If fertilization does not take place, the corpus luteum begins to decrease its secretion of steroids about 10 days after ovulation. This withdrawal of steroids, as previously described, causes necrosis and sloughing of the endometrium following day 28 of the cycle. If fertilization and implantation have occurred, however, these events must obviously be prevented to maintain the pregnancy.
■ Figure 20.46 The secretion of human chorionic gonadotropin (hCG). This hormone is secreted by trophoblast cells during the first trimester of pregnancy, and it maintains the mother's corpus luteum for the first 5>2 weeks. After that time, the placenta becomes the major sex-hormone-producing gland, secreting increasing amounts of estrogen and progesterone throughout pregnancy.
The blastocyst saves itself from being eliminated with the endometrium by secreting a hormone that indirectly prevents menstruation. Even before the sixth day when implantation occurs, the trophoblast cells of the chorion secrete chorionic gonadotropin, or hCG (the h stands for "human"). This hormone is identical to LH in its effects and therefore is able to maintain the corpus luteum past the time when it would otherwise regress. The secretion of estradiol and progesterone is thus maintained and menstruation is normally prevented.
IK All pregnancy tests assay for the presence of hCG in blood or urine because this hormone is secreted by ^ \ ^ the blastocyst but not by the mother's endocrine glands. Modern pregnancy tests detect the beta subunit of hCG, which is unique to hCG and provides the least amount of cross-reaction with other hormones. Accurate and sensitive immunoassays for hCG in pregnancy tests employ antibodies that are produced by a clone of lymphocytes—termed monoclonal antibodies (chapter 15)—against the specific beta subunit of hCG. Home pregnancy kits that use these antibodies are generally accurate in the week following the first missed menstrual period.
The secretion of hCG declines by the tenth week of pregnancy (fig. 20.46). Actually, this hormone is required for only the first 5 to 6 weeks of pregnancy because the placenta itself becomes an active steroid hormone-secreting gland. By the fifth to sixth week, the mother's corpus luteum begins to regress (even in the presence of hCG), but by this time the placenta is secreting more than sufficient amounts of steroids to maintain the endometrium and prevent menstruation.
Clinical Investigation Clues
Remember that Gloria's pregnancy test was negative.
■ What did they specifically test for?
■ If the test came out positive, what physiological mechanism would account for her amenorrhea?
Between days 7 and 12, as the blastocyst becomes completely embedded in the endometrium, the chorion becomes a two-cell-thick structure that consists of an inner cytotrophoblast layer and an outer syncytiotrophoblast layer (see fig. 20.45b). Meanwhile, the inner cell mass (which will become the fetus) also develops two cell layers. These are the ectoderm (which will form the nervous system and skin) and the endoderm (which will eventually form the gut and its derivatives). A third, middle embryonic layer—the mesoderm—is not yet seen at this stage. The embryo at this stage is a two-layer-thick disc separated from the cytotrophoblast of the chorion by an amniotic cavity.
As the syncytiotrophoblast invades the endometrium, it secretes protein-digesting enzymes that create numerous blood-filled cavities in the maternal tissue. The cytotrophoblast then forms projections, or villi (fig. 20.47), that grow into these pools of venous blood, producing a leafy-appearing structure called the chorion frondosum (frond = leaf). This occurs only on the side of the chorion that faces the uterine wall. As the embryonic structures grow, the other side of the chorion bulges into the cavity of the uterus, loses its villi, and takes on a smooth appearance.
Since the chorionic membrane is derived from the zygote, and since the zygote inherits paternal genes that produce proteins foreign to the mother, scientists have long wondered why the mother's immune system doesn't attack the embryonic tissues. The placenta, it seems, is an "immunologically privileged site." Recent studies suggest that this immune protection may be due to FAS ligand, which is produced by the cytotrophoblast. As you may recall from chapter 15, T lymphocytes produce a surface receptor called FAS. The binding of FAS to FAS ligand triggers the apoptosis (cell suicide) of those lymphocytes, thereby preventing them from attacking the placenta.
As the blastocyst implants in the endometrium and the chorion develops, the cells of the endometrium also undergo changes. These changes, including cellular growth and the accumulation of glycogen, are collectively called the decidual reaction. The maternal tissue in contact with the chorion frondosum is called the decidua basalis. These two structures—chorion frondosum (fetal tissue) and decidua basalis (maternal tissue)—together form the functional unit known as the placenta.
Amniotic sac containing amniotic fluid Yolk sac
Villi of chorion frondosum
Umbilical blood vessels
■ Figure 20.47 The extraembryonic membranes. After the syncytiotrophoblast has created blood-filled cavities in the endometrium, these cavities are invaded by extensions of the cytotrophoblast (a). These extensions, or villi, branch extensively to produce the chorion frondosum (b). The developing embryo is surrounded by a membrane called the amnion.
The disc-shaped human placenta is continuous at its outer surface with the smooth part of the chorion, which bulges into the uterine cavity. Immediately beneath the chorionic membrane is the amnion, which has grown to envelop the entire embryo (fig. 20.48). The embryo, together with its umbilical cord, is therefore located within the fluid-filled amniotic sac.
Amniotic fluid is formed initially as an isotonic secretion. Later, the volume is increased and the concentration changed by urine from the fetus. Amniotic fluid also contains cells that are sloughed off from the fetus, placenta, and amniotic sac. Since all of these cells are derived from the same fertilized ovum, all have the same genetic composition. Many genetic abnormalities can be detected by aspiration of this fluid and examination of the cells thus obtained. This procedure is called amniocentesis (fig. 20.49).
Amniocentesis is usually performed at about the sixteenth week of pregnancy. By this time the amniotic sac contains between 175 to 225 ml of fluid. Genetic diseases such as Down
■ Figure 20.48 The amniotic sac and placenta. Blood from the embryo is carried to and from the chorion frondosum by umbilical arteries and veins. The maternal tissue between the chorionic villi is known as the decidua basalis; this tissue, together with the chorionic villi, forms the functioning placenta. The space between chorion and amnion is obliterated, and the fetus lies within the fluid-filled amniotic sac.
syndrome (characterized by three instead of two chromosomes number 21) can be detected by examining chromosomes; diseases such as Tay-Sachs disease, in which degeneration of myelin sheaths results from a defective enzyme, can be detected by biochemical techniques.
The amniotic fluid that is withdrawn contains fetal cells at a concentration too low to permit direct determination of genetic or chromosomal disorders. These cells must therefore be cultured in vitro for 10 to i4 days before they are present in sufficient numbers for the laboratory tests required. A newer method, called chorionic villus biopsy, is now available to detect genetic disorders earlier than permitted by amniocentesis. In chorionic villus biopsy, a catheter is inserted through the cervix to the chorion and a sample of a chorionic villus is obtained by suction or cutting. Genetic tests can be performed directly on the villus sample because it contains much larger numbers of fetal cells than does a sample of amniotic fluid. Chorionic villus biopsy can provide genetic information at 12 weeks' gestation. Amniocentesis, by contrast, cannot provide such information before about 20 weeks.
Major structural abnormalities that may not be predictable from genetic analysis can often be detected by ultrasound. Sound-wave vibrations are reflected from the interface of tissues with different densities—such as the interface between the fetus and amniotic fluid—and used to produce an image. This technique is so sensitive that it can be used to detect a fetal heartbeat several weeks before it can be heard using a stethoscope.
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