Chapt. 29: DEVELOPMENT, INHERITANCE, AND HOMEOSTASIS
When the cloacal membrane appears, the wall of the yolk sac forms a small vascularized outpouching called the allantois (a-LAN-tō-is; allant- = sausage) that extends into the connecting stalk
. In nonmammalian organisms enclosed in an amnion, the allantois is used for gas exchange and waste removal. Because of the role of the human placenta in these activities, the allantois is not a prominent structure in humans (see Figure 29.11a). Nevertheless, it does function in the early formation of blood and blood vessels, and it is associated with the development of the urinary bladder.
Prenatal development is divided into periods of three calendar months each, called trimesters.
1.During the first trimester, the most critical stage of development, all of the major organ-systems begin to form. Because of the extensive, widespread activity, it is also the period when the developing organism is most vulnerable to the effects of drugs, radiation, and microbes. 2.The second trimester is characterized by the nearly complete development of organ systems. By the end of this stage, the fetus assumes distinctively human features. 3.The third trimester represents a period of rapid fetal growth in which the weight of the fetus doubles. During the early stages of this period, most of the organ systems become fully functional.
The blastocyst remains free within the uterine cavity for about 2 days before it attaches to the uterine wall. At this time the endometrium is in its secretory phase.
About 6 days after fertilization, the blastocyst loosely attaches to the endometrium in a process called implantation. As the blastocyst implants, usually in either the posterior portion of the fundus or the body of the uterus, it orients with the inner cell mass toward the endometrium. About 7 days after fertilization, the blastocyst attaches to the endometrium more firmly, endometrial glands in the vicinity enlarge, and the endometrium becomes more vascularized (forms new blood vessels). The blastocyst eventually secretes enzymes and burrows into the endometrium, and becomes surrounded by it.
Lactation (lak-TĀ-shun) is the production and ejection of milk from the mammary glands. A principal hormone in promoting milk production is prolactin (PRL), which is secreted from the anterior pituitary gland. Even though prolactin levels increase as the pregnancy progresses, no milk production occurs because progesterone inhibits the effects of prolactin.
After delivery, the levels of estrogens and progesterone in the mother's blood decrease, and the inhibition is removed. The principal stimulus in maintaining prolactin secretion during lactation is the sucking action of the infant. Suckling initiates nerve impulses from stretch receptors in the nipples to the hypothalamus; the impulses decrease hypothalamic release of prolactin-inhibiting hormone (PIH) and increase release of prolactin-releasing hormone (PRH), so more prolactin is released by the anterior pituitary.
During the first 3 to 4 months of pregnancy, the corpus luteum in the ovary continues to secrete progesterone and estrogens, which maintain the lining of the uterus during pregnancy and prepare the mammary glands to secrete milk. The amounts secreted by the corpus luteum, however, are only slightly more than those produced after ovulation in a normal menstrual cycle. From the third month through the remainder of the pregnancy, the placenta itself provides the high levels of progesterone and estrogens required.
As noted previously, the chorion of the placenta secretes human chorionic gonadotropin (hCG) (kō-rē-ON-ik gō′-nad-ō-TRŌ-pin) into the blood. In turn, hCG stimulates the corpus luteum to continue production of progesterone and estrogens—an activity required to prevent menstruation and for the continued attachment of the embryo and fetus to the lining of the uterus (Figure 29.16a). By the eighth day after fertilization, hCG can be detected in the blood and urine of a pregnant woman. Peak secretion of hCG occurs at about the ninth week of pregnancy (Figure 29.16b). During the fourth and fifth months the hCG level decreases sharply and then levels off until childbirth.
About 16 days after fertilization, mesodermal cells from the primitive node migrate toward the head end of the embryo and form a hollow tube of cells in the midline called the notochordal process
By days 22-24, the notochordal process becomes a solid cylinder of cells called the notochord (NŌ-tō-kord; noto- = back; -chord = cord). This structure plays an extremely important role in induction (in-DUK-shun), the process by which one tissue (inducing tissue) stimulates the development of an adjacent unspecialized tissue (responding tissue) into a specialized one. An inducing tissue usually produces a chemical substance that influences the responding tissue. The notochord induces certain mesodermal cells to develop into the vertebral bodies. It also forms the nucleus pulposus of the intervertebral discs
The fourth through eighth weeks of development are very significant in embryonic development because all major organs appear during this time. The term organogenesis (or′-ga-nō-JEN-e-sis) refers to the formation of body organs and systems.
By the end of the eighth week, all of the major body systems have begun to develop, although their functions for the most part are minimal. Organogenesis requires the presence of blood vessels to supply developing organs with oxygen and other nutrients. However, recent studies suggest that blood vessels play a significant role in organogenesis even before blood begins to flow within them. The endothelial cells of blood vessels apparently provide some type of developmental signal, either a secreted substance or a direct cell-to-cell interaction, that is necessary for organogenesis.
The inner layer of the chorion eventually fuses with the amnion. With the development of the chorion, the extraembryonic coelom is now referred to as the chorionic cavity
By the end of the second week of development, the bilaminar embryonic disc becomes connected to the trophoblast by a band of extraembryonic mesoderm called the connecting (body) stalk (see Figure 29.7). The connecting stalk is the future umbilical cord.
