of the embryo. Some of those cells are specified to become endothelial precursor cells, which will become endothelial cells. After that, they start to delimit a cavity that will form the blood vessel. Since endothelium is not enough, some cells of the mesenchyme transform also into smooth muscle cells, in pericytes, thus to complete the blood vessel. 2. Angiogenesis is the budding from existing vessels (also present in adults, for example during menstrual cycle, wound healing, and tumours). When the process of formation of a vessel is finished by vasculogenesis, there is the sprouting of other vessels through angiogenesis.

Explain the formation of the amniotic cavity, of the primary and secondary yolk sacs and of the extraembryonic mesoderm

1. Very soon we see that, at the level of the epiblast, a cavity called primordial amniotic cavity is formed, which will be opened for a while and then it will close again, with the formation of the amniotic membrane. In the meanwhile, we have the formation of the parietal endoderm (Heuser's membrane), which origins from the hypoblast and that lines the cytotrophoblast. The cavity enclosed by it is called primary yolk sac. At the same time, some cells of the parietal endoderm forms the extraembryonic mesoderm, important because it supports the epithelium of the amnium, the yolk sac, the chorionic villi and the blood vessels. 2. The extraembryonic mesoderm is going to be formed (10-11 days), by the proliferation of mesodermal cells, between the Heuer's membrane and the cytotrophoblast. One day later (11-12 days), we can appreciate 2 things: inside the extraembrionic mesoderm some cavities are formed and the extraembrionic mesoderm is also surrounding the amniotic cavity. All the cavities formed will come together (12-13 days) to form a larger cavity that surrounds the vitelline sac and will take the name of chorionic cavity or extraembrionic coelom. The wall of this last cavity is delimited by a membrane called chorion, made by extraembryonic mesoderm, cytotrophoblast and syncytiorophoblast. 3. In the 13th day there is a constriction of the yolk sac, which make it so that part of the yolk salc is eliminated and the definitive yolk sac is formed. In the meanwhile (14-15 days) the chorionic cavity is growing very much, and the extra emrbrionic mesoderm becomes the body or connective stalk, which suspend the embryo into the chorionic cavity. The connective stalk is the first place where blood vessels will be formed.

Outline the general organization of the embryonic circulation.

1. We need to have some primitive blood vessels associated with the endocardial tube. The formation of the primitive blood vessels begins at the beginning of the 3rd week of development. They start to be visible in the extraembryonic mesoderm of the yolk sac, in the connecting stalk and in the chorion; a couple of days later, we start to see vessels also in the embryonic disc. Blood vessels are formed by 2 mechanisms: by vasculogenesis and by angiogenesis. 2. Vessels of the embryo are connected by 3 symmetric (will be symmetrical just for a while) networks: the embryonic network, the vitelline network, and the umbilical network. 3. Vessels that carry blood to the inflow portion of the cardiac tube are the umbilical veins, those vessels that carry blood from the periphery (placenta) to the heart (richly oxygenated blood); the vitelline veins, which carry blood from the yolk sac to the heart (poorly oxygenated blood); the cardinal veins, which receive blood from segmental veins of the embryo and bring blood to the heart (poorly oxygenated blood) through the cardinal veins. The cardinal veins are divided into an anterior and a posterior cardinal vein, which then come together and form a common cardinal vein. 4. When the blood arrives at the heart is mixed, that's why in the blood there is medium oxygenated blood. This means that all the blood pumped out from the blood will be a mixed oxygenated blood. Vessels that carry blood to the outflow portion of the cardiac tube From the outflow portion of the cardiac tube, at the beginning of development, we have the formation of an aortic sac, which is connected by a system of arteries called pharyngeal arch arteries, to 2 large vessels of the embryo called dorsal aortae, which will become 1 in the future. The blood that goes in the dorsal aortae will go in the vitelline arteries, which will bring it to the vitelline sac and to the primitive intestine; in the umbilical arteries, which will send it to the placenta; in the segmental arteries, which will distribute it to the body of the embryo. So, the fetus never receives fully oxygenated blood because from the heart is pumped only medium oxygenated blood.

Folding of the median plane: tail fold

1. When folding takes place in the tail region, the allantois with part of the body stalk is brought ventrally to the embryo and the folding incorporates again part of the vitelline sac into the embryo, thus to form the hindgut (posterior portion of the primitive posterior tube). The hindgut is closed by the cloacal membrane, which has been shifted ventral. 2. This folding is caused by the growth of the distal part of the neural tube, it does not grow as much as the rostral part, but it still contributes to the folding. The neural tube grows over the cloacal membrane, forming a posterior endodermal pocket: the hindgut. 3. The connective stalk (primordium of the umbilical cord) is now attached to the ventral surface of the embryo and has incorporated a diverticulum (an outpouching of a hollow, or a fluid-filled structure in the body) of the yolk sac, the allantois. 4. At this point the primitive streak is caudal to the cloacal membrane and it will soon be going to disappear.

Outline the transport of the zygote and blastocyst to the uterine cavity

1. When the blastocyst is formed, it is transported to the utherus and it slides into the uterine cavity. Once the spermatozoon has entered in the cytoplasm, after 2 days from fertilization, the corona radiata is completely lost, but there is still the zona pellucida. The embryo will stay for 3 days in the ampullary portion and then it will take 8 hours to cross the isthmus, before being released in the uterine cavity. In the isthumus, the crossing is helped by the releasing of progesterone, produced by the corpus luteum, which relaxes the uterotubal junction. 2. When the blastocyst reaches the uterine cavity, it is still surrounded by the zona pellucida. So, once it reach the uterus, there is a process, called hatching of the blastocyst, to remove the zona pellucida. This process is carried out by microvilli of the trophoblast, which extend on the side of the zona pellucida and then start to release proteases enzymes, thus to digest part of the zona pellucida and let the embryo to protrude from the opening created. After the blastocyst is free from the zona pellucida, it will be able to adhere and then to attach to the mucosa of the uterus. 3. While all of this is going on, the mucosa of the uterus is getting ready for implantation, preparing a suitable cellular and nutritional envirnment and forming an immunoprivileged site.

Describe the steps that lead to the formation of the neural plate and neural tube including secondary neurulation: The neural crest

1. When the neural tube closes, it detaches from the ectoderm that is on the side. At the boundaries of the neural plate, we have the formation of the neural crest cells; when there is the folding of the plates, the crests are lifted dorsally and when the neural tubes close, the neural crests detach from the neural tube. At this point, the crests will undergo an extensive process of migration. 2. The neural crest, then, will give rise to all the peripheral nervous system: Swann cells, melanocytes, sympathetic ganglia, sensory ganglia, some portion of bones of the head... Some of the cells from the neural crest will also migrate in the primary heart field, contributing to the formation of the heart; some others will contribute to the formation of some structures of the eye bulb, like the cornea.

