13a Placentation
What are the steps of implantation?
- Adhesion - Decidualization
What are the general functions of the placenta and embryonic/fetal membranes?
- Protect - Provide nutrition - Provide respiration - Provide excretion - Provide hormones
What two groups of cells emerge in as the blastocyst cavity enlarges?
- Trophoblasts: The tight junction-having peripheral cells. They will eventually form the fetal components of the placenta - Inner cell mass: A group of cells. Will go on to form the embryo (sometimes called the embryoblast)
What happens to the developing embryo after it becomes a morula
As it continues down the uterine tube it undergoes compaction: - The outer cells develop tight junctions -> form a seal that separates the inner cells from the outside world. once the tight junctions form, Na/K ATPases form on the outer cell plasma membrane, which moves fluid into the center of the morula. The fluid forms a space called the blastocoele - usually at this point the cell can be referred to as a blastocyst
Summary of the end of the second week of gestation.
At the end of the second week of gestation: The embryo consists of the: - Amniotic cavity - Bilaminar disc - Primary yolk sac It is connected to the placenta via the connecting stalk. It lies within the extraembryonic coelom or the chorionic cavity. The embryo is receiving maternal blood via the maternal blood vessels with are coming in contact with the cytotrophoblast and syncytiotrophoblast cells and the syncytiotrophoblast cells produce hCG human chorionic gonadotropin.
At what stage is any cell in a bastomere still able to become an embryo?
At the four cell stage. By the eight cell stage, the cells will begin to change
Describe blastocyst adhesion
At the site of implantation, the uterine endometrium expresses HB-EGF (Heparin-bound epidermal growth factor-like factor) which has affinity for heparan sulfate proteoglycans and EGF receptor (EGF-R) on the surface of the trophoblast. The binding of HB-EGF to EGF-R causes autophosphorylation and special microprosses on the uterine epithelium (pinopodes) to interact with microvilli on the trophoblasts. Special cells in the uterine wall called decidual cells and create a decidual zone. Local vascular permeability is observed at the implantation site.
What is the location of the placenta?
Away fro the cervix, usually closer to the fundum.
What happens with bebes and alcohol
Because of the physiochemical properties of alcohol (it is polar and small in size), it can easily diffuse from the maternal blood to the fetal blood. It travels throughout the fetus's body where it can damage the face, brain, or any other organ that is developing at the time of exposure. The fetus cannot metabolize alcohol the same way the mother does, which may result in higher levels of alcohol in the fetus for an extended period of time.
When is the uterine wall receptive to implantation?
Between days 20-23 of the menstrual cycle
What happens around day 11 and 12?
Blastocyst becomes completely embedded into the endometrium. At the embryonic pole the syncytiotrophoblast layer continues to penetrate the endometrial stroma and erodes the lining of the maternal capillaries. The syncytial lacunae become continuous with the maternal capillaries. Large cavities also develop in the extraembryonic mesoderm.
When is a developing embryo a morula
By day 3 the cells within the intact zona pellucida are called a morula.
What is the fetal component of the placenta?
Chorion frondosum (bushy chorion): chorionic villi facing the decidua basalis, chorionic plate and chorionic villi Note: villi facing the decidua capsularis atrophy forming a smooth chorion called the chorion leave.
Chorionic Villus Sampling vs Amniocentesis
Chorionic villus sampling (CVS) is a prenatal test that can detect genetic and chromosomal abnormalities of a fetus. CVS is performed between 10 and 12 weeks gestation, while amniocentesis is performed after 15 weeks. It is more likely that a CVS will involve multiple attempts - a failure to obtain an adequate sample of cells and the woman requiring a repeat test later on - when compared with amniocentesis. Pregnancies complicated by isoimmunization can be followed by serial assessment of the amniotic fluid for bilirubin.
What day does the bastocyst cavity appear?
Day five of gestation (or 19 days after the beginning of the woman's last menstrual period.
