Pregnancy and Human Development: Part A

Pregnancy and Human Development: Part A
28 Pregnancy and Human Development: Part A

Pregnancy Pregnancy: events that occur from fertilization until the infant is born Conceptus: the developing offspring Gestation period: time from the last menstrual period until birth (~280 days) Embryo: conceptus from fertilization through week 8 Fetus: conceptus from week 9 through birth

1-week conceptus Embryo Fertilization 3-week embryo (3 mm)
12-week fetus (90 mm) Figure 28.1

From Egg to Zygote The oocyte is viable for 12 to 24 hours Sperm is viable 24 to 48 hours after ejaculation

For fertilization to occur, coitus must occur no more than
From Egg to Zygote For fertilization to occur, coitus must occur no more than Two days before ovulation 24 hours after ovulation Fertilization: when the sperm’s chromosomes combine with those of a secondary oocyte to form a fertilized egg (zygote)

Accomplishing Fertilization
Ejaculated sperm Leak out of the vagina immediately after deposition Are destroyed by the acidic vaginal environment Fail to make it through the cervix Are dispersed in the uterine cavity or destroyed by phagocytes Few (100 to a few thousand) reach the uterine tubes

Accomplishing Fertilization
Sperm must become motile Sperm must be capacitated before they can penetrate the oocyte Secretions of the female tract weaken acrosome membrane

Acrosomal Reaction and Sperm Penetration
Sperm must breach oocyte coverings Corona radiata and zona pellucida Sperm binds to the zona pellucida and undergoes the acrosomal reaction Enzymes are released to digest holes in the zona pellucida Hundreds of acrosomes release their enzymes to digest the zona pellucida

Acrosomal Reaction and Sperm Penetration
Sperm head approaches the oocyte An acrosomal process forms and binds to receptors Oocyte and sperm membranes fuse Only one sperm is allowed to penetrate the oocyte (monospermy)

Blocks to Polyspermy Upon entry of a sperm, Ca2+ surge from the ER causes the cortical reaction Cortical granules release enzymes (zonal inhibiting proteins, or ZIPs) ZIPs destroy sperm receptors Spilled fluid binds water and swells, detaching other sperm (slow block to polyspermy)

Sperm Granulosa cells of corona radiata Zona pellucida ZP3 molecules
Aided by surface hyaluronidase enzymes, a sperm cell weaves its way past granulosa cells of the corona radiata. 1 Granulosa cells of corona radiata Binding of the sperm to ZP3 molecules in the zona pellucida causes a rise in Ca2+ level within the sperm, triggering the acrosomal reaction. 2 Zona pellucida ZP3 molecules Oocyte plasma membrane Acrosomal enzymes digest holes through the zona pellucida, clearing a path to the oocyte membrane. 3 Oocyte sperm-binding membrane receptors The sperm forms an acrosomal process, which binds to the oocyte’s sperm-binding receptors. 4 Cortical granules The sperm and oocyte plasma membranes fuse, allowing sperm contents to enter the oocyte. 5 Acrosomal process Entry of sperm contents (tail and plasma membrane remain behind) causes a rise in the Ca2+ level in the oocyte’s cytoplasm, triggering the cortical reaction (exocytosis of cortical granules). The result is hardening of the zona pellucida and clipping off of sperm receptors (slow block to polyspermy). 6 Cortical reaction Sperm nucleus Extracellular space Figure 28.2

Completion of Meiosis II and Fertilization
As sperm nucleus moves toward the oocyte nucleus it swells to form the male pronucleus The Ca2+ surge triggers completion of meiosis II  ovum + second polar body Ovum nucleus swells to become a female pronucleus Membranes of the two pronuclei rupture and the chromosomes combine

After the sperm penetrates the secondary oocyte, the
Sperm nucleus Extracellular space Corona radiata After the sperm penetrates the secondary oocyte, the oocyte completes meiosis II, forming the ovum and second polar body. 1 Zona pellucida Second meiotic division of oocyte Second meiotic division of first polar body Male pronucleus Female pronucleus (swollen ovum nucleus) Sperm and ovum nuclei swell, forming pronuclei. 2 Polar bodies Male pronucleus Mitotic spindle Pronuclei approach each other and mitotic spindle forms between them. 3 Centriole Female pronucleus Chromosomes of the pronuclei intermix. Fertilization is accomplished. Then, the DNA replicates in preparation for the first cleavage division. 4 Zygote (a) Figure 28.3a

Embryonic Development
Cleavage Mitotic divisions of zygote First cleavage at 36 hours  two daughter cells (blastomeres) At 72 hours  morula (16 or more cells) At day 3 or 4, the embryo of ~100 cells (blastocyst) has reached the uterus

Embryonic Development
Blastocyst: fluid-filled hollow sphere Trophoblast cells Display factors that are immunosuppressive Participate in placenta formation Inner cell mass Becomes the embryonic disc ( embryo and three of the embryonic membranes)