Explain the importance of studying embryology:

1. illuminates clinically oriented anatomy and explains how normal and abnormal relationship develop (bridge with pediatrics, obstetrics, perinatal medicine and clinical anatomy) 2. cancerogenesis may involve mutations or re-activation of genes involved in key development events 3. during development many defects can rise: genetic and environmental problems that may either kill the fetus or can impair the adult life

Outline the periods and phases of human embryology:

1. medical point of view: 3 trimesters (the ones into which pregnancy is divided) 2. embryological pov: comprehends 3 periods A) period of the egg: from fertilization to implantation (conceptus or pre-implantation embryo), through the phases of zygote, morula and blastocyst B) period of the proper embryo: from implantation (end 1st week) to 8th week C) period of the fetus: from the 8th week (2nd month) to the end of pregnancy. ***We should consider that development continues after birth, because organs are not mature yet (like the lungs and the Central Nervous System = CNS). And after it continues throughout our entire life.

In the production of female gametes, which of the following cells can remain dormant for 12 to 40 years? (A) Primordial germ cell (B) Primary oocyte (C) Secondary oocyte (D) First polar body (E) Second polar body

B. Primary oocytes are formed by month 5 of fetal life and remain dormant until puberty, when hormonal changes in the young woman stimulate the ovarian and menstrual cycles. From 5 to 15 oocytes will then begin maturation with each ovarian cycle throughout the woman's repro- ductive life.

List the components of male reproductive system and female genital tract:

1. penis (contains erectile tissue, becomes enlarged and hard, penetrates vagina, deposits sperm close to cervix). 2. epididymis (holds sperm until ready for ejaculation). 3. testis (where spermatozoa originate -- produce sperm and testosterone) 4. scrotal bursa (where testis and epididymis are contained -- outside the body = lower temp.) 5. Vas Deferens (Long tube which conducts sperm from the testes to the prostate gland (which connects to the urethra) during ejaculation) 6. prostate gland (Secretes an alkaline fluid to neutralise vaginal acids (necessary to maintain sperm viability)) 7. Seminal Vesicle (Secretes fluid containing fructose (to nourish sperm), mucus (to protect sperm) and prostaglandin (triggers uterine contractions)) 8. urethra (transports semen + urine out of the body) 9. ureter (transports urine out of the body) ***In particular, in the testis we can identify the seminiferous tubules, where spermatozoa are formed and mature; then they are carried to the epididymis and they enter the ductus deferens 1. ovaries (where the oocytes mature each month) 2. Fallopian tubes (where only 1 mature oocyte is captured and may encounter fertilization) 3. uterus (either the unfertilized oocyte is expelled out of the body via menstrual periods; or the fertilized cell is implanted in the uterine wall) 4. cervix 5. vagina (Passage leading to the uterus by which the penis can enter (uterus protected by a muscular opening called the cervix)) 6. external genitalia

Explain what is accomplished by the process of fertilization and the characteristics of the zygote:

1• The oocyte completes the second meiotic division, the oocyte has a normal amount of genetic material from the maternal side 2• Zygote is restored to normal diploid number: 23 chromosomes from paternal side and 23 from maternal side 3• Determination of the genetic sex, determined by the spermatozoa (apparently y-sperms are a bit quicker than x-sperms, so there is a slightly higher chance to get fertilized by a y-sperm) 4• There is a creation of variation of the human species through the mingling of paternal and maternal genetic material 5• There is a metabolic activation of the egg and the initiation of the cleavage.

The migration pattern gives rise to different subdivision of the mesoderm:

1• The prechordal plate, formed very early in the rostral region of the embryo, caudal to the oropharyngeal membrane, it is associated with a region of the endoderm called anterior visceral endoderm. That is why it can also be called meso-endodermal structure. This region will be important to induce the development of the head and the rostral portion of the central nervous system. 2• The notochord, formed on the middle line, caudal to the prechordal plate and rostral to the primitive streak. Together with the prechordal plate, it will disappear in the adult human, but they are very important during the early stages of development. 3• The paraxial mesoderm develops on the sides of the notochord. 4• The intermediate mesoderm develops on the sides of the paraxial mesoderm. 5• The lateral plate mesoderm is even more laterally respect to the intermediate mesoderm. 6• Part of the extraembryonic mesoderm will be formed as a further migration; it will form the chorionic cavity. 7• The primitive heart field will be formed rostral to the oropharyngeal membrane When mesodermal cells migrate through the primitive streak, they migrate in different region of the primitive streak depending on which subdivision of the mesoderm they have to form (according to the previous image).

Describe the periods of susceptibility to teratogens:

A teratogen is an agent, chemical, physical or biological that alters fetal morphology or function if the fetus is exposed to it during a critical stage of development -- because there are periods, where the embryo is more susceptible to teratogens (usually the period of the first few month -- 3 to 8 months). 1. Maximum susceptibility period (weeks 3-8, embryonic period) teratogens can affect the development of organs, since this is when proper organogenesis happens (all the systems are really coming in to be -- organs are developed) 2. Maximum susceptibility period (weeks 3-8, embryonic period) teratogens can affect the development of organs, since this is when proper organogenesis happens (all the systems are really coming in to be -- organs are developed) 3. Maximum susceptibility period (weeks 3-8, embryonic period) teratogens can affect the development of organs, since this is when proper organogenesis happens (all the systems are really coming in to be -- organs are developed)

A normal somatic cell contains a total of 46 chromosomes. What is the normal complement of chromosomes found in a sperm? (A) 22 autosomes plus a sex chromosome (B) 23 autosomes plus a sex chromosome (C) 22 autosomes (D) 23 autosomes (E) 23 paired autosomes

A. A normal gamete (sperm in this case) contains 23 single chromosomes. These 23 chromosomes consist of 22 autosomes plus 1 sex chromosome.

Concerning maturation of the female gam- ete (oogenesis), when do the oogonia enter meiosis I and undergo DNA replication to form primary oocytes? (A) During fetal life (B) At birth (C) At puberty (D) With each ovarian cycle (E) Following fertilization

A. All primary oocytes are formed by month 5 of fetal life, so no oogonia are present at birth.

In the production of male gametes, which of the following cells remains dormant for 12 years? (A) Primordial germ cell (B) Primary spermatocyte (C) Secondary spermatocyte (D) Spermatid (E) Sperm

A. Primordial germ cells migrate from the wall of the yolk sac during week 4 of embryonic life and enter the gonad of a genetic male, where they remain dormant until puberty (about age 12 years), when hormonal changes in the young man stimulate the production of sperm.