In addition to inducing mesodermal cells to develop into vertebral bodies, the notochord also induces ectodermal cells over it to form the neural plate
By the end of the third week, the lateral edges of the neural plate become more elevated and form the neural fold (Figure 29.9b). The depressed midregion is called the neural groove (Figure 29.9c). Generally, the neural folds approach each other and fuse, thus converting the neural plate into a neural tube (Figure 29.9d). This occurs first near the middle of the embryo and then progresses toward the head and tail ends. Neural tube cells then develop into the brain and spinal cord. The process by which the neural plate, neural folds, and neural tube form is called neurulation
On the ninth day after fertilization, the blastocyst becomes completely embedded in the endometrium. As the syncytiotrophoblast expands, small spaces called lacunae (la-KOO-nē = little lakes) develop within it
By the twelfth day of development, the lacunae fuse to form larger, interconnecting spaces called lacunar networks (Figure 29.6c). Endometrial capillaries around the developing embryo become dilated and are referred to as maternal sinusoids (SĪ-nū-soyds). As the syncytiotrophoblast erodes some of the maternal sinusoids and endometrial glands, maternal blood and secretions from the glands enter the lacunar networks and flow through them. Maternal blood is both a rich source of materials for embryonic nutrition and a disposal site for the embryo's wastes.
The hormone most recently found to be produced by the placenta is corticotropin-releasing hormone (CRH) (kor′-ti-kō-TRŌ-pin), which in nonpregnant people is secreted only by neurosecretory cells in the hypothalamus. CRH is now thought to be part of the "clock" that establishes the timing of birth.
CRH is now thought to be part of the "clock" that establishes the timing of birth. Secretion of CRH by the placenta begins at about 12 weeks and increases enormously toward the end of pregnancy. Women who have higher levels of CRH earlier in pregnancy are more likely to deliver prematurely; those who have low levels are more likely to deliver after their due date. CRH from the placenta has a second important effect: It increases secretion of cortisol, which is needed for maturation of the fetal lungs and the production of surfactant
In chorionic villi sampling (CVS), a catheter is guided through the vagina and cervix of the uterus and then advanced to the chorionic villi under ultrasound guidance
CVS can identify the same defects as amniocentesis because chorion cells and fetal cells contain the same genome. CVS offers several advantages over amniocentesis: It can be performed as early as 8 weeks of gestation, and test results are available in only a few days, permitting an earlier decision on whether to continue the pregnancy. However, CVS is slightly riskier than amniocentesis; after the procedure there is a 1-2% chance of spontaneous abortion.
Strong evidence implicates cigarette smoking during pregnancy as a cause of low infant birth weight; there is also a strong association between smoking and a higher fetal and infant mortality rate. Women who smoke have a much higher risk of an ectopic pregnancy.
Cigarette smoke may be teratogenic and may cause cardiac abnormalities as well as anencephaly (see Clinical Connection: Anencephaly in Section 29.1). Maternal smoking also is a significant factor in the development of cleft lip and palate and has been linked with sudden infant death syndrome (SIDS). Infants nursing from smoking mothers have also been found to have an increased incidence of gastrointestinal disturbances. Even a mother's exposure to secondhand cigarette smoke (breathing air containing tobacco smoke) during pregnancy or while nursing predisposes her baby to increased incidence of respiratory problems, including bronchitis and pneumonia, during the first year of life.
Sperm that reach the vicinity of the oocyte within minutes after ejaculation are not capable of fertilizing it until about 7 hours later.
During this time in the female reproductive tract, mostly in the uterine tube, sperm undergo capacitation, a series of functional changes that cause the sperm's tail to beat even more vigorously and prepare its plasma membrane to fuse with the oocyte's plasma membrane. During capacitation, sperm are acted on by secretions in the female reproductive tract that result in the removal of cholesterol, glycoproteins, and proteins from the plasma membrane around the head of the sperm cell. Only capacitated sperm are capable of being attracted by and responding to chemical factors produced by the surrounding cells of the ovulated oocyte.
During implantation, the syncytiotrophoblast secretes enzymes that enable the blastocyst to penetrate the uterine lining by digesting and liquefying the endometrial cells.
Eventually, the blastocyst becomes buried in the endometrium and inner one-third of the myometrium.
During fertilization, the genetic material from a haploid sperm cell (spermatozoon) and a haploid secondary oocyte merges into a single diploid nucleus. Of the 200 million sperm introduced into the vagina, fewer than 2 million (1%) reach the cervix of the uterus and only about 200 reach the secondary oocyte.
Fertilization normally occurs in the uterine (fallopian) tube within 12 to 24 hours after ovulation. Sperm can remain viable for about 48 hours after deposition in the vagina, although a secondary oocyte is viable for only about 24 hours after ovulation. Thus, pregnancy is most likely to occur if intercourse takes place during a 3-day window—from 2 days before ovulation to 1 day after ovulation.
The first major event of the third week of development, gastrulation (gas-troo-LĀ-shun), occurs about 15 days after fertilization. In this process, the bilaminar (two-layered) embryonic disc, consisting of epiblast and hypoblast, transforms into a trilaminar (three-layered) embryonic disc consisting of three layers: the ectoderm, mesoderm, and endoderm. These primary germ layers are the major embryonic tissues from which the various tissues and organs of the body develop.
