1. Embryology Ⅰ
Yanqiu Hu

2. Today’s contents Introduction of embryology Gametogenesis
1st week, ovulation to implantation ★
2nd week, bilaminar germ disc ★

3. Ⅰ. Introduction What is embryology?
Embryology is a science that studies the normal development as well as the birth defects of a human being in the maternal uterus.

4. Normal development
Takes 38 weeks from fertilization to parturition of a mature fetus.
Three periods are divided:
1) Preembryonic period: First 2 weeks, Fertilization to formation of the bilaminar germ disc.

5. 2) Embryonic period: Weeks 3 – 8, Primordial of all major organs develop from the three germ layers.

6. 3) Fetal period: Week 9 – birth.
Growth of the organ systems

7. Birth defects
The study of the congenital malformations is the Teratology
Causes
1) Genetic factors (25%)
2) Environmental factors (teratogens) (10%)
3) Interaction of genetic and environmental
factors (65%)

9. The genetic control of early embryonic development

10. Chapter 1. Gametogenesis
A process of formation and maturation of the gametes (sperm and ovum)

11. 1.1 At the end of 3rd week, PGCs in the wall of the yolk sac and start to migrate to the developing gonad through the hindgut.

12. Spermatogenesis
Oogenesis
2n, 46
2n, 46
n, 23
n, 23
n, 23
n, 23

13. 1.2 The purpose of gametogenesis
Reduction of the number of chromosomes to half by meiosis, which occurs only in germ cell.
Alteration of the shape of germ cells for fertilization

14. 1.3 Differences between male and female
Male Female
Has stem cells No stem cells
Continuous after puberty Pause twice
4 sperm from a spermatocyte Only 1 ovum fromed
Equal division Unequal division
Functional mature and No
capacitation

15. 1.5 Functional mature and capacitation of sperm
Functional mature: getting ability of forward
movement and potential of fertilizing eggs in the
epididymis
Capacitation: obtaining capacity of fertilizing an
egg in the female reproductive tract through
removal of a glycoprotein coat from the plasma
membrane of the acrosomal region by enzyme
(b-amylase).

16. Abnormal gametes
For fertilization to occur, egg and sperm must meet between 2 – 48 hrs after sex.
Egg is only fertilizable for 48 hrs.

17. Chapter 2. First week of development
Ovulation to implantation

18. 2.1 Fertilization
-- union of a sperm and an oocyte to form a fertilized egg (zygote)
Time: hours after ovulation.
Site: ampulla of uterine tube.

19. 2.1.1. Fertilization precursors
Women: Ovulation  Secondary oocyte released.
Accompanying the oocyte:
Corona radiata
Zona pellucida

20. 2.1.1. Fertilization precursors
Men:
~10 hour journey for sperm
Some sperm are gradually released into the cervical canal; which increases the chances of fertilization.
200~300 million sperm  3 million make it to uterus  300~500 sperm reach the fertilization site.

21. 2.1.2. Important events in fertilization
Before:
Capacitation
Acrosome reaction: release of enzymes from acrosome induced by, and needed for sperm penetration of, the zona pellucida.
removal of a glycoprotein coat from the plasma
membrane of the acrosomal region by enzyme
(amylase).

22. Capacitation
Takes 5-7 hours. This maturation of sperm is called capacitation
Capacitation is facilitated by substances in the female tract. Removal of a glycoprotein coat from the plasma membrane of the acrosomal region by enzyme (β-amylase).
Capacitation is required for the acrosomal reaction to take place.

23. The acrosome reaction Sperm bind to zona pellucida
The acrosome reaction provides the sperm with an enzymatic drill to get through zona pellucida
red
blue
pink

24. 2.1.2. Important events in fertilization
During:
Penetration of corona radiata, zona pellucida, oocyte cell membrane.
Recognition: a zona protein ZP3 is responsible for species-specific fertilization.

26. After entry:
Zona reaction: zona becomes impenetrable to other sperm (monospermy), through enzyme release from cortical granules of oocyte to change structure and composition of the zona.
Oocyte finishes 2nd meiosis.
Formation and merge of male and female pronuclei.

28. 2.2. Cleavage
24 hr after fertilization, the mammalian zygote (1 cell) begins cleavage.
a series of mitotic divisions of fertilized egg
It is difficult to count the cells in a morula; the embryo shown here probably has between 20 and 30 cells.

29. Morula
solid ball of cells
Zygote
Blastocyst
with blastocoele cavity

31. 2.3 Blastocyst formation
Formation: morula → cell number↑→ spaces appearing →blastocyst

32. 3) Blastocele: will form exocoelomic cavity
1) Inner cell mass (embryoblast): develops into embryo proper.
2) Trophoblast: absorbs nutrients. Will go on to contribute to fetal membrane systems.
3) Blastocele: will form exocoelomic cavity

33. Trophoblast forms syncytiotrophoblast- erodes into endometrium
Implantation - embedding of blastocyst into endometrium begins at day 5~6
Trophoblast forms syncytiotrophoblast- erodes into endometrium
Cytotrophoblast - carries nutrients to inner cell mass
Lacunae and primary villi formed by trophoblast
All of these form placental tissues
Fig 28-3

34. 2.4. Implantation Blastocyst “floats” in the uterus for 2-3 days
Blastocyst implants 5-6 days after ovulation

35. hCG Implantation completed by 14 days after ovulation
hCG from the placental chorion signals the hypothalamus, pituitary, and corpus luteum that implantation has occurred
Hormone levels are maintained which prevents uterine sloughing (menses)
hCG

36. Endometrial epithelium grows around implanted blostocyst
Chorion – developed from trophoblasts after implantation, continues this hormonal stimulus

37. The chorion develops fingerlike villi, which:
Become vascularized
Extend to the embryo as umbilical arteries and veins
Lie immersed in maternal blood

38. Ectopic implantation

39. Chapter 3. Second week of development
Bilaminar Germ Disc

40. Day 7

41. Day 8

42. Day 9

43. Day 12

44. 3.5 Extraembryonic mesoderm
extraembryonic coelom somatopleuric
splanchnopleuric
chorionic cavirty
connecting (body) stalk