In the process of meiosis, DNA replication of each chromosome occurs, forming a structure consisting of two sister chromatids attached to a single centromere. What is this structure? (A) A duplicated chromosome (B) Two chromosomes (C) A synapsed chromosome (D) A crossover chromosome (E) A homologous pair (D) A crossover chromosome (E) A homologous pair

A. The structure formed is a duplicated chromosome. DNA replication occurs, so that the amount of DNA is doubled (2 × 2N = 4N). However, the chromatids remain attached to the centromere, forming a duplicated chromosome.

Where do primordial germ cells initially develop? (A) In the gonads at week 4 of embryonic development (B) In the yolk sac at week 4 of embryonic development (C) In the gonads at month 5 of embryonic development (D) In the yolk sac at month 5 of embryonic development (E) In the gonads at puberty

B. Primordial germ cells, the predecessors to gametes, are first seen in the wall of the yolk sac at week 4 of embryonic development, and they migrate into the gonads at week 6.

Describe the initial development of the uteroplacental circulation

By the end of the 2nd week of development there is the development of the uteroplacental circulation. The cytotrophoblast starts to grow and forms the primary villi, covered by syncytiotrophoblast and then, at the core of them, the extraembryinoic mesoderm enters to form the secondary villi and then, from the extraembryonic mesoderm, we have the formation of blood vessels that form the fetal placental microcrculation. The villi are bathed by the blood present in the lacunae and the wall of the villi will become very thin, thus to allow an exchange of nutrients, gases, wastes between blood from the fetus with things that are in the maternal blood present in the lacunae.

Define the concept of phylotypic stage

By the end of the 4th week, the human embryo is at the phylotypic stages, which means that it looks like others embryo of the same phylum (vertebrates).

List and highlight the importance of the major key transitions of animal evolution

By the end of the 4th week, the human embryo is at the phylotypic stages, which means that it looks like others embryo of the same phylum (vertebrates). Major key transitions can be noted in animal evolution: 1. Formation of tissues, without differentiation of cells, lots of functions can't be performed 2. Symmetry 3. Body cavities 4. Segmentation of the body The 2 major macroscopic events of the 4th week are the folding of the embryo with the formation of the body cavities and the segmentation of the body.

Which of the following describes the num- ber of chromosomes and amount of DNA in a gamete? (A) 46 chromosomes, 1N (B) 46 chromosomes, 2N (C) 23 chromosomes, 1N (D) 23 chromosomes, 2N (E) 23 chromosomes, 4N

C. Gametes contain 23 chromosomes and 1N amount of DNA, so that when two gametes fuse at fertilization, a zygote containing 46 chromosomes and 2N amount of DNA is formed.

A young woman enters puberty with approximately 40,000 primary oocytes in her ovary. About how many of these pri- mary oocytes will be ovulated over the entire reproductive life of the woman? (A) 40,000 (B) 35,000 (C) 480 (D) 48 (E) 12

C. Over her reproductive life, a woman will ovulate approximately 480 oocytes. A woman will ovulate 12 primary oocytes per year, provided that she is not using oral contraceptives, does not become pregnant, or does not have any anovulatory cycles. Assuming a 40-year reproductive period gives 40 × 12 = 480.

How much DNA does a primary spermato- cyte contain? (A) 1N (B) 2N (C) 4N (D) 6N (E) 8N

C. Type B spermatogonia give rise to primary spermatocytes by undergoing DNA replication, thereby doubling the amount of DNA (2 × 2N = 4N) within the cell.

During meiosis, pairing of homologous chromosomes occurs, which permits large seg- ments of DNA to be exchanged. What is this process called? (A) Synapsis (B) Nondisjunction (C) Alignment (D) Crossing over (E) Disjunction

D. Synapsis (pairing of homologous chromosomes) is a unique event that occurs only during meiosis I in the production of gametes. Synapsis is necessary so that crossing over, whereby large segments of DNA are exchanged, can occur.

Fetal sex can be diagnosed by noting the presence or absence of the Barr body in cells obtained from the amniotic fluid. What is the etiology of the Barr body? (A) Inactivation of both X chromosomes (B) Inactivation of homologous chromosomes (C) Inactivation of one Y chromosome (D) Inactivation of one X chromosome (E) Inactivation of one chromatid

D. The Barr body is formed from inactivation of one X chromosome in a female. All somatic cells of a normal female will contain two X chromosomes. The female has evolved a mechanism for permanent inactivation of one of the X chromosomes presumably because a double dose of X chromosome products would be lethal.

During ovulation, the secondary oocyte resides at what specific stage of meiosis? (A) Prophase of meiosis I (B) Prophase of meiosis II (C) Metaphase of meiosis I (D) Metaphase of meiosis II (E) Meiosis is completed at the time of ovulation

D. The secondary oocyte is arrested in metaphase of meiosis II about 3 hours before ovulation, and it remains in this meiotic stage until fertilization.

Describe the characteristics of the mucosa of the uterus at the end of the endometrial cycle

Decidual reaction 1. The mucosa of the uterus, in the second phase of the uterine cycle, starts to undergo to a decidual reaction, which is carried on only if there is the implantation of the blastocyst. This reaction consists in the changes in the lamina propria of the endometrium. The fibroblasts in contact with the syncytiotrophoblast become bigger, rounded and epitheliod-like. They are filled with fluids, lipids and glycogen and they form a massive celular matrix surrounding the embryo and later occupying the whole endometrium. Many of the changes are related to the formation of an immunologically privileged site (leukocytes secrete interleukin 2). 2. When implantation takes place, the endometrium undergoes the decidua reaction and from that moment we call it decidua. We can divide the decidua into a decidua basalis, the one on the side where the embryo is implanted; into a decidua capsularis, a very thin layer of mucosa cells, which keeps on covering the amniotic sac, even when it starts to protude into the uterus, and into the decidua parietalis, which is the decidua on the other side respect to where the uterus has been implanted.

Outline the steps of the folding of the embryo and its outcomes: formation of the primitive intestinal tube, formation of the body cavities, formation of the body wall.