Gastrulation involves the rearrangement and migration of cells from the epiblast. The first evidence of gastrulation is the formation of the primitive streak, a faint groove on the dorsal surface of the epiblast that elongates from the posterior to the anterior part of the embryo (Figure 29.7a). The primitive streak clearly establishes the head and tail ends of the embryo, as well as its right and left sides. At the head end of the primitive streak a small group of epiblastic cells forms a rounded structure called the primitive node.
Another secretion of the trophoblast is human chorionic gonadotropin (hCG), which has actions similar to LH.
Human chorionic gonadotropin rescues the corpus luteum from degeneration and sustains its secretion of progesterone and estrogens. These hormones maintain the uterine lining in a secretory state, preventing menstruation. Peak secretion of hCG occurs about the ninth week of pregnancy, at which time the placenta is fully developed and produces the progesterone and estrogens that continue to sustain the pregnancy. The presence of hCG in maternal blood or urine is an indicator of pregnancy and is detected by home pregnancy tests.
The heart forms from splanchnic mesoderm in the head end of the embryo on days 18 and 19. This region of mesodermal cells is called the cardiogenic area
In response to induction signals from the underlying endoderm, these mesodermal cells form a pair of endocardial tubes (see Figure 20.19). The tubes then fuse to form a single primitive heart tube. By the end of the third week, the primitive heart tube bends on itself, becomes S-shaped, and begins to beat. It then joins blood vessels in other parts of the embryo, connecting stalk, chorion, and yolk sac to form a primitive cardiovascular system.
Stem cells are unspecialized cells that have the ability to divide for indefinite periods and give rise to specialized cells. In the context of human development, a zygote (fertilized ovum) is a stem cell. Because it has the potential to form an entire organism, a zygote is known as a totipotent stem cell
Inner cell mass cells, called pluripotent stem cells, can give rise to many (but not all) different types of cells. Later, pluripotent stem cells can undergo further specialization into multipotent stem cells, stem cells with a specific function. Examples include keratinocytes that produce new skin cells, myeloid and lymphoid stem cells that develop into blood cells, and spermatogonia that give rise to sperm. Pluripotent stem cells currently used in research are derived from (1) the embryoblast of embryos in the blastocyst stage that were destined to be used for infertility treatments but were not needed and from (2) nonliving fetuses terminated during the first 3 months of pregnancy
If there is a question about the normal progress of a pregnancy, fetal ultrasonography (ul-tra-son-OG-ra-fē) may be performed. By far the most common use of diagnostic ultrasound is to determine a more accurate fetal age when the date of conception is unclear.
It is also used to confirm pregnancy, evaluate fetal viability and growth, determine fetal position, identify multiple pregnancies, identify fetal-maternal abnormalities, and serve as an adjunct to special procedures such as amniocentesis. During fetal ultrasonography, a transducer, an instrument that emits high-frequency sound waves, is passed back and forth over the abdomen. The reflected sound waves from the developing fetus are picked up by the transducer and converted to an on-screen image called a sonogram (see Table 1.3). Because the urinary bladder serves as a landmark during the procedure, the patient needs to drink liquids before the procedure and not void urine to maintain a full bladder.
sexual reproduction is the process by which organisms produce offspring by making sex cells called gametes
Male gametes are called sperm (spermatozoa) and female gametes are called secondary oocytes.
Because monozygotic (identical) twins develop from a single fertilized ovum, they contain exactly the same genetic material and are always the same sex.
Monozygotic twins arise from separation of the developing cells into two embryos, which in 99% of the cases occurs before 8 days have passed. Separations that occur later than 8 days are likely to produce conjoined twins, a situation in which the twins are joined together and share some body structures.
As the neural tube forms, some of the ectodermal cells from the tube migrate to form several layers of cells called the neural crest
Neural crest cells give rise to all sensory neurons and postganglionic neurons of the peripheral nerves, the adrenal medullae, melanocytes (pigment cells) of the skin, arachnoid mater, and pia mater of the brain and spinal cord, and almost all of the skeletal and connective tissue components of the head.
In addition to embryonic folding, development of somites, and development of the neural tube, four pairs of pharyngeal arches (fa-RIN-jē-al) or branchial arches (BRANG-kē-al; branch = gill) begin to develop on each side of the future head and neck regions (Figure 29.13) during the fourth week. These four paired structures begin their development on the 22nd day after fertilization and form swellings on the surface of the embryo. Each pharyngeal arch consists of an outer covering of ectoderm and an inner covering of endoderm, with mesoderm in between. Within each pharyngeal arch there is an artery, a cranial nerve, cartilaginous skeletal rods that support the arch, and skeletal muscle tissue that attaches to and moves the cartilage rods.
On the ectodermal surface of the pharyngeal region, each pharyngeal arch is separated by a groove called a pharyngeal cleft (Figure 29.13a). The pharyngeal clefts meet corresponding balloonlike outgrowths of the endodermal pharyngeal lining called pharyngeal (branchial) pouches. Where the pharyngeal cleft and pouch meet to separate the arches, the outer ectoderm of the cleft contacts the inner endoderm of the pouch and there is no mesoderm between (Figure 29.13b).
