1. Chapter 28 - Development

2. SPERM MIGRATION
In upper 1/3 of fallopian tubes within 24 hrs of ovulation & 48 hrs of insemination.
Only about 3000 of original 300 million make it.

3. CAPACITATION Even after arrival at egg, sperm can’t fertilize.
Fluids in female soften plasma membrane and dilute inhibitory factors that prevent the
acrosome from working
Ca++ diffuses in and enhances tail lashing

4. CAPACITATION
Timing – sperm live only 6 days, so can’t get pregnant more than a week before ovulation, nor more than 14 hours after [ egg won’t be viable long enough]

5. Fertilization
Several sperm needed to enzymatically penetrate cell layers around the ovum – acrosome reaction – exocytosis of acrosome
Only 1 sperm enters the ovum - tail & midpiece disintegrate, egg releases enzymes to destroy sperm surface receptors – prevents polyspermy.

6. Fertilization

7. MEIOSIS Oocyte now completes meiosis.
23 male chromosomes + 23 female chromosomes join to form zygote.

8. TRIMESTERS
1 - Fertilization through week 12 – most sensitive time-over half die
2 - Weeks 13 through 24 – complete most organ development – near end IS possible to survive birth

9. TRIMESTERS
weeks – birth - growth continues to point where survival is more likely – brain liver & kidneys have to develop further AFTER birth.
Single births usually 40 weeks, twins 35 weeks 

10. CLEAVAGE/PRE-EMBRYONIC DEVELOPMENT
Mitotic cell divisions produce identical copies.
Morula - solid ball of cells.
Blastocyst - hollow ball - cavity is called blastocoel.
Made of trophoblast [outer layer] & Inner cell mass

11. Human Morula

12. Human Blastocyst

13. CLEAVAGE/PRE-EMBRYONIC DEVELOPMENT
Becomes implanted in endometrium - inner cell mass facing endometrial wall. TAKES ABOUT 1 WEEK.
Trophoblast gives rise to chorion - forms placenta with chorionic villi – secretes hCG to maintain corpus luteum.

14. CLEAVAGE/PRE-EMBRYONIC DEVELOPMENT
Inner cell mass forms embryo and other extra-embryonic membranes

15. IMPLANTATION ~ 6 days after ovulation
Trophoblast cells facing wall fuse  syncytiotrophoblast – grows into uterus
Uterus responds by growing over and burying entire blastocyst
Ideally blastocyst implants HIGH on the wall.
Trophoblast grow into placenta

16. IMPLANTATION ~ 6 days after ovulation
Placenta - nutritive bridge between mother & fetus –by end of 3rd month
Produces large amounts of hCG - peaks at wk 8-9, drops to constant level at wk 16, stimulates corpus luteum to continue secretion of ES and P
Later, the placenta secretes its own P and ES to maintain pregnancy, and relaxin to aid in delivery and prevent premature contractions

17. EXTRA-EMBRYONIC MEMBRANES
Chorion - outer -derived from trophoblast of blastocyst.
Forms placenta.
Secretes estrogens, progesterone, relaxin.
Amnion - for protection.
Filled with amniotic fluid.
Shock absorber for fetus.
Repository for fetal urine, etc.

19. EXTRA-EMBRYONIC MEMBRANES
Allantois - blood vessels to placenta and umbilicus.
Later becomes ligament attached to urinary bladder.
Yolk sac - has little yolk, but source of early blood cells & primordial gametes.

20. EMBRYOGENESIS
Embryoblast flattens into embryonic disc in amniotic cavity – 2 layers – epiblast [toward cavity, hypoblast – away from]
Primitive streak forms along midline with primitive groove running down it  bilateral symmetry, front and back defined

21. EMBRYONIC DEVELOPMENT - Gastrulation
Results in 3 germ layers - ectoderm, endoderm, mesoderm.
Epiblast will become ectoderm [skin, etc. and CNS]
Migration of epiblast cells into and through the primitive groove to replace hypoblast cells with endoderm – will be gut lining

22. EMBRYONIC DEVELOPMENT - Gastrulation
Then a third layer migrates in between the two = mesoderm [will become mesenchyme – basis of connective tissues, and muscle