HY Embryology 03

1. The embryoblast differentiates into two distinct cell layers: the dorsal epiblast and the ventral hypoblast. The epiblast and hypoblast together forma flat, ovoid-shaped disk known as the bilaminar embryonic disk

2. Within the epiblast, clefts develop and eventually coalesce to form the amniotic cavity

3. Hypoblast cells migrate and line the inner surface of the cytotrophoblast and eventually delimit a space called the definitive yolk sac

4. The epiblast and hypoblast fuse to form the prochordal plate, which markes the future site of the mouth

Two distinct cell layers:
1) The dorsal epiblast
2)The ventral hypoblast

A flat, ovoid shaped disk formed from the dorsal epiblast and ventral hypoblast

Within the epiblast, clefts develop and eventually coalesce to form the amniotic cavity

Hypoblast cells migrate and line the inner surface of the cytotrophoblast and eventually delimit a space called the definitive yolk sac

The prochordal plate, which is formed from fusion of the epiblast and hypoblast

1. The syncytiotrophoblast continues its growth into the endometrium to make contact with endometrial blood vessels and glands

2. The syncytiotrophoblast does not divide mitotically. The cytotrophoblast does divide mitotically, adding to the growth of the syncytiotrophoblast

3. The syncytiotrophoblast produces human chorionic gonadotropin (hCG)

4. Primary chorionic villi formed by the cytotrophoblast protrude into the syncytiotrophoblast

The syncytiotrophoblast

1. Is a new layer of cells derived from the epiblast

2. Extraembryonic somatic mesoderm (somatopleuric mesoderm) lines the cytotrophoblast, formed the connecting stalk, and covers the amnion

3. The conceptus is suspended by the connecting stalk within the chorionic cavity

4. The wall of the chorionic cavity is called the chorion and consists of three components: extraembryoonic somatic mesoderm, cytotrophoblast, and syncytiotrophoblast

5. Extraembryonic visceral mesoderm (splanchnopleuric mesoderm) covers the yolk sac

1. Somatopleuric mesoderm

2. Lines the cytotrophoblast, forms the connecting stalk, and covers the amnion

Connecting stalk

Suspends the conceptus in the chorionic cavity

The wall of the chorionic cavity

1. Extraembryonic somatic mesoderm

2. Cytotrophoblast

3. Syncytiotrophoblast

Extraembryonic visceral mesoderm (splanchnopleuric mesoderm)

1. hCG is a glycoprotein produced by the syncytiotrophoblast that sitmultaes the production of progesteron by the corpus luteum of the ovary (ie, maintains corpus luteum function). This is clinically significant because progesterone produced by the corpus luteum is essential for the maintenance of prgnancy until week 8. The placena then takes over progesterone production

2. hCG can be assayed in maternal blood at day 8 or maternal urine at day 10 and is the bases of pregnancy testing

3. hCG is detectable throughout a pregnancy

May predict a spontaneous abortion or indicate an ectopic pregnancy

May indicate multiple pregnancy, hydatidiform mole, or gestational trophoblastic neoplasia (such as choriocarcinoma)

The syncytiotrophoblast

1. hCG from the syncytiotrophoblast stimulates the production of progesterone by the corpus luteum of the ovary

2. This is essential for the maintenance of pregnancy until week 8

3. After that the placenta takes over progesterone production

hCG

-Day 8 in maternal blood
-Day 10 in maternal urine

1. MIfepristone; Mifeprex
2. Will initiate menstruation when taken within 8-10 weeks of the start of the last menstrual period. If implantation of a conceptus has occurs, the conceptus will be sloughed along with the endometrium
3. RU-486 is a progesterone-receptor antagonist (blocker) used in conjunction with misoprostol (Cytotec; a PGE1 analogue) and is 96% effective at terminating pregnancy

1. A blighted blastocyst (ie, blastocyst growth is prevented) leads to death of the embryo. This is followed by hyperplastic proliferation of the trophoblast.
2. A hydatidiform mole (complete or partial) represents an abnormal placenta characeteized by marked enlargement of chorionic villi
3. A complete mole usually has apparently normal 46,XX karyotype, but both nuclear chromosomes are of paternal origin. This results from fertilization of an “empty” egg (ie, absent or inactivated maternal chromosomes) by a haploid sperm that then duplicates (46,YY moles do not occur, because this karyotype is lethal)
4. A partial mole usually has a triploid karyotype (69,XXX; 69,XYY) due o the fertilization of an ovum (one set of haploid maternal chromosomes) by two sperm (two sets of haploid paternal chromosomes)
5. A complete mole (no embryo present) is distinguished from a partial mole (embryo present) by the amount of chorionic villous involvement
6. Hallmarks of a complete mole include gross, generalized edema of chorionic villi forming grape-like, transparent vesicles, hyperplastic proliferation of surrounding trophoblastic cells, and absence of an embryo/fetus
7. Clinical signs diagnostic of a mole include preeclampsia during the first trimester, elevated hCG (>100,000 mIU/mL), and an enlarged uterus with bleeding.
8. Follow-up visits after a mole are essential because 3-5% of moles develop into gestational trophoblastic neoplasia

1. GTN is a malignant tumor of the trophoblast that may occur following a normal or ectopic pregnancy, abortion, or a hydatidiform mole
2. With a high degree of suspicion, elevated hCG levels are diagnostic
3. Nonmetastatic GTN (ie, 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

A complete mole usually has apparently normal 46,XX karyotype, but both nuclear chromosomes are of paternal origin. This results from fertilization of an “empty” egg (ie, absent or inactivated maternal chromosomes) by a haploid sperm that then duplicates (46,YY moles do not occur, because this karyotype is lethal)

A partial mole usually has a triploid karyotype (69,XXX; 69,XYY) due o the fertilization of an ovum (one set of haploid maternal chromosomes) by two sperm (two sets of haploid paternal chromosomes)

A complete mole (no embryo present) is distinguished from a partial mole (embryo present) by the amount of chorionic villous involvement

-Gross, generalized edema of chorionic villi forming grape-like, transparent vesicles
-Hyperplastic proliferation of surrounding trophoblastic cells,
-Absence of an embryo/fetus

-Preeclampsia during the first trimester
-Elevated hCG (>100,000 mIU/mL)
-An enlarged uterus with bleeding