When the oropharyngeal membrane ruptures during the fourth week, the pharyngeal region of the pharynx is brought into contact with the stomodeum. In a developing embryo, the last part of the hindgut expands into a cavity called the cloaca
On the outside of the embryo is a small cavity in the tail region called the proctodeum (prok-tō-DĒ-um; procto- = anus) (Figure 29.12c). Separating the cloaca from the proctodeum is the cloacal membrane (see Figure 29.8). During embryonic development, the cloaca divides into a ventral urogenital sinus and a dorsal anorectal canal. As a result of tail folding, the cloacal membrane moves downward and the urogenital sinus, anorectal canal, and proctodeum move closer to their final positions. When the cloacal membrane ruptures during the seventh week of development, the urogenital and anal openings are created.
The organs that produce gametes are called gonads; these are the testes in the male and the ovaries in the female.
Once sperm have been deposited in the female reproductive tract and a secondary oocyte has been released from the ovary, fertilization can occur.
Following formation of the primitive streak, cells of the epiblast move inward below the primitive streak and detach from the epiblast (Figure 29.7b) in a process called invagination
Once the cells have invaginated, some of them displace the hypoblast, forming the endoderm (endo- = inside; -derm = skin). Other cells remain between the epiblast and newly formed endoderm to form the mesoderm (meso- = middle). Cells remaining in the epiblast then form the ectoderm (ecto- = outside). The ectoderm and endoderm are epithelia composed of tightly packed cells; the mesoderm is a loosely organized connective tissue (mesenchyme). As the embryo develops, the endoderm ultimately becomes the epithelial lining of the gastrointestinal tract, respiratory tract, and several other organs. The mesoderm gives rise to muscles, bones, and other connective tissues, and the peritoneum. The ectoderm develops into the epidermis of the skin and the nervous system.
The acrosome, helmetlike structure that covers the head of a sperm, contains several enzymes. Acrosomal enzymes and strong tail movements by the sperm help it penetrate the cells of the corona radiata and come in contact with the zona pellucida.
One of the glycoproteins in the zona pellucida, called ZP3, acts as a sperm receptor. Its binding to specific membrane proteins in the sperm head triggers the acrosomal reaction, the release of the contents of the acrosome. The acrosomal enzymes digest a path through the zona pellucida as the lashing sperm tail pushes the sperm cell onward. Although many sperm bind to ZP3 molecules and undergo acrosomal reactions, only the first sperm cell to penetrate the entire zona pellucida and reach the oocyte's plasma membrane fuses with the oocyte.
The passage of sperm through the rest of the uterus and then into the uterine tube results mainly from contractions of the walls of these organs.
Prostaglandins in semen are believed to stimulate uterine motility at the time of intercourse and to aid in the movement of sperm through the uterus and into the uterine tube.
The extraembryonic mesoderm, together with the two layers of the trophoblast (the cytotrophoblast and syncytiotrophoblast), forms the chorion
The chorion surrounds the embryo and, later, the fetus (see Figure 29.11a). Eventually it becomes the principal embryonic part of the placenta, the structure for exchange of materials between mother and fetus. The chorion also protects the embryo and fetus from the immune responses of the mother in two ways: (1) It secretes proteins that block antibody production by the mother. (2) It promotes the production of T lymphocytes that suppress the normal immune response in the uterus. Finally, the chorion produces human chorionic gonadotropin (hCG), an important hormone of pregnancy
During the fourth week after fertilization, the embryo undergoes very dramatic changes in shape and size, nearly tripling its size. It is essentially converted from a flat, two-dimensional trilaminar embryonic disc to a three-dimensional cylinder, a process called embryonic folding
The cylinder consists of endoderm in the center (gut), ectoderm on the outside (epidermis), and mesoderm in between. The main force responsible for embryonic folding is the different rates of growth of various parts of the embryo, especially the rapid longitudinal growth of the nervous system (neural tube). Folding in the median plane produces a head fold and a tail fold; folding in the horizontal plane results in the two lateral folds. Overall, due to the foldings, the embryo curves into a C-shape.
Following implantation, the endometrium is known as the decidua. The decidua separates from the endometrium after the fetus is delivered, much as it does in normal menstruation. Different regions of the decidua are named based on their positions relative to the site of the implanted blastocyst
The decidua basalis is the portion of the endometrium between the embryo and the stratum basale of the uterus; it provides large amounts of glycogen and lipids for the developing embryo and fetus and later becomes the maternal part of the placenta. The decidua capsularis is the portion of the endometrium located between the embryo and the uterine cavity. The decidua parietalis (par-ri-e-TAL-is) is the remaining modified endometrium that lines the noninvolved areas of the rest of the uterus. As the embryo and later the fetus enlarges, the decidua capsularis bulges into the uterine cavity and fuses with the decidua parietalis, thereby obliterating the uterine cavity. By about 27 weeks, the decidua capsularis degenerates and disappears.
During the formation of the blastocyst two distinct cell populations arise: the embryoblast and trophoblast
The embryoblast (EM-brē-ō-blast), or inner cell mass, is located internally and eventually develops into the embryo. The trophoblast (TRŌF-ō-blast; tropho- = develop or nourish) is the outer superficial layer of cells that forms the spherelike wall of the blastocyst. It will ultimately develop into the outer chorionic sac that surrounds the fetus and the fetal portion of the placenta, the site of exchange of nutrients and wastes between the mother and fetus. Around the fifth day after fertilization, the blastocyst "hatches" from the zona pellucida by digesting a hole in it with an enzyme, and then squeezing through the hole. This shedding of the zona pellucida is necessary in order to permit the next step, implantation (attachment) into the vascular, glandular endometrial lining of the uterus.