During the 4th week the embryo starts to fold. We have the formation of a tube within a tube: the outer one is due to the formation of the body wall, which now delimits the embryo and the intraembryonic coelom; the second one is formed by the folding of the embryo, which causes a partial incorporation of the vitelline sac inside the body of the embryo, forming a long tube called primitive intestine. There is a differential growth of various embryonic structures: • The embryonic disc and the amnion grow vigorously, while the yolk sac hardly grows at all. • The developing notochord, the neural tube and the somites stiffen the dorsal axis of the embryo: most of the folding is concentrated in the thin, flexible outer rim of the disc. The cranial, caudal, and lateral margins of the disc fold completely under the dorsal axial structure and give rise to the ventral surface of the body. Folding takes place into planes. The trilaminar disc becomes a "cylindrical embryo": there are foldings on the median/sagittal plane (longitudinal folds, rostral and caudal) and foldings on the horizontal/transverse plane (lateral folds). Folds come together in the region of the future umbilicus.

Approximately how many sperm will be ejaculated by a normal fertile male during sexual intercourse? (A) 10 million (B) 20 million (C) 35 million (D) 100 million (E) 350 million

E. A normal fertile male will ejaculate about 3.5 mL of semen containing about 100 million sperm/mL (3.5 mL × 100 million = 350 million).

Which of the following chromosome com- positions in a sperm normally results in the production of a genetic female if fertilization occurs? (A) 23 homologous pairs of chromosomes (B) 22 homologous pairs of chromosomes (C) 23 autosomes plus an X chromosome (D) 22 autosomes plus a Y chromosome (E) 22 autosomes plus an X chromosome

E. A sperm contains 22 autosomes and 1 sex chromosome. The sex chromosome in sperm may be either the X chromosome or the Y chromosome. The sex chromosome in a secondary oocyte is only the X chromosome. If an X-bearing sperm fertilizes a secondary oocyte, a genetic female (XX) is produced. Therefore, sperm is the arbiter of sex determination.

When does formation of primary spermato- cytes begin? (A) During week 4 of embryonic life (B) During month 5 of fetal life (C) At birth (D) During month 5 of infancy (E) At puberty

E. At birth, a male has primordial germ cells in the testes that remain dormant until puberty, at which time they differentiate into type A spermatogonia. At puberty, some type A spermatogo- nia differentiate into type B spermatogonia and give rise to primary spermatocytes by undergo- ing DNA replication.

Clinical drop: What is an ectopic pregnancy?

Ectopic pregnancy: All the other pregnancies that are not carried out in the posterior wall of the uterine cavity are defined as ectopic pregnancy. 95% of them takes place in the uterine tube, causing uterine tube rupture and haemorrhage; but they can also develop in the ovary, in the cervical canal, in the abdominal or on the rectouterine pouch. ETP can cause abnormal uterine bleeding, pelvic pain, abdominal pain, massive first trimester bleeding. ETP at the right uterine tube can be confused with appendix inflammation. HCG is produced at a slower rate and to make sure of possible ETP, transvaginal ultrasonography is very helpful for early tubal pregnancy. Risk factors are pelvic inflammatory disease (which produce scar tissue in the tubes), endometriosis, smoking (which affects the cilia).

Outline the transport of the egg and sperm up to the site of fertilization:

Egg transport to site of fertilization: 1. The ovulated complex slides out from the ovary and it is captured by the fimbriae of the tube, which gently sweeps the surface of the ovary, capturing the complex. 2. After the capture, it is transported along the surface of the tube for 3-4 days. 3. The transport is carried out by the contraction of the muscular wall of the uterine tube and movement of cilia. 4. The transport is made by 2 phases: a slow transport along the ampullary region (where the egg will be fertilized), slow because it tries to favour the meeting of the spermatozoa with the oocyte and it lasts around 72 hours; then there is a rapid phase (8 hours) during which the egg or the zygote is transported from the isthmus of the uterine tube to the uterine cavity. Sperm transport to site of fertilization: 1. The spermatozoa are in the epididymis and when there is an ejaculation there is a rapid transit from the tail of the epididymis to the ductus deferens and then into the male urethra and then into the female vagina. 2. The sperm is made by seminal fluid (2 - 6 ml, pH 7.2 - 7.8), which is enriched by prostatic secretion (acidic, citric acid, acid phosphates, magnesium, zinc) and by seminal vesicle secretion( basic, fructose main source of energy for spermatozoa). 3. An ejaculation usually contains around 300 million of spermatozoa. 4. The spermatozoa may retain their function in the female reproductive tract for around 80 hours. Then they encounter several barriers until they reach the egg (only one).

Correlate the ovarian cycle and the uterine cycle and their hormonal regulation:

Events of the Ovarian and Uterine Cycle 1. At sexual maturity, each of a woman's ovaries contains about 200,000 immature eggs, called primary oocytes. A primary oocyte is diploid (2n), and is arrested in prophase I of meiosis. A layer of follicle cells surrounds each primary oocyte. Together, an oocyte and its follicle cells make up a follicle. 2. An ovarian cycle lasts about 28 days, beginning at the first day of menstruation, or menses. During the first seven days of the cycle, six to twelve primary oocytes begin to mature. As the follicles develop, the follicle cells communicate with oocytes and pass them nutrients through pores called gap junctions. 3. Each oocyte grows larger and the surrounding follicle cells divide, proliferating to produce thousands of follicle cells in a single follicle. 4. By day 7, all but one of the developing follicles begins to degenerate. The remaining follicle continues to develop, and its follicle cells continue to pump it with nourishment and also supply it with proteins and informational molecules needed for early stages of development. 5. The maturing primary oocyte completes meiosis I and divides into two haploid (n) cells. Each of these cells receives half the chromosomes. However, one cell, called a polar body, receives very little cytoplasm. The other, now a secondary oocyte, enters meiosis II and arrests there until fertilization. 6. At day 14, ovulation occurs, and the secondary oocyte erupts from the ovary. The oviduct contains microscopic cilia that beat and draw in the released oocytes. This immature egg enters an oviduct, where it may become fertilized by a sperm cell and complete meiosis. 7. The follicle cells that are left behind develop into a small mass of endocrine tissue called the corpus luteum. The corpus luteum remains in the ovary for two weeks, secreting the hormones estrogen and progesterone. At the end of the ovarian cycle, if the woman is not pregnant, the corpus luteum disintegrates. 8. Each month, the ovarian cycle is tightly coordinated with the uterine cycle. In the uterine cycle, the lining of the uterus builds up and then sloughs off. The cycle begins with the sloughing of the uterine lining. This is the first day of menses, also called menstruation. 9. After menstruation, the uterine lining starts to grow again and to prepare for implantation of an embryo. During this phase of the uterine cycle, up until ovulation, the uterine lining proliferates. Capillary beds supply this tissue with nutrients. 10. Just before menstruation occurs again, the capillary beds degenerate and no longer deliver the surrounding tissue with nutrients. During menstruation, this tissue dies and sloughs off through the vagina to the outside of the body.

