Pregnancy and Human Development: Part B 28 Pregnancy and Human Development: Part B

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

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

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

(a) 17 days. The flat three-layered embryo has completed 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

(b) 20 days. The neural folds form by folding of the 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

(c) 22 days. The neural folds have closed, 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

(d) End of week 4. Embryo undercutting is complete. Somites Dermatome Neural tube (ectoderm) 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

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)

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

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

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

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

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

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

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)

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

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

(a) 17 days. The flat three-layered embryo has completed 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

(b) 20 days. The neural folds form by folding of the 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

(c) 22 days. The neural folds have closed, 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

(d) End of week 4. Embryo undercutting is complete. Somites Dermatome Neural tube (ectoderm) 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

Epiblast ECTODERM MESODERM ENDODERM Notochord Somite Intermediate mesoderm Lateral plate mesoderm Somatic mesoderm Splanchnic mesoderm • 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) Nucleus pulposus of inter- vertebral discs • Sclerotome: vertebrae and ribs • Dermatome: dermis of dorsal body region • Myotome: trunk and limb musculature • Kidneys • Gonads • 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 Epithelial lining and glands of digestive and respiratory tracts Figure 28.13

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

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

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)

Figure 28.14a 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

Figure 28.14b Aortic arch Newborn 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 Umbilical arteries Low oxygenation Very low oxygenation Medial umbilical ligaments Urinary bladder (b) Figure 28.14b

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

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

Table 28.1 (1 of 3)

Table 28.1 (2 of 3)

Table 28.1 (3 of 3)

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)

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

(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

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

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

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

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

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

During the last few weeks of pregnancy 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

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

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

vigorous contractions of uterus 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

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

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

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

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

Perineum (c) Expulsion Figure 28.18c

Stages of Labor: Placental Stage Strong contractions continue, causing detachment of the 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

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

Adjustments of the Infant to Extrauterine Life Neonatal period: four-week period immediately after birth Physical status is assessed 1–5 minutes after birth Apgar score: 0–2 points each for Score of 8–10: healthy Heart rate Respiration Color Muscle tone Reflexes

Respiratory rate: ~45 per minute for first two weeks, then declines First Breath  CO2  central acidosis  stimulates respiratory control centers to trigger the 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

Unstable period lasting 6–8 hours after birth Transitional Period Unstable period lasting 6–8 hours after birth Alternating periods of activity and sleep Vital signs may be irregular during activity Stabilizes with waking periods occurring every 3–4 hours

Occlusion of Fetal Blood Vessels Umbilical arteries and vein constrict and become fibrosed Proximal umbilical arteries  superior vesical arteries to urinary bladder Distal umbilical arteries  medial umbilical ligaments

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

Production of milk by the mammary glands Toward the end of pregnancy Lactation Production of milk by the mammary glands Toward the end of pregnancy Placental estrogens, progesterone, and lactogen stimulate the hypothalamus to release prolactin-releasing factors (PRFs) Anterior pituitary releases prolactin

Suckling initiates a positive feedback mechanism 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

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

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

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

Assisted Reproductive Technology Surgical removal of oocytes following hormone stimulation Fertilization of oocytes Return of fertilized oocytes to the woman’s body

Assisted Reproductive Technology In vitro fertilization (IVF) Oocytes and sperm are incubated in culture dishes for several days Embryos (two-cell to blastocyst stage) are transferred to uterus for possible implantation

Assisted Reproductive Technology Zygote intrafallopian transfer (ZIFT Fertilized oocytes are transferred to the uterine tubes Gamete intrafallopian transfer (GIFT) Sperm and harvested oocytes are transferred together into the uterine tubes