As the embryonic tissue invades the uterine wall, maternal uterine vessels are eroded and maternal blood fills spaces, called lacunae (Figure 29.10) within the invading tissue. By the end of the second week of development, chorionic villi (kō-rē-ON-ik VIL-ī) begin to develop. These fingerlike projections consist of chorion (syncytiotrophoblast surrounded by cytotrophoblast) that projects into the endometrial wall of the uterus (Figure 29.10a). By the end of the third week, blood capillaries develop in the chorionic villi (Figure 29.10b). Blood vessels in the chorionic villi connect to the embryonic heart by way of the umbilical arteries and umbilical vein through the connecting (body) stalk, which will eventually become the umbilical cord (
The fetal blood capillaries within the chorionic villi project into the lacunae, which unite to form the intervillous spaces (in′-ter-VIL-us) that bathe the chorionic villi with maternal blood. As a result, maternal blood bathes the chorion-covered fetal blood vessels. Note, however, that maternal and fetal blood vessels do not join, and the blood they carry does not normally mix. Instead, oxygen and nutrients in the blood of the mother's intervillous spaces, the spaces between chorionic villi, diffuse across the cell membranes into the capillaries of the villi. Waste products such as carbon dioxide diffuse in the opposite direction.
After fertilization, rapid mitotic cell divisions of the zygote called cleavage take place.
The first division of the zygote begins about 24 hours after fertilization and is completed about 6 hours later. Each succeeding division takes slightly less time. By the second day after fertilization, the second cleavage is completed and there are four cells. By the end of the third day, there are 16 cells. The progressively smaller cells produced by cleavage are called blastomeres. Successive cleavages eventually produce a solid sphere of cells called the morula. The morula is still surrounded by the zona pellucida and is about the same size as the original zygote
Each pharyngeal arch is a developmental unit and includes a skeletal component, muscle, nerve, and blood vessels. In the human embryo, there are four obvious pharyngeal arches. Each of these arches develops into a specific and unique component of the head and neck region. For example, the first pharyngeal arch is often called the mandibular arch because it forms the jaws (the mandible is the lower jawbone).
The first sign of a developing ear is a thickened area of ectoderm, the otic placode (PLAK-ōd), or future internal ear, which can be distinguished about 22 days after fertilization. A thickened area of ectoderm called the lens placode, which will become the eye, also appears at this time (see Figure 29.13a). By the middle of the fourth week, the upper limbs begin their development as outgrowths of mesoderm covered by ectoderm called upper limb buds (see Figure 8.16b). By the end of the fourth week, the lower limb buds develop. The heart also forms a distinct projection on the ventral surface of the embryo called the heart prominence (see Figure 8.16b). At the end of the fourth week the embryo has a distinctive tail
Currently, chorionic villi testing and amniocentesis are the only useful ways to obtain fetal tissue for prenatal testing of gene defects. While these invasive procedures pose relatively little risk when performed by experts, much work has been done to develop noninvasive prenatal tests, which do not require the penetration of any embryonic structure. The goal is to develop accurate, safe, more efficient, and less expensive tests for screening a large population.
The first such test developed was the maternal alpha-fetoprotein (AFP) test (AL-fa fē′-tō-PRŌ-tēn). In this test, the mother's blood is analyzed for the presence of AFP, a protein synthesized in the fetus that passes into the maternal circulation. The highest levels of AFP normally occur during weeks 12 through 15 of pregnancy. Later, AFP is not produced, and its concentration decreases to a very low level both in the fetus and in maternal blood. A high level of AFP after week 16 usually indicates that the fetus has a neural tube defect, such as spina bifida or anencephaly. Because the test is 95% accurate, it is now recommended that all pregnant women be tested for AFP. A newer test (Quad AFP Plus) probes maternal blood for AFP and three other molecules. The test permits prenatal screening for Down syndrome, trisomy 18, and neural tube defects; it also helps predict the delivery date and may reveal the presence of twins.
By about the 17th day after fertilization, the mesoderm adjacent to the notochord and neural tube forms paired longitudinal columns of paraxial mesoderm
The mesoderm lateral to the paraxial mesoderm forms paired cylindrical masses called intermediate mesoderm. The mesoderm lateral to the intermediate mesoderm consists of a pair of flattened sheets called lateral plate mesoderm. The paraxial mesoderm soon segments into a series of paired, cube-shaped structures called somites (SŌ-mīts = little bodies). By the end of the fifth week, 42-44 pairs of somites are present. The number of somites that develop over a given period can be correlated to the approximate age of the embryo.
Uterine contractions occur in waves (quite similar to the peristaltic waves of the gastrointestinal tract) that start at the top of the uterus and move downward, eventually expelling the fetus. True labor begins when uterine contractions occur at regular intervals, usually producing pain. As the interval between contractions shortens, the contractions intensify. Another symptom of true labor in some women is localization of pain in the back that is intensified by walking
The most reliable indicator of true labor is dilation of the cervix and the "show," a discharge of a blood-containing mucus into the cervical canal. In false labor, pain is felt in the abdomen at irregular intervals, but it does not intensify and walking does not alter it significantly. There is no "show" and no cervical dilation.