Outline the importance of gastrulation

Gastrulation is the process that establishes the three definitive germ layers of the embryo (ectoderm, intraembryonic mesoderm, and endoderm), forming a trilaminar embryonic disk by day 21 of development. These three germ layers give rise to all the tissues and organs of the adult. During the second week of development, the embryo has became a bilaminar structure, but during the 3rd week the embryo become a trilaminar structure and from these 3 laminae we will have the development of all the structure of the human body. The process of bringing the embryo to be trilaminar is called gastrulation. The gastrulation is the process that occupy the whole 3rd week and then it finishes at the beginning of the 4th week.

Summarize some basic molecular events involved in the rostrocaudal and mediolateral patterning of the embryo.

How cell signals determines body organization 1. The caudal and rostral part of the embryo is determined by the expression and inhibition of some factors in some region of the embryonic disk. In the early phase of development, proteins such as Nodal, BMPs, Wnts, FGFs retinoic acid, are expressed in the same concentration everywhere. But when the prechordal plate forms, it starts to communicate with the epiblast and produces some factors, like Cerberus, that starts to inhibit the expression of those factor in the rostral region of the embryo. 2. In the organization of the body, we can say that we have some Body organizers, one for the head, one for the trunk and one for the tail. These organizers make so that the levels of Wnt, Nodal and BMP are different according to the area (see image). 3. Cranio-caudal axis is also controlled by Hox genes expression. Hox genes are expressed along specific pathway from rostral to caudal and this is true in every animal. Hox genes are transcription factors that activate or repress expression of other genes. They are important for segmentation. 4. There are some molecules that are expressed with a rostral to caudal gradient, but at the same way, there are some others expressed medio to lateral (or dorsal to ventral, because when the embryo folds, we can't use the term "lateral" anymore). For example, some of them are more concentrated close to the midline, like Noggin, Nodal, Chordin; and then they decrease in concentration in the lateral part; while others are more concentrated laterally (Wnts, BMPs). These gradients of molecules are very important to drive the subdivision of mesoderm.

Clinical drop: What is mittelschmerz?

Mittelschmerz: It can happen that a woman, at the moment of ovulation (14 days of cycle, 38h after LH surge) can suffer of abdominal and/or pelvic pain. It can be caused by some bleeding, at the moment of ovulation, in the abdominal peritoneal cavity, which can cause irritation of the peritoneum. Also, the enlargement of the follicle before ovulation can somehow exert a pressure on the surrounding structures, causing pain.

Define the organogenetic period

Organogenetic period (4th - 8th weeks) By the end of the 8th week, all the major internal and external structures are established. The organs are not working yet, they are just at the beginning and have a minimal organ function, an exception is the cardiovascular system. The primitive heart starts to beat around the 21st - 22nd day of gestation and it can be heard by Doppler ultrasonography during the 5th week (approximately 7 weeks after the last known menstrual period). The heart beats 54 million times before birth.

Hormones that control the uterine cycle:

Ovarian Hormones Control the Uterine Cycle: 1. The ovarian and uterine cycles are tightly coordinated. Hormones secreted by the ovary at different phases of the ovarian cycle trigger changes in the uterine lining. For example, at the beginning of the cycles, the levels of estrogen and progesterone are too low to maintain the uterine lining, and menses begins. 2. About a week into the ovarian cycle, the developing follicle increases its secretion of estrogen, and estrogen levels in the body begin to rise. This hormone triggers the cells of the uterine lining to proliferate, and the lining becomes thicker. 3. Just before ovulation, the level of estrogen in the body has reached its peak. Afterward, the follicle cells remaining in the ovary develop into the corpus luteum, a structure that releases estrogen and progesterone. The hormones maintain the uterine lining at a peak thickness and preparedness for embryo implantation. 4. At the end of the cycle, if the egg is not fertilized or has not implanted, the corpus luteum breaks down and stops releasing estrogen and progesterone. Without these hormones, the uterine lining also breaks down, initiating menses.

Explain the difference between developmental potency and developmental fate

Until the 16-cells stage, cells can still transform, they can become either part of the inner cell mass, either part of the trophoblast. This leads to the developmental potency and fate: the first corresponds to all the types of cells to which a precursor can give rise; the second is the type of cells to which normally a precursor can give rise.

Explain the events taking place at ovulation

Ovulation (day 14) 1. It takes place around 38 hours after LH and FSH surge. Ovulation is a sort of inflammatory reaction: the follicular cells and the theca cells start to produce a lot of inflammatory proteins and enzymes, which start to digest the wall of the follicles and the surface of the ovary. Thus, there is a loosening of cumulus oophorus cells and part of the follicular complex slides outside the ovary. The ovulated complex consists of the secondary oocyte, the zona pellucida, the portion of the cumulus oophorus that was attached to the oocyte, which takes the name of corona radiata and a sticky matrix (some of the fluid of the antral cavity, quite sticky due to the hyaluronic acid, and some others follicular cells). 2. It can happen that a woman at the moment of ovulation can suffer from abdominal and/or pelvic pain of variable amount. This is called Mittelschmerz. The cause of this is probably due to some small bleeding in the abdominal peritoneal cavity, at the moment of the opening of the ovary, which can cause irritation of the peritoneum. Also, the enlargement of the follicle before ovulation can somehow exert a pressure on the surrounding structures, producing pain. 3. The rest of the cells of the oocyte remains in the cortex of the ovary (theca interna and externa and most of the granulosa cells) and they keep producing hormones. They are called the corpus luteum (due to its yellowish appearance).

