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 • 3 - 25 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