Once a sperm cell enters a secondary oocyte, the oocyte first must complete meiosis II. It divides into a larger ovum (mature egg) and a smaller second polar body that fragments and disintegrates)
The nucleus in the head of the sperm develops into the male pronucleus, and the nucleus of the fertilized ovum develops into the female pronucleus
Placentation (plas-en-TĀ-shun) is the process of forming the placenta (pla-SEN-ta = flat cake), the site of exchange of nutrients and wastes between the mother and fetus.
The placenta also produces hormones needed to sustain the pregnancy (see Figure 29.16). The placenta is unique because it develops from two separate individuals, the mother and the fetus. By the beginning of the twelfth week, the placenta has two distinct parts: (1) the fetal portion formed by the chorionic villi of the chorion and (2) the maternal portion formed by the decidua basalis of the endometrium (Figure 29.11a). When fully developed, the placenta is shaped like a pancake (Figure 29.11b). Functionally, the placenta allows oxygen and nutrients to diffuse from maternal blood into fetal blood while carbon dioxide and wastes diffuse from fetal blood into maternal blood. The placenta also is a protective barrier because most microorganisms cannot pass through it.
The head fold brings the developing heart and mouth into their eventual adult positions. The tail fold brings the developing anus into its eventual adult position. The lateral folds form as the lateral margins of the trilaminar embryonic disc bend ventrally. As they move toward the midline, the lateral folds incorporate the dorsal part of the yolk sac into the embryo as the primitive gut, the forerunner of the gastrointestinal tract
The primitive gut differentiates into an anterior foregut, an intermediate midgut, and a posterior hindgut (Figure 29.12c). The fates of the foregut, midgut, and hindgut are described in Section 24.16. Recall that the oropharyngeal membrane is located in the head end of the embryo (see Figure 29.8). It separates the future pharyngeal (throat) region of the foregut from the stomodeum (stō-mō-DĒ-um; stomo- = mouth), the future oral cavity. Because of head folding, the oropharyngeal membrane moves downward and the foregut and stomodeum move closer to their final positions.
Oxytocin causes release of milk into the mammary ducts via the milk ejection reflex (Figure 29.19). Milk formed by the glandular cells of the breasts is stored until the baby begins active suckling. Stimulation of touch receptors in the nipple initiates sensory nerve impulses that are relayed to the hypothalamus. In response, secretion of oxytocin from the posterior pituitary increases. Carried by the bloodstream to the mammary glands, oxytocin stimulates contraction of myoepithelial (smooth muscle-like) cells surrounding the glandular cells and ducts.
The resulting compression moves the milk from the alveoli of the mammary glands into the mammary ducts, where it can be suckled. This process is termed milk ejection (let-down). Even though the actual ejection of milk does not occur until 30-60 seconds after nursing begins (the latent period), some milk stored in lactiferous sinuses near the nipple is available during the latent period. Stimuli other than suckling, such as hearing a baby's cry or touching the mother's genitals, also can trigger oxytocin release and milk ejection. The suckling stimulation that produces the release of oxytocin also inhibits the release of PIH; this results in increased secretion of prolactin, which maintains lactation.
After the birth of the baby, the placenta detaches from the uterus and is therefore termed the afterbirth. At this time, the umbilical cord is tied off and then severed.
The small portion (about an inch) of the cord that remains attached to the infant begins to wither and falls off, usually within 12 to 15 days after birth. The area where the cord was attached becomes covered by a thin layer of skin, and scar tissue forms. The scar is the umbilicus (um-BIL-i-kus) or navel.
Also during the third week of development, two faint depressions appear on the dorsal surface of the embryo where the ectoderm and endoderm make contact but lack mesoderm between them.
The structure closer to the head end is called the oropharyngeal membrane (or-ō-fa-RIN-jē-al; oro- = mouth; -pharyngeal = pertaining to the pharynx) (Figure 29.8a, b). It breaks down during the fourth week to connect the mouth cavity to the pharynx and the remainder of the gastrointestinal tract. The structure closer to the tail end is called the cloacal membrane (klō-Ā-kul = sewer), which degenerates in the seventh week to form the openings of the anus and urinary and reproductive tracts.
Alcohol is by far the number-one fetal teratogen. Intrauterine exposure to even a small amount of alcohol may result in fetal alcohol syndrome (FAS), one of the most common causes of mental retardation and the most common preventable cause of birth defects in the United States.
The symptoms of FAS may include slow growth before and after birth, characteristic facial features (short palpebral fissures, a thin upper lip, and sunken nasal bridge), defective heart and other organs, malformed limbs, genital abnormalities, and central nervous system damage. Behavioral problems, such as hyperactivity, extreme nervousness, reduced ability to concentrate, and an inability to appreciate cause-and-effect relationships, are common.