Explain the phases of gametogenesis:

PHASE 1: in the flat undifferentiated embryo some cells start to form: primordial germ cells; then they migrate. PHASE 2: increasing number of those primordial germ cells through mitosis. PHASE 3: meiosis (to reduce the amount of genetic material at 50% in order to make fertilization possible) PHASE 4: maturation of egg and sperm (we should consider how at the beginning of their life cycle spermatozoa are rounded cells and then they mature becoming tapered cells)

Hormones that control the ovarian cycle:

Pituitary and Ovarian Hormones Control the Ovarian Cycle: 1. The ovarian cycle is controlled by the interplay of hormones from the pituitary gland and from the ovary itself. A few days before the beginning of the cycle, the anterior pituitary begins to increase its secretion of two hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). 2. FSH and LH stimulate ovarian follicles to grow. As the follicles grow, they begin to secrete estrogen. During this phase of the cycle, the increasing levels of estrogen feed back on the pituitary to inhibit the release of additional FSH and LH. During the next week, the levels of FSH and LH drop. 3. Beginning around day 12, the increasing levels of estrogen suddenly have the opposite effect on the pituitary gland. Instead of exerting a negative feedback on the pituitary, these hormones now exert a positive feedback, stimulating the pituitary to release FSH and large amounts of LH. 4. LH reaches a peak at day 14 of the ovarian cycle. This LH surge triggers the mature follicle to rupture and release the eggÑthe process of ovulation. LH then triggers the remaining follicle cells to differentiate into the corpus luteum, which secretes estrogen and progesterone. 5. The corpus luteum remains in the ovary, secreting estrogen and progesterone, for the last two weeks of the cycle. At this point in the cycle, these hormones again inhibit the release of FSH and LH. A decline in FSH and LH restricts follicles from beginning to develop during the second half of the cycle. 6. LH (or a hormone produced by an implanted embryo) is required to maintain the life of the corpus luteum. At the end of the cycle, if an embryo has not implanted, the corpus luteum degenerates. When the corpus luteum degenerates, it no longer releases estrogen and progesterone.

Explain the relationship between somites and neural tube development

Segmentation and neural tube 1. Segmentation of the paraxial mesoderm affects also the neural tube which is medial to the paraxial mesoderm. The region of the neural tube near the paraxial mesoderm is going to become the spinal cord. There are different somites on both side of the future spinal cords and neurons and sensory ganglia starts to innervate the derivatives of those somites, therefore the spinal cords will organize in a segmented way. 2. The derivatives of the somites, sclerotome, dermatome, and myotome, will be innervated by the neural tube, which means that every single derivative of the sclerotome, dermatome and myotome will be innervated as well. Each segment innervates the cutaneous territory (dermatome), the muscular territory (myotome) and the bones-tendinous territory (sclerotome).

What are developmental defects during gastrulation?

Sirenomielia; ethanol as a teratogen (holoprosencephaly); Shh signaling pathways and the primary cilium 1. Gastrulation continues until the begin of the 4th week, that happens because the process does not take place all together, but it proceeds rostral to caudal. During the 4th week, where there is the gastrulation of the caudal region, there can be some environmental factors or genetic mutations or some others problems (like maternal diabetes with elevated insulin level) that can cause caudal dysplasia or sirenomielia. This condition can also cause defects in the last portion of digestive tract and in the urinary tract. 2. The end of gastrulation can be very critical because can generate caudal dysplasia, but also at the beginning of gastrulation there is a critical period. If something happens at the begins of the third week the embryo is very likely to die. 3. Ethanol can be a very powerful teratogen: it alters the expression of genes of the middle line of the body. This can cause the failure in the activation of Hox genes and a deficiency in the activation of the Shh pathway. If those mechanism don't get activated there is an alteration in the development of the body, because what should be separated does not get separated, causing pathology such as Cyclopia and Holoprosencephaly.

What are developmental defects that occur during the 'breaking down' of the symmetry of the embryo?

Situs inversus and Kartagener syndrome Anomalies in the ciliary movement: Mutation in the Lrd gene can cause the Situs Inversus Totalis, which means that all the organs are switched to the wrong side. From the functional point of view nothing is wrong with those with this problem. The problems may arise if the breaking down of the symmetry takes place a little bit later, due, for example, to genetic pathologies that affect all the cilia in the body, we have a condition which is called Kartagener syndrome, that cause situs inversus totalis and immotile cilia all over the body. These immotile cilia don't allow the respiratory cilia and the sperm cilia to work properly, that causes the accumulus of mucus and the consequently damaging of the lungs tract.

Explain the structural and functional maturation of sperm:

Spermatogenesis is classically divided into three phases: A. Spermatocytogenesis 1. Primordial germ cells (46, 2N) from the wall of the yolk sac arrive in the testes at week 6 and remain dormant until puberty. At puberty, primordial germ cells differentiate into type A spermatogonia (46, 2N). 2. Type A spermatogonia undergo mitosis to provide a continuous supply of stem cells throughout the reproductive life of the male. Some type A spermatogonia differentiate into type B spermatogonia (46, 2N). B. Meiosis 1. Type B spermatogonia enter meiosis I and undergo DNA replication to form primary spermatocytes (46, 4N). 2. Primary spermatocytes complete meiosis I to form secondary spermatocytes (23, 2N). 3. Secondary spermatocytes complete meiosis II to form four spermatids (23, 1N). C. Spermiogenesis 1. Spermatids undergo a postmeiotic series of morphological changes to form sperm (23, 1N). These changes include the (a) formation of the acrosome, (b) condensation of the nucleus, and (c) formation of head, neck, and tail. The total time of sperm formation (from spermatogonia to spermatozoa) is about 64 days. 2. Newly ejaculated sperm are incapable of fertilization until they undergo capacitation, which occurs in the female reproductive tract and involves the unmasking of sperm glycosyltransferases and the removal of adherent plasma proteins coating the surface of the sperm.

Compare male and female gametogenesis and summarize their differences

Spermatogenesis vs. Oogenesis 1. During spermatogenesis, primary spermatocytes go through the first cell division of meiosis to produce secondary spermatocytes. These are haploid cells. 2. Secondary spermatocytes then quickly complete the meiotic division to become spermatids, which are also haploid cells. 3. The four haploid cells produced from meiosis develop a flagellum tail and compact head piece to become mature sperm cells, capable of swimming and fertilizing an egg. 4. The compact head, which has lost most of its cytoplasm, is key in the formation of a streamlined shape. 5. The middle piece of the sperm, connecting the head to the tail, contains many mitochondria, providing energy to the cell. 6. The sperm cell essentially contributes only DNA to the zygote. 1. On the other hand, the egg provides the other half of the DNA, but also organelles, building blocks for compounds such as proteins and nucleic acids, and other necessary materials. 2. The egg, being much larger than a sperm cell, contains almost all of the cytoplasm a developing embryo will have during its first few days of life. 3. Therefore, oogenesis is a much more complicated process than spermatogenesis. 4. Oogenesis begins before birth and is not completed until after fertilization. 5. Oogenesis begins when oogonia (singular, oogonium), which are the immature eggs that form in the ovaries before birth and have the diploid number of chromosomes, undergo mitosis to form primary oocytes, also with the diploid number. 6. Oogenesis proceeds as a primary oocyte undergoes the first cell division of meiosis to form secondary oocytes with the haploid number of chromosomes. 7. A secondary oocyte only undergoes the second meiotic cell division to form a haploid ovum if it is fertilized by a sperm. 8. The one egg cell that results from meiosis contains most of the cytoplasm, nutrients, and organelles. 9. This unequal distribution of materials produces one large cell, and one cell with little more than DNA. 10. This other cell, known as a polar body, eventually breaks down. 11. The larger cell undergoes meiosis II, once again producing a large cell and a polar body. 12. The large cell develops into the mature gamete. 13. The unequal distribution of the cytoplasm during oogenesis is necessary as the zygote that results from fertilization receives all of its cytoplasm from the egg. So the egg needs to have as much cytoplasm as possible.

