Embryonic and Fetal Stages

1. Embryonic and Fetal Stages

2. Embryonic Stage • Beginning of 2nd week through 8th week • Placenta forms • Main internal organs develop • Major external body structures appear

3. 2nd Week • Blastocyst completes implantation • Amniotic cavity forms between inner cell mass and trophoblast • Inner cell mass flattens and becomes embryonic disc • Primary germ layers form • Embryo now called gastrula • Connecting stalk attaches embryo to placenta Gastrula

4. Ectodermal Cells • Nervous system • Epidermis • Hair, nails, glands of skin • Linings of mouth and anal canal

5. Mesodermal Cells • Muscle • Bone • Bone marrow • Blood • Blood vessels • Lymphatic vessels • Connective tissues • Internal reproductive organs • Kidneys • Epithelial linings of body cavities

6. Endodermal Cells • Epithelial linings of digestive tract • Respiratory tract • Urinary bladder • Urethra

7. By 4th Week • Trophoblast produces proteolytic enzymes that break down endometrial tissue and provide nutrients for the embryo • Chorion • Trophoblast plus second layer of cells • Provides outermost covering around fetus • Produces chorionic villi • Help attach embryo to uterine wall • Lacunae form • Spaces between chorionic villi • Fill with maternal blood

8. During 4th Week • Head and jaws appear • Heart beats • Tiny buds form – will become limbs

9. During 4th week • Embryonic disc becomes cylindrical

10. 5th – 7th Weeks • Head grows rapidly • Face becomes more humanlike • Limbs elongate • Fingers and toes form

11. End of 7th Week • Main internal organs established

12. End of 8th Week • Chorionic villi that aren’t in contact with endometrium degenerate • Placental membrane • Separates maternal blood in lacuna from embryonic blood • Gases and other substances exchanged

13. Fetal Stage • End of 8th week-birth • Beginning of fetal stage • Head is disproportionately large • Lower limbs relatively short

14. 3rd Month • 3rd month • Body lengthening accelerates • Growth of head slows • External reproductive organ distinguishable as male or female • Eyelids fuse together

15. 4th Month • 4th month • Body grows rapidly • 20 cm and 170g • Lower limbs lengthen considerably • Skeleton continues to ossify • Fetus has hair, nipples, nails

16. 5th Month • Growth slows • Lower limbs reach final proportion • Skeletal muscles contract – woman may feel movement • White mixture of sebum and dead epidermal cells covers fetus (protects from amniotic fluid) • Lanugo covers skin • Fine, downy hair • 30 cm, 450 g

17. 6th Month • Gains substantial amount of weight • Eyebrows and eyelashes appear • Skin wrinkled, translucent, and red from blood vessels

18. Final Trimester • Fetal brain cell rapidly form networks • Layer of fat beneath skin • Testes of male descend • Digestive and respiratory systems mature • skin becomes smoother (more fat deposited) • eyelids reopen • 40 cm

19. At Birth • 50 cm long • 2.7-3.6 kg • lost lanugo • positioned upside down with head towards cervix

20. Odds of Birth • Of 100 oocytes exposed to sperm: • 84 fertilized • 69 implant in uterus • 42 survive one week or longer • 37 survive 2 weeks or longer • 31 born alive • Many that do not survive have chromosomal abnormalities that are too severe to maintain life