Amniocentesis (am′-nē-ō-sen-TĒ-sis; amnio- = amnion; -centesis = puncture to remove fluid) involves withdrawing some of the amniotic fluid that bathes the developing fetus and analyzing the fetal cells and dissolved substances. It is used to test for the presence of certain genetic disorders, such as Down syndrome (DS), hemophilia, Tay-Sachs disease, sickle cell disease, and certain muscular dystrophies.
The test is usually done at 14-18 weeks of gestation. All gross chromosomal abnormalities and over 50 biochemical defects can be detected through amniocentesis. It can also reveal the baby's gender; this is important information for the diagnosis of sex-linked disorders, in which an abnormal gene carried by the mother affects her male offspring only
The actual connection between the placenta and embryo, and later the fetus, is through the umbilical cord (um-BIL-i-kal = navel), which develops from the connecting stalk
The umbilical cord consists of two umbilical arteries that carry deoxygenated fetal blood to the placenta, one umbilical vein that carries oxygen and nutrients acquired from the mother's intervillous spaces into the fetus, and supporting mucous connective tissue called Wharton's jelly (WOR-tons) derived from the allantois. A layer of amnion surrounds the entire umbilical cord and gives it a shiny appearance
About 8 days after fertilization, the trophoblast develops into two layers in the region of contact between the blastocyst and endometrium
These are a syncytiotrophoblast that contains no distinct cell boundaries and a cytotrophoblast between the embryoblast and syncytiotrophoblast that is composed of distinct cells. The two layers of trophoblast become part of the chorion (one of the fetal membranes) as they undergo further growth
About the twelfth day after fertilization, the extraembryonic mesoderm develops
These mesodermal cells are derived from the yolk sac and form a connective tissue layer (mesenchyme) around the amnion and yolk sac (Figure 29.6c). Soon a number of large cavities develop in the extraembryonic mesoderm, which then fuse to form a single, larger cavity called the extraembryonic coelom
Dizygotic (fraternal) twins are produced from the independent release of two secondary oocytes and the subsequent fertilization of each by different sperm.
They are the same age and in the uterus at the same time, but genetically they are as dissimilar as any other siblings. Dizygotic twins may or may not be the same sex.
In the third week of development, small spaces appear in the lateral plate mesoderm. These spaces soon merge to form a larger cavity called the intraembryonic coelom
This cavity splits the lateral plate mesoderm into two parts called the splanchnic mesoderm and somatic mesoderm (Figure 29.9d). Splanchnic mesoderm (SPLANK-nik = visceral) forms the heart and the visceral layer of the serous pericardium, blood vessels, the smooth muscle and connective tissues of the respiratory and digestive organs, and the visceral layer of the serous membrane of the pleurae and peritoneum. Somatic mesoderm (sō-MAT-ik; soma- = body) gives rise to the bones, ligaments, blood vessels, and connective tissue of the limbs and the parietal layer of the serous membrane of the pericardium, pleurae, and peritoneum.
At the beginning of the third week, angiogenesis (an′-jē-ō-JEN-e-sis; angio- = vessel; -genesis = production), the formation of blood vessels, begins in the extraembryonic mesoderm in the yolk sac, connecting stalk, and chorion
This early development is necessary because there is insufficient yolk in the yolk sac and ovum to provide adequate nutrition for the rapidly developing embryo. Angiogenesis is initiated when mesodermal cells differentiate into hemangioblasts (hē-MAN-jē-ō-blasts). These then develop into cells called angioblasts, which aggregate to form isolated masses of cells referred to as blood islands (see Figure 21.32). Spaces soon develop in the blood islands and form the lumens of blood vessels. Some angioblasts arrange themselves around each space to form the endothelium and the tunics (layers) of the developing blood vessels. As the blood islands grow and fuse, they soon form an extensive system of blood vessels throughout the embryo.
When the morula enters the uterine cavity on day 4 or 5, a glycogen-rich secretion from the glands of the endometrium of the uterus passes into the uterine cavity and enters the morula through the zona pellucida.
This fluid, called uterine milk, along with nutrients stored in the cytoplasm of the blastomeres of the morula, provides nourishment for the developing morula. At the 32-cell stage, the fluid enters the morula, collects between the blastomeres, and reorganizes them around a large fluid-filled cavity called the blastocyst cavity.Once the blastocyst cavity is formed, the developing mass is called the blastocyst. Though it now has hundreds of cells, the blastocyst is still about the same size as the original zygote.
