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1. 28 Pregnancy and Human Development: Part B

2. Organogenesis • Gastrulation sets the stage for organogenesis: formation of body organs and systems • At eighth week • All organ systems are recognizable • End of the embryonic period

3. Specialization of Ectoderm • Neurulation • First major event of organogenesis • Gives rise to brain and spinal cord • Ectoderm over the notochord forms the neural plate • Neural plate folds inward as a neural groove with neural folds

4. Specialization of Ectoderm • By the 22nd day, neural folds fuse into a neural tube • Anterior end  brain; the rest  spinal cord • Neural crest cells  cranial, spinal, and sympathetic ganglia, and adrenal medulla

5. Head Amnion Amniotic cavity Neural plate Ectoderm Left Right Mesoderm Cut edge of amnion Primitive streak Notochord Endoderm Tail Yolk sac (a) 17 days. The flat three-layered embryo has completed gastrulation. Notochord and neural plate are present. Figure 28.10a

6. Neural groove Somite Neural fold Intermediate mesoderm Neural crest Lateral plate mesoderm • Somatic mesoderm Coelom • Splanchnic mesoderm (b) 20 days. The neural folds form by folding of the neural plate, which then deepens, producing the neural groove. Three mesodermal aggregates form on each side of the notochord (somite, intermediate mesoderm, and lateral plate mesoderm). Figure 28.10b

7. Surface ectoderm Neural crest Neural tube Somite Notochord (c) 22 days. The neural folds have closed, forming the neural tube which has detached from the surface ectoderm and lies between the surface ectoderm and the notochord. Embryonic body is beginning to undercut. Figure 28.10c

8. Neural tube (ectoderm) Dermatome Somite Myotome Sclerotome Epidermis (ectoderm) Kidney and gonads (intermediate mesoderm) Gut lining (endoderm) Splanchnic mesoderm Somatic mesoderm • Visceral serosa • Limb bud • Smooth muscle of gut • Parietal serosa • Dermis Peritoneal cavity (coelom) (d) End of week 4. Embryo undercutting is complete. Somites have subdivided into sclerotome, myotome, and dermatome, which form the vertebrae, skeletal muscles, and dermis respectively. Body coelom present. Figure 28.10d

9. Specialization of Endoderm • Embryonic folding begins with lateral folds • Next, head and tail folds appear • Endoderm tube forms epithelial lining of the GI tract • Organs of the GI tract become apparent, and oral and anal openings perforate • Mucosal lining of respiratory tract forms from pharyngeal endoderm (foregut)

10. Tail Head Amnion Yolk sac (a) Ectoderm Trilaminar embryonic disc Mesoderm Endoderm Figure 28.11a

11. Future gut (digestive tube) Lateral fold (b) Figure 28.11b

12. Somites (seen through ectoderm) Tail fold Head fold Yolk sac (c) Figure 28.11c

13. Neural tube Notochord Primitive gut Foregut Yolk Hindgut sac (d) Figure 28.11d

14. Pharynx Parathyroid glands and thymus Thyroid gland Esophagus Trachea Connection to yolk sac Right and left lungs Stomach Liver Umbilical cord Pancreas Gallbladder Small intestine Allantois Large intestine 5-week embryo Figure 28.12

15. Specialization of Mesoderm • First evidence is appearance of the notochord • Three mesoderm aggregates appear lateral to notochord • Somites, intermediate mesoderm, and double sheets of lateral plate mesoderm

16. Specialization of Mesoderm • Somites (40 pairs) each have three functional parts • Sclerotome cells: produce vertebra and rib at each level • Dermatome cells: form dermis of the skin on the dorsal part of the body • Myotome cells: form skeletal muscles of the neck, trunk, and limbs (via limb buds)

17. Specialization of Mesoderm • Intermediate mesoderm forms gonads and kidneys • Lateral mesoderm consists of somatic and splanchnic mesoderm

18. Specialization of the Mesoderm • Somatic mesoderm forms the: • Dermis of the skin in the ventral region • Parietal serosa of the ventral body cavity • Bones, ligaments, and dermis of limbs • Splanchnic mesoderm forms: • The heart and blood vessels • Most connective tissues of the body