Explain the origin of teratomas:

Teratomas are a type of germ cell tumors and include components/tissues (eg. eyes, hair, tooth, etc.) derived by the 3 embryonic layers (ectoderm, mesoderm and endoderm): they can form in the gonads or in extragonadal site. PGCs are pluripotent germ cells that can give rise to different tissues. Some of the tumors that originate in the PGCs can have tissues from the body on them (embryonic tissues) = Teratomas.

Outline the phases of implantation of the blastocyst in the endometrium (apposition, adhesion, invasion)

The 3 stages of implantation are apposition, adhesion and invasion. Apposition and adhesion: 1. The apposition of the blastocyst can only occur in a period of time known as the implantation window, a specific period of days in which the mucosa of the uterus is really primed to attach the blastocyst. This reception-ready phase last around 4 days and it happens 6 days after the LH surge. When there is the apposition, it is not an attachment, which means that the embryo can still be eliminated. 2. Apposition and adhesion take place through a signaling process between the trophoblast and the endometrium. They start to express adhesion molecules: on the trophoblast side, appear some microvilla, instead on the endometrium there will be some changes, such as a thinner glycocalix, microvilli disappear to prepare a flattened surface and pinopodes appear. The blastocyst is going to be implanted on the side of the inner cell mass. Moreover there is also a process of regulation of immunotolerance: the embryo is a semi-allograft (A transplant, half of whose genes come from another individual). 3. In the adhesion process, there is the involvement of cytokines (Cytokines are a broad and loose category of small proteins important in cell signalling), by the endometrium (such as the LIF, Leukemia inhibiting factor) and receptors, expressed on the trophoblast. The endometrium, in the last phase of the cycle, is infiltrated by leukocytes, which produce interleukin 2 that prevents recognition of the embryo as a foreign body. Invasion 1. Below the endometrium of the mucosa, we can find the myometrium, the muscle layer. The blastocyst hast to invade the endometrium, without invading the myometrium. The process of invasion is basically an erosion process. 2. During invasion, the trophoblast can be divided into cytotrophoblast and syncytiotrophoblast, which is the part that comes in contact with the endometrium and that starts to undergo several changes, like the fusion of the outer trophoblast. The syncytiotrophoblast is highly invasive, it produces a lot of digestive enzymes, which will digest first the epithelium and then the lamina propria of the endometrium. At day 7 the syncytiotrophoblast has just started approaching the endometrium and it isn't developed that much. In the 8th day, instead, it has penetrated deeply in the endometrium and almost all the blastocyst has entered in the endometrium. At day 9, the blastocyst is completely enveloped by the syncytiotrophoblast and it has completely entered into the endometrium. During the invasion, there is the formation of lots of capillaries and glands. And at the 9th day, the syncytiotrophoblast has also reached the level of the capillaries. 3. In the point from where the blastocyst entered, now a coagulation plug is formed. The invasion of the embryo is an inflammation process that can cause some bleeding as well. Thus, during this period (after 9 days), the woman can experience a possible spotting, a small quantyty of blood lost, which can be misinterpreted as an abnormal menstrual period. 4. During the 9th day, in the syncytiotrophoblast, we can start noticing some lacunae, and since it is very erosive, it will start eroding also the capillaries that at that point will have accumulated glycogens and other nutrients. Then the content of the vessels flows inside of the lacunae: maternal blood enter in contact with the blastocyst. 5. The process of invasion of the endometrium is regulated by lots of factors, among them the progesterone, produced by the corpus luteum, which in the meanwhile has became an endocrine organ. From the capillaries, progesterone is brought to the embryo, thus to regulate the endometrial functions and the proteolytic activity of the trophoblast. At the same time the syncytiotrophoblast produces the Human chorionic gonadotropin (hCG), which is used to maintain the activity of the corpus luteum. The hCG can be found in maternal blood by day 8 and in urine by day 10. 6. Moreover, the syncytiotrophoblast releases some angiogenic factors (VEGF, PDGF, PAF, VCAM...), which are used to promote an increasing vascularization of the endometrium, so that more blood can flow into the lacunae. 7. In the meanwhile, the production of digestive factors has to be stopped, since we don't want the syncytiotrophoblast to reach the myometrium. This is why the endometrium (the decidual cells, a distinctive cell population observed in endometrium during pregnancy) produces different cytokines and growth factors that are able to regulate the process of invasion.

List the derivatives of the endodermal layer

The endoderm will form the mucosa of the intestinal tube, the pancreas, the mucosa of the respiratory tract, the liver, the urinary bladder, the pharynx...

Clinical drop: What is epithelio-mesenchymal transition?

The invagination of the primitive streak is formed because gastrulation is started. This means that some cells of the epiblast starts to migrate toward the middle line and invaginate below the epiblast, causing a depression called primitive streak. There are several factors that guide the migration of cells towards the primitive streak, one of these is a grow factor called FGF 8(fibroblast grow factor 8). The cells which are undergoing migration are epithelial cells, so in order to be able to migrate, it is necessary that those cells undergo an epithelio-mesenchymal transition (EMT), which permits a change from cell-to-cell to cell-to substrate adhesion, the size of the cells also changes and then there is a cytoskeletal reorganization, with the formation of lamellipodia and filopodia. Due to all these necessary changes, this process is quite difficult and complex. When the cells reach the right position, they undergo an opposite transition, a mesenchymal-epithelio transition.

List the normal and abnormal sites of implantation.

The normal site of implantation for an embryo is the posterior wall of the uterine cavity. All the other pregnancies that are not implanted there are called ectopic pregnancy (ETP).

Clinical drop: What are chordomas?

The notochord is going to disappear early in the embryo development. The only remanence of it in our body are the nucleus polposus of the intervertebral disks. But it may happen that some pieces of the notochord remain, either at the base of the skull or somewhere in the vertebral column. If this happens, they can give rise to chordomas, malignant cancers that may take years to grow, they have a very slow growth. This means that when they are discovered they will be quite large, fact that can cause compression of the brain or/and of the spinal structures. The most common symptoms are headache and double vision.

