General Embryology 3

1. General Embryology 3 Lei Lei Aug.29, 2006

2. Today’s contents • 3rd month to birth: Fetal membrane and placenta • Birth defects and prenatal diagnosis

3. Chapter 6. Thirdmonth to birth: the fetus and placenta Ⅰ. Development of the fetus Ⅱ. Fetal membranes: Chorion Ⅲ. Placenta★

4. Ⅰ. Fetal development • GL: greatest length, before 4 wks • CRL: crown-rump length • CHL: crown-heel length Table. 6.1 Expected date of delivery, EDD Year+1 Month-3 or +9 Date-7

5. A. Monthly change

6. The face becomes more human looking. The eyes move to ventral aspect of the face, the ears come to lie close to their definitive position. Limbs, primary ossification centers Sex can be determined

7. Fetus lengthens rapidly, but the weight increases little. Fetus is covered with lanugo hair. Mother can feel the movement of the fetus.

8. The skin is reddish and has wrinkled appearance. The respiratory system and the CNS have not differentiated sufficiently.

9. Ⅱ. Fetal membranes • Amnion ★ • Yolk sac • Allantois • Umbilical cord • Chorion ★

11. After implantation, endometrium becomes decidua, which is shed during parturition Decidua parietalis Decidua basalis Decidua capsularis

12. Decidua: forms 3 distinct regions (3 weeks) Decidua Basalis- implantation site Decidua Capsularis- enclosing the conceptus Decidua Parietalis- remainder of uterus

13. 1. Amnion Thin, transparent membrane made up of an amniogenic cell layer and outer extraembryonic mesoderm layer.

14. Amniotic fluid • is a dialysate of blood, first secreted by the amniotic membrane. By 16 weeks of gestation, the fetal urine is also a major contributor to the amniotic fluid. • about 1- 1.5 liter at full term.

15. Functions of amniotic fluid • 1) provide a protective cushion and absorb jolts; • 2) allow for fetal movement and prevent adherence of the fetus; • 3) maintain body temperature of the embryo; • 4) help to dilate and clean the cervical canal making childbirth easier. • 5) Prenatal diagnosis: biochemical assay ,chromosome or gene analysis.

16. 30ml/10wk → 350ml/20wk → 1L/38wk • Totally renewed every 3h Circulation of the amniotic fluid

17. Abnormalities • polyhydramnios, amniotic fluid >2 liter, suggesting anencephaly, hydrocephalus, or esophageal atresia. • oligohydramnios, amniotic fluid <500 ml, due to renal agenesis or urethral atresia.

18. 2. Yolk sac • Inner endoderm + outer extraembryonic mesoderm

19. Its significance in development is • a. Some endoderm cells of it form primordial germ cells.

20. b. Mesoderm cells of its wall differentiate into primitive blood cells.

21. 3. Allantois • 1) Formation: caudal part of yolk sac → diverticulum extending into body stalk • 2) Significance: blood vessels in its wall → umbilical arteries and vein.

22. allantois-outpocketing of embryo’s gut, incorporated into umbilical cord, forms blood vessels of umbilical cord

23. Contents: body stalk, yolk sac stalk, allantois, 2 arteries and 1 vein, mucoid C.T. umbilical vessels 4. Umbilical cord • Formation: body stalk → connecting the embryo or fetus to the placenta, 50 cm long at birth.

25. very long (＞80cm) → encircling neck or extremities very short (＜35cm) → prematurely detaching of placenta and bleeding Abnormalities

26. 5. Chorion Chorion -- made up of the syncytiotrophoblast, cytotrophobalst and extraembryonic mesoderm. a. Chorionic plate b. Villi：primary, secondary, tertiary

27. A. Chorionic plate

29. Chorionic plate Villi chorionic plate: trophoblast + extraembryonic mesoderm

30. Chorion frondosum, adjacent to decidua basalis with numerous, large and branched villi, forming fetal component of placenta. • chorionlaeve：The chorion associated with the decidua capsularis, the villi of which become degenerated.

31. leafy smooth

32. B. Villi ① Primary villus cytotrophoblast Primary Villi:

33. ②Secondary villus Secondary Villi: cytotrophoblast + extraembryonic mesoderm

34. ③ tertiary villus Tertiary Villi: cytotrophoblast + extraembryonic mesoderm+ blood vessels

36. Abnormalities Excessive proliferation of the trophoblast may lead the hydatidiformmole with large amounts of vesicles and dead embryo. The malignant proliferation of trophoblast cells is called the chorion carcinoma.

37. hydatidiform mole

38. Ⅲ. Placenta • the structure by which exchange of material between fetus and mother takes place • size: round, disc-shaped, 15-20 cm in D, 2.5 cm thickness, 500g in weight

39. Maternal portion fetal portion • Components: fetal portion ← chorion frondosum • Maternal portion ← decidua basalis

40. fetal surface: smooth, covered by amniotic membrane • mother surface: rough, 15-30 cotyledons

41. The cotyledons are partially separated by the decidual septa.

42. Chorionic plate→stem villi→cytotrophoblastic shell Basal plate→placental septa→cotyledonsFree villi + intervillous spaces (maternal blood) 1). Structure

43. 2). Blood circulation of placenta spiral arteriesumbilical arteries mother intervillous spaces capillaries fetus uterine veinsumbilical vein

45. 3). Placental barrier

46. Placental barrier: the structure between fetal and maternal blood. Components: • a. Endothelium and its basal lamina of the villous capillary. • b. Connective tissue in the core of the villus. • c. Cytotrophoblast and its basal lamina. • d. Syncytiotrophoblast layer.

47. 4). Functions of placenta • Exchange of gases, nutrients, metabolites between the maternal and fetal blood. • Protection • Hormone production: all from syncytiotrophoblast Human chorionic gonadotropin (HCG) Human placental lactogen (HPL) Progesterone and estrogen

48. Ⅳ. Twins

49. multiple birth

50. Conjoined twins