19. Head Amnion Amniotic cavity Neural plate Ectoderm Left Right Mesoderm Cut edge of amnion Primitive streak Notochord Endoderm Tail Yolk sac (a) 17 days. The flat three-layered embryo has completed gastrulation. Notochord and neural plate are present. Figure 28.10a

20. Neural groove Somite Neural fold Intermediate mesoderm Neural crest Lateral plate mesoderm • Somatic mesoderm Coelom • Splanchnic mesoderm (b) 20 days. The neural folds form by folding of the neural plate, which then deepens, producing the neural groove. Three mesodermal aggregates form on each side of the notochord (somite, intermediate mesoderm, and lateral plate mesoderm). Figure 28.10b

21. Surface ectoderm Neural crest Neural tube Somite Notochord (c) 22 days. The neural folds have closed, forming the neural tube which has detached from the surface ectoderm and lies between the surface ectoderm and the notochord. Embryonic body is beginning to undercut. Figure 28.10c

22. Neural tube (ectoderm) Dermatome Somite Myotome Sclerotome Epidermis (ectoderm) Kidney and gonads (intermediate mesoderm) Gut lining (endoderm) Splanchnic mesoderm Somatic mesoderm • Visceral serosa • Limb bud • Smooth muscle of gut • Parietal serosa • Dermis Peritoneal cavity (coelom) (d) End of week 4. Embryo undercutting is complete. Somites have subdivided into sclerotome, myotome, and dermatome, which form the vertebrae, skeletal muscles, and dermis respectively. Body coelom present. Figure 28.10d

23. Epiblast ECTODERM MESODERM ENDODERM Intermediate mesoderm Lateral plate mesoderm Notochord Somite Somatic mesoderm Splanchnic mesoderm Nucleus pulposus of inter- vertebral discs Epithelial lining and glands of digestive and respiratory tracts • Epidermis, hair, nails, glands of skin • Brain and spinal cord • Neural crest and derivatives (sensory nerve cells, pigment cells, bones and blood vessels of the head) • Parietal serosa • Dermis of ventral body region • Connective tissues of limbs (bones, joints, and ligaments) • Wall of digestive and respiratory tracts (except epithelial lining) • Visceral serosa • Heart • Blood vessels • Sclerotome: vertebrae and ribs • Dermatome: dermis of dorsal body region • Myotome: trunk and limb musculature • Kidneys • Gonads Figure 28.13

24. Development of Fetal Circulation • First blood cells arise in the yolk sac • By the end of the third week • Embryo has a system of paired vessels • Vessels forming the heart have fused

25. Development of Fetal Circulation • Unique vascular modifications • Umbilical arteries and umbilical vein • Three vascular shunts • All are occluded at birth

26. Development of Fetal Circulation • Vascular shunts • Ductus venosus: bypasses liver (umbilical vein  ductus venosus  IVC) • Foramen ovale: opening in interatrial septum; bypasses pulmonary circulation • Ductus arteriosus: bypasses pulmonary circulation (pulmonary trunk  ductus arteriosus  aorta)

27. Fetus Aortic arch Superior vena cava Ductus arteriosus Ligamentum arteriosum Pulmonary artery Pulmonary veins Heart Lung Foramen ovale Fossa ovalis Liver Ductus venosus Ligamentum venosum Hepatic portal vein Umbilical vein Ligamentum teres Inferior vena cava Umbilicus Abdominal aorta Common iliac artery Umbilical arteries Medial umbilical ligaments Urinary bladder Umbilical cord Placenta High oxygenation Moderate oxygenation Low oxygenation (a) Very low oxygenation Figure 28.14a

28. Newborn Aortic arch Superior vena cava Ductus arteriosus Ligamentum arteriosum Pulmonary artery Pulmonary veins Heart Lung Foramen ovale Fossa ovalis Liver Ductus venosus Ligamentum venosum Hepatic portal vein Umbilical vein Ligamentum teres Inferior vena cava Umbilicus Abdominal aorta High oxygenation Common iliac artery Moderate oxygenation Low oxygenation Umbilical arteries Very low oxygenation Medial umbilical ligaments Urinary bladder (b) Figure 28.14b

