I. EMBRYOBLAST (Figure 3-1). The embryoblast differentiates into two distinct cell layers, the epiblast and hypoblast, forming a bilaminar embryonic disk.
A. Epiblast. Clefts develop within the epiblast to form the amniotic cavity.
B. Hypoblast cells migrate along the cytotrophoblast, forming the yolk sac.
C. The prochordal plate, formed by the fusion of epiblast and hypoblast cells, marks the future site of the mouth.
A. The syncytiotrophoblast continues its growth into the endometrium to make contact with endometrial blood vessels and glands.
1. The syncytiotrophoblast does not divide mitotically.
2. The syncytiotrophoblast produces human chorionic gonadotropin (HCG).
B. The cytotrophoblast does divide mitotically, adding to the growth of the syncytiotrophoblast. Primary chorionic villi protrude into the syncytiotrophoblast.
III. EXTRAEMBRYONIC MESODERM is a new layer of cells derived from the epiblast.
A. Extraembryonic somatic mesoderm (somatopleuric mesoderm) lines the cytotrophoblast, forms the connecting stalk, and covers the amnion (see Figure 3-1).
1. The conceptus is suspended by the connecting stalk within the chorionic cavity.
2. The wall of rhe chorionic cavity is called the chorion and consists of three components: (1) extraembryonic somatic mesoderm, (2) cytotrophoblast, and (3) syncytiotrophoblast.
B. Extraembryonic visceral mesoderm (splanchnopleuric mesoderm) covers the yolk sac.
IV. CLINICAL CORRELATIONS
A. Human chorionic gonadotropin (HCG)
1. HCG is a glycoprotein produced by the syncytiotrophoblast, which stimulates the production of progesterone by the corpus luteum (i.e., maintains corpus luteum function). This is clinically significant because progesterone produced by the corpus luteum is essential for the maintenance of pregnancy until week 8. The placenta then takes over the production of progesterone.
Endometrial blood vessel
Cytotrophoblast Amniotic cavity
Extraembryonic visceral mesoderm (splanchnopleuric)
Primary chorionic villi
Extraembryonic somatic mesoderm (somatopleuric)
Figure 3-1. Diagram of a day-14 blastocyst highlighting the formation of the bilaminar embryonic disk and the completion of implantation within the ondometrium.
2. HCG can be assayed in maternal blood at day 8 or in maternal urine at day 10 and is the basis of pregnancy testing.
3. HCG is detectable throughout a pregnancy. Low HCG values may predict a spontaneous abortion or indicate an ectopic pregnancy. Elevated HCG values may indicate a multiple pregnancy, hydatidiform mole, or gestational trophoblastic neoplasia.
B. Hydatidiform mole. A blighted blastocyst leads to death of the embryo; this is followed by hyperplastic proliferation of the trophoblast, resulting in a vesicular or polycystic mass called a hydatidiform mole. Clinical signs diagnostic of a mole include preeclampsia during the first trimester, elevated HCG levels (>100,000 mlU/ml), and an enlarged uterus with bleeding. Follow-up visits are essential because 3%-5% of moles develop into gestational trophoblastic neoplasia.
C. Gestational trophoblastic neoplasia (GTN; or choriocarcinoma). GTN is a malignant tumor of the trophoblast that may occur after a normal or ectopic pregnancy, abortion, or hydatidiform mole. With a high degree of suspicion, elevated HCG levels are diagnostic. Nonmetastatic GTN (i.e., confined to the uterus) is the most common form of the neoplasia, and treatment is highly successful. However, the prognosis of metastatic GTN is poor if it spreads to the liver or brain.
D. Oncofetal antigens arc ccll surfacc antigens that normally appear only on embiyoiiic cells; however, for unknown reasons, they re-express themselves in human malignant cells. Monoclonal antibodies directed against specific oncofetal antigens provide an avenue for cancer therapy.
1. Carcinoembryonic antigen (CEA) is associated with colorectal carcinoma.
2. a-Fetoprotein is associated with hepatoma and germ cell tumors.
E. RU-486 (mifepristone) will initiate menstruation when taken within 8-10 weeks of the previous menses. If implantation of a conceptus has occurred, the conceptus will be sloughed along with the endometrium. RU-486 which blocks the progesterone receptor is used in conjunction with prostaglandins and is 96% effective at terminating pregnancy.
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