Neurulation

1. Neurulation • First major event of organogenesis • Gives rise to brain and spinal cord • Ectoderm folds inward as a neural groove with neural folds • By 22nd d, neural folds fuse into neural tube • Anterior end  brain; the rest  spinal cord

2. 21 d after conception, folds of tissue on back of developing embryo are rapidly growing together to form neural tube. Just a day later, growth is almost complete. If tissue fails to close completely development of spine, muscle and skin in this region is affected and the baby will be born with spina bifida. Figure 28.10c

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

4. Development of Fetal Circulation • Vascular shunts • Ductusvenosus: bypasses liver (umbilical vein ductusvenosus IVC) • Foramen ovale: opening in interatrial septum; bypasses pulmonary circulation • Ductusarteriosus: bypasses pulmonary circulation (pulmonary trunk ductusarteriosus aorta)

5. Israeli Prime Minister Ariel Sharon suffered a significant Hemorrhagic stroke today, according to a hospital here. He had been scheduled to undergo repair of a patent foramen ovale defect tomorrow. Sharon, 77, had suffered a cerebral hemorrhage with “massive bleeding” and was undergoing brain surgery.

6. 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

7. 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

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

9. 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

10. 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

11. 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

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

13. 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

14. 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

15. 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

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

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

18. 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

19. Perineum (c) Expulsion Figure 28.18c

20. Stages of Labor: Placental Stage • Strong contractions continue, causing detachment of placenta and compression of uterine blood vessels • Delivery of the afterbirth (placenta and membranes) occurs ~30 minutes after birth • All placenta fragments must be removed to prevent postpartum bleeding

21. Uterus Placenta (detaching) Umbilical cord (d) Placental Figure 28.18d

22. First Breath •  CO2 central acidosis  stimulates respiratory control centers to trigger first inspiration • Requires tremendous effort: airways are tiny and the lungs are collapsed • Surfactant in alveolar fluid helps reduce surface tension • Respiratory rate: ~45 per minute for first two weeks, then declines

23. Occlusion of Fetal Blood Vessels • Umbilical vein becomes the ligamentum teres • Ductus venosus  ligamentum venosum • Foramen ovale  fossa ovalis • Ductus arteriosus  ligamentum arteriosum

24. Lactation • Colostrum • Yellowish secretion rich in vitamin A, protein, minerals, and IgA antibodies • Released the first 2–3 days • Followed by true milk production • Suckling initiates a positive feedback mechanism • Oxytocin causes the letdown reflex

25. Inhibits hypothalamic neurons that release dopamine. Hypothalamus releases prolactin releasing factors (PRF) to portal circulation. Start Stimulation of mechanoreceptors in nipples by suckling infant sends afferent impulses to the hypothalamus. Hypothalamus sends efferent impulses to the posterior pituitary where oxytocin is stored. Anterior pituitary secretes prolactin to blood. Oxytocin is released from the posterior pituitary and stimulates myoepithelial cells of breasts to contract. Prolactin targets lactiferous glands. Milk production Alveolar glands respond by releasing milk through ducts of nipples. Figure 28.19

26. Advantages of Breast Milk • Fats and iron easily absorbed; amino acids easily metabolized, compared with cow’s milk • Beneficial chemicals: IgA, complement, lysozyme, interferon, and lactoperoxidase • Interleukins and prostaglandins prevent overzealous inflammatory responses

27. Advantages of Breast Milk • Natural laxative effect helps eliminate bile-rich meconium, helping to prevent physiological jaundice • Encourages bacterial colonization of the large intestine