(a) Zygote (fertilized egg) (b) 4-cell stage 2 days (c) Morula (a solid ball of blastomeres). 3 days (d) Early blastocyst (Morula hollows out, fills with fluid, and “hatches” from the zona pellucida). 4 days Zona pellucida Degenerating zona pellucida (e) Implanting blastocyst (Consists of a sphere of trophoblast cells and an eccentric cell clus- ter called the inner cell mass). 7 days Sperm Blastocyst cavity Uterine tube Fertilization (sperm meets and enters egg) Oocyte (egg) Ovary Uterus Blastocyst cavity Ovulation Endometrium Trophoblast Inner cell mass Cavity of uterus Figure 28.4

Blastocyst floats for 2–3 days
Implantation Blastocyst floats for 2–3 days Implantation begins 6–7 days after ovulation Trophoblast adheres to a site with the proper receptors and chemical signals Inflammatory-like response occurs in the endometrium

Endometrium Uterine endometrial epithelium Inner cell mass Trophoblast
Blastocyst cavity Lumen of uterus (a) Figure 28.5a

Trophoblasts proliferate and form two distinct layers
Implantation Trophoblasts proliferate and form two distinct layers Cytotrophoblast (cellular trophoblast): inner layer of cells Syncytiotrophoblast: cells in the outer layer lose their plasma membranes, invade and digest the endometrium

Endometrial stroma with blood vessels and glands Syncytiotrophoblast
Cytotrophoblast Inner cell mass (future embryo) Lumen of uterus (c) Figure 28.5c

Implantation The implanted blastocyst is covered over by endometrial cells Implantation is completed by the twelfth day after ovulation

Endometrial stroma with blood vessels and glands Syncytiotrophoblast
Cytotrophoblast Lumen of uterus (d) Figure 28.5d

Hormonal Changes During Pregnancy
Human chorionic gonadotropin (hCG) Secreted by trophoblast cells, later the chorion Prompts corpus luteum to continue secretion of progesterone and estrogen hCG levels rise until the end of the second month, then decline as the placenta begins to secrete progesterone and estrogen

Human chorionic gonadotropin Estrogens Progesterone Gestation (weeks)
Ovulation and fertilization Birth Figure 28.6

Formation of the placenta from embryonic and maternal tissues
Placentation Formation of the placenta from embryonic and maternal tissues Embryonic tissues Mesoderm cells develop from the inner cell mass and line the trophoblast Together these form the chorion and chorionic villi

Placentation Maternal tissues
Decidua basalis (stratum functionalis between chorionic villi and stratum basalis of endometrium) develops blood-filled lacunae

Placentation The chorionic villi
Grow into blood-filled lacunae (intervillous spaces) Vascularized by umbilical arteries and veins Lie immersed in maternal blood

Figure 28.7 (a-c) Endometrium Amniotic cavity Lacuna (intervillous
space) containing maternal blood Primary germ layers Chorionic villus • Ectoderm Maternal blood vessels Chorion • Mesoderm Proliferating syncytiotrophoblast Amnion • Endoderm Cytotrophoblast Forming body stalk Amniotic cavity Yolk sac Allantois Bilayered embryonic disc Extraembryonic mesoderm • Epiblast • Hypoblast Endometrial epithelium Lumen of uterus Chorion being formed Extraembryonic coelom (a) Implanting 71/2-day blastocyst. The syncytiotrophoblast is eroding the endometrium. Cells of the embryonic disc are now separated from the amnion by a fluid-filled space. (b) 12-day blastocyst. Implantation is complete. Extraembryonic mesoderm is forming a discrete layer beneath the cytotrophoblast. (c) 16-day embryo. Cytotrophoblast and associated mesoderm have become the chorion, and chorionic villi are elaborating. The embryo exhibits all three germ layers, a yolk sac and an allantois, which forms the basis of the umbilical cord. Figure 28.7 (a-c)

Placentation Decidua capsularis: part of the endometrium at the uterine cavity face of the implanted embryo Placenta is fully formed and functional by the end of the third month Placenta also secretes human placental lactogen, human chorionic thyrotropin, and relaxin

41/2-week embryo. The decidua capsularis, decidua basalis,
Maternal blood Chorionic villus Umbilical blood vessels in umbilical cord Amnion Amniotic cavity Yolk sac Extraembryonic coelom Chorion Lumen of uterus Decidua capsularis (d) 41/2-week embryo. The decidua capsularis, decidua basalis, amnion, and yolk sac are well formed. The chorionic villi lie in blood-filled intervillous spaces within the endometrium. The embryo is now receiving its nutrition via the umbilical vessels that connect it (through the umbilical cord) to the placenta. Figure 28.7d

Placenta Decidua basalis Chorionic villi Yolk sac Amnion Amniotic
cavity Umbilical cord Decidua capsularis Uterus Extraembryonic coelom Lumen of uterus (e) 13-week fetus. Figure 28.7e