What are the contents of the placenta?
Decidua basali = maternal portion of the placenta Villus corion - Fetal part
What are the three maternal components of the placenta?
Decidua basalis: maternal component of the placenta Decidua capsularis: the superficial layer covering the fetus and its chorionic sac Decidua parietalis: the rest of the lining of the uterine cavity not occupied by the fetus
describe Gastrulation
During gastrulation, cell movements result in a massive reorganization of the embryo from a simple spherical ball of cells, the blastula, into a three-layered organism. The primitive node regresses towards the anus. During gastrulation, many of the cells at or near the surface of the embryo move to a new, more interior location. Cilia.
What are Hofbauer cells?
Fetal macrophages - Reside within the chorionic villus. They can be detected as early as 4 weeks post-conception. They are implicated in placental morphogenesis, immune regulation, control of stromal water content, and the transfer of ions and serum proteins across the maternal-fetal barrier. Derangements in Hofbauer cell homeostasis are associated with infection, inflammation, and inadequate placental development.
What happens during the 3rd and 4th week?
Four membranes can be seen: the amnion, yolk sac, allantois, and chorion. These membranes lie outside the embryo and are called extraembryonic membranes. These membranes protect and nourish the embryo and later the fetus.
Infections are bad
Infections known to produce congenital defects have been described with the acronym TORCH (Toxoplasma, others, rubella, cytomegalovirus , herpes). The "others" category has rapidly expanded to include several viruses known to cause neonatal disease. Traditionally, the only viral infections of concern during pregnancy were those caused by rubella virus, CMV, and herpes simplex virus (HSV). Other viruses now known to cause congenital infections include parvovirus B19 , varicella-zoster virus (VZV), West Nile virus, measles virus, enteroviruses, adenovirus, human immunodeficiency virus (HIV), and Zika virus. Also of importance is hepatitis E virus because of the high mortality rate associated with infection in pregnant women.
Amnion
Innermost membranous sac surrounding the entire developing embryo, enclosing it in a fluid-filled space called the amniotic cavity. The fluid is a protective cushion.
Why does implantation take place? How many steps are there in implantation? How long is the window for implantation?
Invasion happens because the embryo needs nutrients from the mother. There are two steps. There is a 4 day window for implantation.
When does the blastocyst emerge from the zona pellucida
It "hatches" day 5 in the uterine cavity. If conditions are appropriate it will implant around the end of the first week
What is the structure of the embryo as it enters the uterus
It is a blastocyst cosisting of: - Inner cell mass (embryoblast) - Trophoblast (outside layer) - Blastocyst cavity (Blastocoele)
What is the first thing that happens to the zygote after fertilization?
It undergoes a series of reduction divisions to form blastomeres. During these division the daughter cells each become smaller. By day 3 the cells within the intact zona pellucida are called a morula
At what point is the fertilized egg called a zygote?
Once the two pronuclei combine -> Zygote
Describe Decidualization
Processes of the trophoblast penetrate the uterine luminal cells. These cells undergo apoptosis, and a reduction in desmosomes facilitates the embryo penetration. A secondary decidual zone appears in response to prostaglandin 2, expressing Matrix metalioproteinases (MMPs), tissue inhibitors of MMPs (TIMPs), plasminogen activators (PAs) and inhibitors to regulate this remodel. In general: the decidual cells close to the invading syncytiotrophasts degenerate and release glycogen and lipids providing nutrients for embryonic development and an immunoprotective environment
Allantois
The allantois is a membrane whose blood vessels become part of the umbilical cord
Chorion
The chorion is the outermost membrane, which rests against the uterine cavity. It develops from trophoblasts and becomes the embryo's major contribution to the placenta.
What has occured by the end of the second week of gestation?
The fetal components of the placenta have come into contact with maternal blood.
What are the fetal villi?