As the amniotic cavity enlarges, a single layer of squamous cells forms a domelike roof above the epiblast cells called the amnion
Thus, the amnion forms the roof of the amniotic cavity, and the epiblast forms the floor. Initially, the amnion overlies only the bilaminar embryonic disc. However, as the embryonic disc increases in size and begins to fold, the amnion eventually surrounds the entire embryo, creating the amniotic cavity that becomes filled with amniotic fluid.Most amniotic fluid is initially derived from maternal blood. Later, the fetus contributes to the fluid by excreting urine into the amniotic cavity. Amniotic fluid serves as a shock absorber for the fetus, helps regulate fetal body temperature, helps prevent the fetus from drying out, and prevents adhesions between the skin of the fetus and surrounding tissues. The amnion usually ruptures just before birth; it and its fluid constitute the "bag of waters." Embryonic cells are normally sloughed off into amniotic fluid. They can be examined in a procedure called amniocentesis, which involves withdrawing some of the amniotic fluid that bathes the developing fetus and analyzing the fetal cells and dissolved substances
After the male and female pronuclei form, they fuse, producing a single diploid nucleus, a process known as syngamy
Thus, the fusion of the haploid (n) pronuclei restores the diploid number (2n) of 46 chromosomes. The fertilized ovum now is called a zygote
Also on the eighth day after fertilization, cells at the edge of the hypoblast migrate and cover the inner surface of the blastocyst wall. The migrating columnar cells become squamous (flat) and then form a thin membrane referred to as the exocoelomic membrane
Together with the hypoblast, the exocoelomic membrane forms the wall of the yolk sac, the former blastocyst cavity during earlier development. As a result, the bilaminar embryonic disc is now positioned between the amniotic cavity and yolk sac. Since human embryos receive their nutrients from the endometrium, the yolk sac is relatively empty and small, and decreases in size as development progresses. Nevertheless, the yolk sac has several important functions in humans: supplies nutrients to the embryo during the second and third weeks of development; is the source of blood cells from the third through sixth weeks; contains the first cells (primordial germ cells) that will eventually migrate into the developing gonads, differentiate into the primitive germ cells, and form gametes; forms part of the gut (gastrointestinal tract); functions as a shock absorber; and helps prevent drying out of the embryo
The fusion of a sperm cell with a secondary oocyte sets in motion events that block polyspermy, fertilization by more than one sperm cell
Within a few seconds, the cell membrane of the oocyte depolarizes, which acts as a fast block to polyspermy—the inability of a depolarized oocyte to fuse with another sperm. Depolarization also triggers the intracellular release of calcium ions, which stimulate exocytosis of secretory vesicles from the oocyte. The molecules released by exocytosis inactivate ZP3 and harden the entire zona pellucida, events called the slow block to polyspermy.
Like those of the trophoblast, cells of the embryoblast also differentiate into two layers around 8 days after fertilization:
a hypoblast (primitive endoderm) and epiblast (primitive ectoderm). Cells of the hypoblast and epiblast together form a flat disc referred to as the bilaminar embryonic disc. Soon, a small cavity appears within the epiblast and eventually enlarges to form the amniotic cavity
The fetal period
begins at week nine and continues until birth. During this time, the developing human is called a fetus
True labor can be divided into three stages
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From fertilization through the eighth week of development, the
embryonic period, the developing human is called an embryo
The first week of development is characterized by several significant events including
fertilization, cleavage of the zygote, blastocyst formation, and implantation.
Relaxin, a hormone produced first by the corpus luteum of the ovary and later by the placenta, increases the flexibility of the pubic symphysis and ligaments of the sacroiliac and sacrococcygeal joints and helps dilate the uterine cervix during labor. Both of these actions ease delivery of the baby. A third hormone produced by the chorion of the placenta is
human chorionic somatomammotropin (hCS) (sō′-ma-tō-MAM-ō-trō-pin), also known as human placental lactogen (hPL). The rate of secretion of hCS increases in proportion to placental mass, reaching maximum levels after 32 weeks and remaining relatively constant after that. It is thought to help prepare the mammary glands for lactation, enhance maternal growth by increasing protein synthesis, and regulate certain aspects of metabolism in both mother and fetus. For example, hCS decreases the use of glucose by the mother and promotes the release of fatty acids from her adipose tissue, making more glucose available to the fetus.
Pregnancy
is a sequence of events that begins with fertilization, proceeds to implantation, embryonic development, and fetal development, and ideally ends with birth about 38 weeks later, or 40 weeks after the mother's last menstrual period.
A teratogen
is any agent or influence that causes developmental defects in the embryo. In the following sections we briefly discuss several examples.
Embryology
is the study of development from the fertilized egg through the eighth week.
Development biology
is the study of the sequence of events from the fertilization of a secondary oocyte by a sperm cell to the formation of an adult organism.
Prenatal development
is the time from fertilization to birth and includes both the embryonic and fetal periods.
Each somite differentiates into three regions:
myotome (MĪ-ō-tōm), a dermatome, and a sclerotome (SKLER-ō-tōm) (see Figure 10.17b). The myotomes develop into the skeletal muscles of the neck, trunk, and limbs; the dermatomes form connective tissue, including the dermis of the skin; and the sclerotomes give rise to the vertebrae and ribs.
For fertilization to occur, a sperm cell first must penetrate two layers:
the corona radiata, the granulosa cells that surround the secondary oocyte, and the zona pellucida, the clear glycoprotein layer between the corona radiata and the oocyte's plasma membrane
At about 4 weeks after fertilization, the head end of the neural tube develops into three enlarged areas called primary brain vesicles
the prosencephalon (pros′-en-SEF-a-lon) or forebrain, mesencephalon (mes′-en-SEF-a-lon) or midbrain, and rhombencephalon (rom′-ben-SEF-a-lon) or hindbrain. At about 5 weeks, the prosencephalon develops into secondary brain vesicles called the telencephalon (tel′-en-SEF-a-lon) and diencephalon (dī-en-SEF-a-lon), and the rhombencephalon develops into secondary brain vesicles called the metencephalon (met′-en-SEF-a-lon) and myelencephalon (mi-el-en-SEF-a-lon). The areas of the neural tube adjacent to the myelencephalon develop into the spinal cord.