29. Events of Fetal Development • Fetal period: weeks 9 through 38 • Time of rapid growth of body structures established in the embryo

30. Umbilical cord Chorionic villi Amniotic sac Umbilical vein Yolk sac Cut edge of chorion (a) Embryo at week 7, about 17 mm long. Figure 28.15

31. Table 28.1 (1 of 3)

32. Table 28.1 (2 of 3)

33. Table 28.1 (3 of 3)

34. Effects of Pregnancy: Anatomical Changes • Reproductive organs become engorged with blood • Chadwick’s sign: the vagina develops a purplish hue • Breasts enlarge and areolae darken • Pigmentation of facial skin many increase (chloasma)

35. Effects of Pregnancy: Anatomical Changes • The uterus expands, occupying most of the abdominal cavity • Lordosis occurs with the change in the center of gravity • Weight gain of ~13 kg (28 lb) • Relaxin causes pelvic ligaments and the pubic symphysis to relax to ease birth passage

36. (a) Before conception (Uterus the size of a fist and resides in the pelvis.) (b) 4 months (Fundus of the uterus is halfway between the pubic symphysis and the umbilicus.) (c) 7 months (Fundus is well above the umbilicus.) (d) 9 months (Fundus reaches the xiphoid process.) Figure 28.16

37. Effects of Pregnancy: Metabolic Changes • Placental hormones • Human placental lactogen (hPL), or human chorionic somatomammotropin (hCS) •  maturation of the breasts, fetal growth, and glucose sparing in the mother • Human chorionic thyrotropin (hCT) •  maternal metabolism • Parathyroid hormone and vitamin D levels are high throughout pregnancy

38. Effects of Pregnancy: Physiological Changes • GI tract • Morning sickness due to elevated levels of estrogen and progesterone • Heartburn and constipation are common • Urinary system •  Urine production due to  metabolism and fetal wastes • Stress incontinence may occur as bladder is compressed

39. Effects of Pregnancy: Physiological Changes • Respiratory system • Estrogens may cause nasal edema and congestion • Tidal volume increases • Dyspnea (difficult breathing) may occur later in pregnancy

40. Effects of Pregnancy: Physiological Changes • Cardiovascular system • Blood volume increases 25–40% • Blood pressure and pulse rise • Venous return from lower limbs may be impaired, resulting in varicose veins

41. Parturition • Parturition giving birth to the baby • Labor events that expel the infant from the uterus

42. Initiation of Labor • During the last few weeks of pregnancy • Fetal secretion of cortisol stimulates the placenta to secrete more estrogen • Causes production of oxytocin receptors by myometrium • Antagonizes calming effects of progesterone, leading to Braxton Hicks contractions in uterus

43. Initiation of Labor • Surfactant protein A (SP-A) from fetal lungs causes softening of the cervix • Fetal oxytocin causes the placenta to produce prostaglandins • Oxytocin and prostaglandins: powerful uterine muscle stimulants

44. Initiation of Labor • Maternal emotional and physical stress • Activates the hypothalamus, causing oxytocin release from posterior pituitary • Positive feedback mechanism occurs

45. Estrogen Oxytocin (+) from placenta from fetus and mother’s posterior pituitary Induces oxytocin receptors on uterus Stimulates uterus to contract Stimulates placenta to make (+) Prostaglandins Stimulate more vigorous contractions of uterus Figure 28.17

46. Stages of Labor: Dilation Stage • Longest stage of labor: 6–12 hours or more • Initial weak contractions: • 15–30 minutes apart, 10–30 seconds long • Become more vigorous and rapid • Cervix effaces and dilates fully to 10 cm • Amnion ruptures, releasing amniotic fluid • Engagement occurs: head enters the true pelvis

47. Umbilical cord Placenta Uterus Cervix Vagina (a) Dilation (early) Figure 28.18a

48. Pubic symphysis Sacrum (b) Dilation (late) Figure 28.18b

49. Stages of Labor: Expulsion Stage • Strong contractions every 2–3 minutes, about 1 minute long • Urge to push increases (in absence of local anesthesia) • Crowning occurs when the largest dimension of the head distends vulva • Delivery of infant

50. Perineum (c) Expulsion Figure 28.18c