Embryology%20and%20Development.pdf - Embryology and...

Embryology and Development
Fertilization
1. Fusion of sperm and egg nuclei complete fertilization.
Ovum structure: corona radiata: granulosa cells
zona pellucida: polysaccharide shell
membrane: changes to prevent multiple sperm from
fertilizing ova
a. sperm penetrates corona radiata
b. attaches to zona pellucida
c. acrosome of sperm releases trypsin like enzymes
d. sperm penetrates zona pellucida and attaches to cell membrane
e. membranes fuse - sperm nuclei enters oocyte
f. egg membrane depolarizes (Na diffuses in, Ca diffuses out)
g. cortical granules released
h. zona pellucida hardens - prevents multiple fertilizations
i. secondary oocyte divides (unequal)
j. polar body formed, egg and sperm nuclei fuse
2. The resulting cell is called a zygote .
3. 18-36 hours after fertilization the zygote cleaves.
Early Cell Division
1. The cells represented in the cleavage stage have the ability to form all the cells
necessary for normal development (stem cells).
2. Cell number may be decreased, increased, or altered without effect to the embryo.
3. The 16 cell stage is termed the Morula (app. 3 days).
4. The cells of the Morula are pluripotent !
5. Morula enters the uterine cavity at 3 days but does not attach.
6. After about 7 days gestation, Morula becomes a blastocyst and implants into the
uterine wall.
7. The trophoblast cells erode the uterine lining and form pockets called lacunae.
8. The trophoblast is now called the chorion .
9. The trophoblast, and later the chorion, secrete hCG, human chorionic gonadotropin .
a. HCG prevents immune attack from mother
b. HCG maintains the corpus luteum
c. At same time FSH and LH from pituitary gland suppressed - no menses
Corpus luteum of ovulation - regresses if no pregnancy, size 1-2 cm
If pregnant, size 2-3 cm
Embryonic Stage (Weeks 2-8)
1. Cells of the inner cell mass flatten to form the embryonic disk (twins can develop at
10-14 days).

6. Following is a summary of cell types produced by each germ layer: