Chapter 28

1. Chapter 28 Pregnancy and Human Development G.R. Pitts, J.R. Schiller, and James F. Thompson, Ph.D.

2. Pregnancy • Events from fertilization to birth • Conceptus  Embryo  Fetus : the developing offspring • Gestation period: the time during which development occurs

3. Fertilization • Capacitation: the process in the femal reproductive tract whereby the ejaculated sperm become capable of fertilizing the egg • Acrosomal membrane must become fragile • Acrosomal reaction: release of the digestive enzymes (acrosin, other proteases) from the sperms’ acrosome • Hundreds of sperm must participate

4. Fertilization • If timing is ideal, sperm reach the egg in the upper third of the uterine tube • Sperm move by flagellar action but also receive an assist from uterine tube peristalsis

5. Prevention of Polyspermy • Penetration of the egg membrane by the first sperm causes the membrane to depolarize (Na+ influx) (fast block) • Cell membrane depolarization triggers release of stored Ca++ from the endoplasmic reticulum • Ca++ causes cortical reaction resulting in formation of fertilization membrane from cortical vesicles (slow block) • Polyploid zygotes cannot survive • Similar to the events of an action potential at the synaptic end bulb and muscle contraction

6. Early Events of Fertilization • Upon entry of sperm, the secondary oocyte: • Completes meiosis II • Casts out the second polar body • The ovum nucleus swells, and the two nuclei approach each other • When fully swollen, the two nuclei are called pronuclei • Pronuclei burst • Fertilization – when the pronuclei come together (Blastomeres)

7. Preembryonic Development • Zygote undergoes cleavage to morula and on to blastocyst; should be completed in the uterine tube

8. Implantation • Blastocyst “floats” in the uterus for 2-3 days • Blastocyst implants 6-7 days after fertilization

9. Implantation • the trophoblasts then proliferate and form two distinct layers • Cytotrophoblast – cells of the inner layer that retain their cell boundaries • Syncytiotrophoblast – cells in the outer layer that lose their plasma membranes and invade the endometrium • implantation completed by 14 days after ovulation • hCG from the placental chorion signals the hypothalamus, pituitary, and corpus luteum that implantation has occurred • steroid hormone levels are maintained which prevents uterine sloughing (menses) hCG

10. Implantation • Endometrial epithelium grows around implanted blostocyst • Chorion – develops from trophoblasts after implantation, continues hCG stimulus hCG

11. Placenta produces hCG, estrogen, progesterone, etc. • hCG maintains the corpus luteum which produces estrogen and progesterone (positive feedback) • hCG informs the hypothalamus and pituitary that implantation has occurred • eventually, the placenta produces its own estrogen & progesterone to support uterine proliferation

12. Placentation • The chorion develops fingerlike villi, which: • become vascularized • extend to the embryo as umbilical arteries and veins • lie immersed in maternal blood

13. The Placenta • Before becoming three-layered, the inner cell mass subdivides into the upper epiblast and lower hypoblast • These layers form two of the four embryonic membranes

14. Placentation • The embryo is supported by three external embryonic membranes: chorion, allantois, and amnion.

15. Chorion (outer membrane) forms from the embryonic trophoblast • chorion forms the bulk of the placenta • chorionic villi develop and become vascularized • villi are surrounded by maternal blood vessels in the uterine lacunae • nutrients, gases, and wastes are exchanged by diffusion between the maternal and fetal circulations

16. Allantois • Allantois – a small outpocketing at the caudal end of the yolk sac • structural base for the umbilical cord • becomes part of the urinary bladder • Yolk sac – hypoblast cells that form a sac on the ventral surface of the embryo • forms part of the digestive tube • produces earliest blood cells and vessels • is the source of primordial germ cells

17. Umbilical cord with fetal blood vessels develops from allantois

18. Amnion (inner membrane) envelops and protects embryo • amnion – epiblast cells form a transparent membrane filled with amniotic fluid – a maternal plasma filtrate • amniotic fluid comes from maternal blood, and, later, fetal urine adds to it • amniotic fluid acts as a liquid shock absorber to protect the fetus • helps maintain uterine internal homeostatis • amniotic fluid may be sampled to determine certain aspects of fetal health

19. Gastrulation • During the 3rd week, a primitive streak appears • this raised dorsal groove establishes the longitudinal axis of the embryo • The first cells that enter the groove form the endoderm

20. Gastrulation • the cells that follow push laterally between the cells forming the mesoderm • the cells that remain on the embryo’s dorsal surface form the ectoderm • the two-layered embryonic disc becomes a three-layered embryo • the primary germ layers form: ectoderm, mesoderm, and endoderm

21. The Primary Germ Layers • form populations of stem cells from which all body tissues and organs are derived • Ectoderm – forms structures of the nervous system and skin epidermis • Endoderm – forms epithelial linings of the digestive, respiratory, and urogenital systems • Endoderm and ectoderm give rise to the epithelial tissues • Mesoderm – forms all other tissues (all connective tissues, bone, all types of muscle, blood and blood vessels, the gonads and the adrenal cortex)

22. Body Plan and Tissues Develop, Then Organ Systems Develop • Head, Trunk, and Limb Buds Develop • The Notochord and Neural Tube organize on the dorsal surface • The Peritoneal Cavity (coelom) and Primitive Gut (archenteron) organize beneath the ventral surface

23. Organogenesis • Neurulation – the first event of organogenesis gives rise to the brain and spinal cord • induced by the notochord • Ectoderm over the notochord thickens, forming the neural plate • the neural plate folds inward as a neural groove with prominent neural folds

24. Organogenesis • by the 22nd day, the neural folds fuse into a neural tube, which pinches off into the body • the anterior end becomes the brain; the rest becomes the spinal cord • associated neural crest cells give rise to the cranial, spinal, and sympathetic ganglia of the PNS

25. Endoderm Specializations • embryonic folding begins with lateral folds • next, head and tail folds appear • an endoderm tube forms the epithelial lining of the GI tract

26. Endoderm Specializations • organs of the GI tract become apparent, and oral and anal openings perforate • endoderm forms the epithelium linings of the hollow organs of the digestive and respiratory tracts

27. Mesoderm Specializations • three mesoderm aggregates appear lateral to the notochord • the somites produce the vertebrae, ribs, dermis of the skin, and skeletal muscles of the neck, trunk, and limbs • intermediate mesoderm forms the gonads and the kidneys • lateral mesoderm • somatic mesoderm forms dermis of the skin in the ventral region parietal serosa of the ventral body cavity, bones, ligaments, and dermis of the limbs • splanchnic mesoderm forms the heart, blood vessels and most connective tissues of the body

28. Fetal Circulation Patterns • Umbilical vein routes oxygenated nutrient-laden blood first to the liver, then to the general circulation • Ductus venosus – the venous shunt which bypasses the liver

29. Fetal Circulation Patterns • 3 shunts transfer oxygenated blood from the right to the left side of the heart to bypass the pulmonary circulation • Foramen ovale – opening in the interatrial septum • Interventricular foramen – opening in the interventricular septum • Ductus arteriosus – anastamosis transfers blood from the pulmonary trunk to the aorta

30. Maternal Changes During Pregnancy • Chadwick’s sign – the vagina develops a purplish hue • Breasts enlarge and their areolae darken • The uterus expands, occupying most of the abdominal cavity • Lordosis is common due to the change of the body’s center of gravity

31. Maternal Changes During Pregnancy • Relaxin causes pelvic ligaments and the pubic symphysis to relax • Typical weight gain is about 29 pounds • GI tract – morning sickness occurs due to elevated levels of estrogen and progesterone • Urinary system – urine production increases to handle the additional fetal wastes • Respiratory system – edematous and nasal congestion may occur • Dyspnea (difficult breathing) may develop late in pregnancy • Cardiovascular system – blood volume increases 25-40% • Venous pressure from lower limbs is impaired, resulting in varicose veins

32. Regulation of Parturition (birth) • Labor and Delivery are regulated cooperatively by hormones and the ANS • Relaxin is secreted by the corpus luteum; it helps to soften the cervix and relax the pelvic ligaments in preparation for childbirth

33. Regulation of Parturition (birth) • Cortisol from fetus increases estrogen • Estrogen peaks during the last weeks of pregnancy increasing oxytocin receptors and antagonzing P4 causing myometrial weakness and irritability • Weak Braxton Hicks contractions may take place • As birth nears, the fetus produces oxytocin and the placenta produces prostaglandins causing uterine contractions • Emotional and physical stress: • activates the hypothalamus • sets up a positive feedback mechanism, releasing more oxytocin

34. Parturition • Dilation  Expulsion of Neonate  Expulsion of Placenta • Eventually conscious motor commands add the “push” for delivery

35. Dilation • from the onset of labor until the cervix is fully dilated (10 cm) • initial contractions are 15–30 minutes apart and 10–30 seconds in duration • the cervix thins (effaces) and dilates • the amnion ruptures, releasing amniotic fluid (breaking of the water) • engagement occurs as the infant’s head enters the true pelvis • the head rotates face down • Longest part of parturition (6-12 h)

36. Expulsion • from full dilation to delivery of the infant • strong contractions occur every 2–3 minutes and last about 1 minute • the urge to push increases in labor without local anesthesia • crowning occurs when the largest dimension of the head is distending the vulva

37. Expulsion • placental delivery is accomplished within 30 minutes of birth • afterbirth – the placenta and its attached fetal membranes • all placenta fragments must be removed to prevent postpartum bleeding

38. Extrauterine Life • once carbon dioxide is no longer eliminated by the placenta, central acidosis occurs • this excites the respiratory centers to trigger the first inspiration • this requires tremendous effort – airways are tiny and the lungs are collapsed • once the lungs inflate, surfactant in alveolar fluid helps reduce surface tension • umbilical arteries and vein constrict and soon become fibrosed

39. Lactation • the production of milk by the mammary glands • estrogens, progesterone, and lactogen stimulate the hypothalamus to release a prolactin-releasing factor • the anterior pituitary responds by releasing prolactin • Colostrum • a yellowish solution rich in vitamin A, protein, minerals, and IgA antibodies • is released the first 2–3 days • is followed by true milk production

40. Lactation • Advantages of breast milk for the infant • fats and iron are better absorbed • its amino acids are metabolized more efficiently than those of cow’s milk • beneficial chemicals are present – IgA, other immunoglobulins, complement, lysozyme, interferon, and lactoperoxidase • interleukins and prostaglandins are present, which prevent overzealous inflammatory responses • its natural laxatives help cleanse the bowels of meconium

41. Lactation • Other advantages of breast feeding for the infant • improved maternal-child bond • improved neurological development • appropriate jaw, teeth and overall facial development as well as speech development • reduced risks for breast cancer and ovarian cancer

42. Weaning • The transition from milk to other forms of nutrition • should begin between 6 and 12 months after birth • sometimes a difficult transition for both mother and child

43. Parental Care and Socialization • Mother provides milk • Father and siblings and, perhaps, other relatives may provide additional food, care, support and protection • Maturity and socialization develop slowly over a period of years, even decades!

44. End Chapter 28