Calculate the date of birth:

We can date pregnancy either counting from the fertilization date or the onset of the last menstrual period. 1. Traditionally, determining the first day of the LMP (last menstrual period) is the first step in establishing the EDD (expected day of delivery). 2. By convention, the EDD is 280 days after the first day of the LMP. Because this practice assumes a regular menstrual cycle of 28 days, with ovulation occurring on the 14th day after the beginning of the menstrual cycle, this practice does not account for inaccurate recall of the LMP, irregularities in cycle length, or variability in the timing of ovulation. 3. Truth is, the best tool to determine exactly the developmental stage of an embryo and the EDD is through using ultrasound. 4. Ultrasound measurement of the embryo or fetus in the first trimester (up to and including 13 6/7 weeks of gestation) is the most accurate method to establish or confirm gestational age.

Illustrate the exacting requirements of the relationship between the embryo and the mother

With embryonic adnexa we mean the placenta and the extraembryonic membrane. The embryonic adnexa is there because the embryo must establish a "parasitic" relationship with the mother for acquiring oxygen and nutrients and eliminating wastes, moreover it must avoid the embryo to be rejected as a foreign body.

What are developmental defects of the diaphragm?

congenital diaphragmatic hernia 1. During the formation of the diaphragm, we need the contribution of other structures like the mesentery of the future oesophagus, of the pleuro-peritoneal folds and of the mesenchyme of the body wall to grow. 2. But it may happen that the closure of the communication of the cavities does not take place on one of the 2 sides, leading to a congenital diaphragmatic hernia (CDH). If there is an opening, some of the organs of the abdominal region, due to a high pressure, protrudes in the thorax, leading to compression of one of the 2 lungs, which will be hypoplaxic, which will cause dyspnea and maybe polyhydramnios.

Summarize the most important steps of the progressive embedding of the blastocyst in the uterine wall

o formation of the cytotrophoblast and syncytiotrophoblast o formation of the trophoblastic lacune o formation of the initial uteroplancental circulation

Explain the structural and functional maturation of egg:

o maturation of the oocyte-follicle unit o hormonal regulation of folliculogenesis 1. Primordial germ cells (46, 2N) from the wall of the yolk sac arrive in the ovary at week 6 and differentiate into oogonia (46, 2N), which populate the ovary through mitotic division. 2. Oogonia enter meiosis I and undergo DNA replication to form primary oocytes (46, 4N). All primary oocytes are formed by month 5 of fetal life. No oogonia are present at birth. 3. Primary oocytes remain dormant in prophase (dictyotene) of meiosis I from month 5 of fetal life until puberty. After puberty, 5 to 15 primary oocytes begin maturation with each ovarian cycle, with usually only 1 reaching full maturity in each cycle. 4. During the ovarian cycle and triggered by the luteinizing hormone (LH) surge, a primary oocyte completes meiosis I to form two daughter cells: the secondary oocyte (23, 2N) and the first polar body, which degenerates. 5. The secondary oocyte promptly begins meiosis II but is arrested in metaphase of meiosis II about 3 hours before ovulation. The secondary oocyte remains arrested in metaphase of meiosis II until fertilization occurs. 6. At fertilization, the secondary oocyte completes meiosis II to form a mature oocyte (23, 1N) and a second polar body. 7. Approximate number of oocytes A. Primary oocytes: At month 5 of fetal life, 7 million primary oocytes are present. At birth, 2 million are present (5 million have degenerated). At puberty, 40,000 are present (1.96 million more have degenerated). B. Secondary oocytes: Twelve secondary oocytes are ovulated per year, up to 480 over the entire reproductive life of the woman (40 years × 12 secondary oocytes per year = 480). This number (480) is obviously overly simplified since it is reduced in women who take birth control pills (which prevent ovulation), in women who become pregnant (ovulation stops during pregnancy), and in women who may have anovulatory cycles.

How do PGCs give rise to the gonads?

• Formation of primordial germ cells (PGC) and their migration • Increase in number of PGM by mitosis • Reduction in chromosomal number by meiosis 1. During its early life (2nd week), the embryo is bilaminar. 2. In the epiblast (one of the two layers), we can notice the appearance of primordial germ cells (PGCs). The germ cells form inside the Epiblast (intraembryonal). 3. In phase 1, the embryo has also developed an extraembryonal yolk sac (vitelline sac) outside, where PGCs migrate (from inside to outside) during the 3rd week of pregnancy in order to give the embryo time to develop. 4. PCGs migrate back to the internal embryo (they reach the gonads) around the 5th or 6th week (from inside to outside back to inside). While they move, PGCs proliferate (they proliferate mainly in the gonads, but also during their migration). 1. In the ovary, PGCs are invested by support cells and become oogonia (they are 7 million and go down to 400 ovulated). 2. by the 5th month they have all entered the prophase of the first meiotic (of meiosis 1) = that is when they are called primary oocytes (surrounded by the somatic cells), which are then called primordial follicle (when they are associated with the somatic cells that surround them). 3. Meiosis I is completed at ovulation and meiosis II starts immediately after. If the oocyte is not fertilized, it will never complete meiosis II. 1. In males PGCs are located in the seminiferous tubules. They are named spermatogonia and they are dormant until puberty (there are very few spermatogonia compared to oogonia). 2. Only few of them proliferate before puberty and, anyways, not after the 6th week. Proliferation of PGCs during the early embryonic period. 3. They dormant (sleep) very early = from the 6th week until puberty. 4. Intensive proliferation after puberty throughout life. After puberty and throughout life spermatogonia intensively proliferate. Males are fertile also at an old age, whereas women are not. *** Males are fertile also at an old age ≠ from females 1. Reduction in the number of chromosomes 2. Reassortment of paternal and maternal chromosomes 3. Further redistribution of paternal and maternal genetic information during the crossing over of the first meiotic division 4. So mitosis and meiosis have different pacing for male and female gonocytes. 5. Formation of viable gametes: 4 in males, 1 in females -- the other 3 will eventually die, because there's only 1 viable female gamete.

Describe the decidual reaction

•The endometrial changes during pregnancy are known as the decidual reaction, and the altered endometrial lining is known as the decidua. •The decidual reaction involves remodeling of the extracellular matrix with changes in collagen, proteoglycans, and glycoproteins. •Decidualization also involves alterations in local immune cells and processes and changes in maternal spiral arteries