The fetal villi lie in the intervillus spaces which are separated by connective tissue septa that extend from the decidua basalis(maternal side). These connective tissue septae divide the intervillous space into lobes or cotyledons. These can only be seen on the maternal side of the placenta. The baby has a "smooth ride" why the mom's ride is "rough"
Where does the blood from the spiral arteries go?
The intervillus space
What is the limitation of the placenta and the fetus working alone?
The placenta cannot synthesize estrogens from progesterone. The fetal adrenal cortex can't syntehsize progesterone. However, the placenta can make progesterone, which it gives to the fetus to produce androgens (DHEAs) and cortisol, which can be converted to estrogens by the placenta. This is the purpose of the fetoplacental unit
What happens on day 13?
The spaces in the extraembryonic mesoderm soon coalesce to form the extraembryonic coelom (chorionic cavity). This cavity is surrounded by extraembryonic somatopleuric and splanchnopleuric mesoderm. The bilaminar disc is attached to the placenta by the connecting stalk. The uteroplacental circulation has begun. Primary villi form: cytotrophoblasts proliferate and penetrate the syncytiotrophoblasts to form columns. Additional cells from the hypoblast migrate along the inside of the exocolemic membrane to form a new cavity, the secondary/definitive yolk sac.
Where is the mRNA coming from during the blastocyst phase?
The stored mRNA from the oocyte. At this point the blastocyst's machinery isn't working.
What happens as the embryo continues to implant? (around day 10)
The tissue between the cytotrophoblast and the exocoelomic membrane expands to become extraembryonic mesoderm (derived from the yolk sac cells)
What happens on day six if the uterine conditions are good?
The trophoblast cells at the embryonic pole (by the embryoblast (ICM) will penetrate the uterine mucosa. They start as a single layer, but then begin to form more than one layer.
What happens after implantation?
The trophoblast differentiates into 2 layers: - Inner layer called cytotrophoblasts. These cells are mono-nucleated and mitotically active. - Outer layer called syncytiotrophoblasts. These cells are fusions of cytotrophoblast cells, and are thus multi-nucleated and are found. These are the ones that invade the endometerium to become the fetal portion of the placenta. The amniotic cavity is now forming near the endometrial side. At the same time, the inner cell mass gives rise to the embryo by becoming two layers of cells -> the bilaminar disc made of an epiblast near the amniotic cavity and a hypoblast near the yolk sack (what was the blastocyst cavity) A new layer of cells forms around the yolk sac called the exocoelomic membrane.
yolk sac
The yolk sac remains quite small and does not provide nourishment in humans. The yolk sac in humans is a site of early blood cell formation. It also contains primordial germ cells that migrate to the testes or ovaries.
How do cytotrophoblasts ensure constant blood from the mother?
They form a plug by invading and replacing the endothelium of the mother's artery and ensuring blood flow at low pressure. This results in the creation of the uteroplacental artery.
What do syncytiotrophoblasts do?
They release proteases to erode branches of the spiral arteries to form lacunae of maternal blood. (note: these lacunae are not contained within any endothelial-lined structure. They're just chilling out in the open for the little vampire to steal). At this point them mother and baby's blood is only separated by syncytiotrophoblast cells. The syncytiotrophoblast secrete hCG into the lacunae, which regulates the secretion of estrogen and progesterone from the corpus luteum.
Consider the fact that there are particularly detrimental to the development of a fetus.
Typical full term pregnancy is 38 wks. These days we can deliver a baby at 25 weeks, but they will need a lot of support (think surfactant)
What occurs at the chorionic villi?
fetal blood circulates through them, allowing for exchange of oxygen/carbon dioxide, nutrients and wastes between the baby and mother - Stem villus originates from chorionic plate and branches - Maternal blood in intervillous space - Fetal blood in vessels within villi
Screw the first slide - Fertilization
one
decidual cells
polyhedral connective tissue cells around the implantation site with accumulated glycogen and lipids
Consider twins
two