Maternal and embryonic blood supplies do not intermix
Placentation Maternal and embryonic blood supplies do not intermix Embryonic placental barriers include: Membranes of the chorionic villi Endothelium of embryonic capillaries

Placenta Chorionic villi Decidua basalis Maternal arteries Maternal
veins Umbilical cord Myometrium Stratum basalis of endometrium Uterus Lumen of uterus Maternal portion of placenta (decidua basalis) Decidua capsularis Chorionic villus containing fetal capillaries Fetal portion of placenta (chorion) Maternal blood in lacuna (intervillous space) Fetal arteriole Umbilical arteries Fetal venule Umbilical vein Amnion Connection to yolk sac Umbilical cord Figure 28.8

Embryonic Development: Gastrula to Fetus
Germ Layers During implantation, the blastocyst starts to convert to a gastrula Inner cell mass develops into the embryonic disc (subdivides into epiblast and hypoblast) The three primary germ layers and the extraembryonic membranes develop

Extraembryonic Membranes
Amnion: epiblast cells form a transparent sac filled with amniotic fluid Provides a buoyant environment that protects the embryo Helps maintain a constant homeostatic temperature Allows freedom of movement and prevents parts from fusing together Amniotic fluid comes from maternal blood, and later, fetal urine

Yolk sac: a sac that hangs from the ventral surface of the embryo Forms part of the digestive tube Source of the earliest blood cells and blood vessels

Allantois: a small outpocketing at the caudal end of the yolk sac Structural base for the umbilical cord Becomes part of the urinary bladder Chorion: helps form the placenta Encloses the embryonic body and all other membranes

Figure 28.7 (a-c) Endometrium Amniotic cavity Lacuna (intervillous
space) containing maternal blood Primary germ layers Chorionic villus • Ectoderm Maternal blood vessels Chorion • Mesoderm Proliferating syncytiotrophoblast Amnion • Endoderm Cytotrophoblast Forming body stalk Amniotic cavity Yolk sac Allantois Bilayered embryonic disc Extraembryonic mesoderm • Epiblast • Hypoblast Endometrial epithelium Lumen of uterus Chorion being formed Extraembryonic coelom (a) Implanting 71/2-day blastocyst. The syncytiotrophoblast is eroding the endometrium. Cells of the embryonic disc are now separated from the amnion by a fluid-filled space. (b) 12-day blastocyst. Implantation is complete. Extraembryonic mesoderm is forming a discrete layer beneath the cytotrophoblast. (c) 16-day embryo. Cytotrophoblast and associated mesoderm have become the chorion, and chorionic villi are elaborating. The embryo exhibits all three germ layers, a yolk sac and an allantois, which forms the basis of the umbilical cord. Figure 28.7 (a-c)

41/2-week embryo. The decidua capsularis, decidua basalis,
Maternal blood Chorionic villus Umbilical blood vessels in umbilical cord Amnion Amniotic cavity Yolk sac Extraembryonic coelom Chorion Lumen of uterus Decidua capsularis (d) 41/2-week embryo. The decidua capsularis, decidua basalis, amnion, and yolk sac are well formed. The chorionic villi lie in blood-filled intervillous spaces within the endometrium. The embryo is now receiving its nutrition via the umbilical vessels that connect it (through the umbilical cord) to the placenta. Figure 28.7d

Gastrulation Occurs in week 3, in which the embryonic disc becomes a three-layered embryo with ectoderm, mesoderm, and endoderm Begins with appearance of primitive streak, a raised dorsal groove that establishes the longitudinal axis of the embryo

(e) Bilayered embryonic disc, superior view Mesoderm Endoderm
Amnion Bilayered embryonic disc Head end of bilayered embryonic disc Yolk sac (b) Frontal section (c) 3-D view (a) (d) Section view in (e) Primitive streak Head end Epiblast Cut edge of amnion Yolk sac (cut edge) (f) days Hypoblast Endoderm Right Left Ectoderm Primitive streak Tail end (e) Bilayered embryonic disc, superior view Mesoderm Endoderm (g) 16 days Figure 28.9

Cells begin to migrate into the groove
Gastrulation Cells begin to migrate into the groove The first cells form the endoderm Cells that follow push laterally, forming the mesoderm Cells that remain on the embryo’s dorsal surface form the ectoderm Notochord: rod of mesodermal cells that serves as axial support

(e) Bilayered embryonic disc, superior view Mesoderm Endoderm
Amnion Bilayered embryonic disc Head end of bilayered embryonic disc Yolk sac (b) Frontal section (c) 3-D view (a) (d) Section view in (e) Primitive streak Head end Epiblast Cut edge of amnion Yolk sac (cut edge) (f) days Hypoblast Endoderm Right Left Ectoderm Primitive streak Tail end (e) Bilayered embryonic disc, superior view Mesoderm Endoderm (g) 16 days Figure 28.9

Primary Germ Layers The primitive tissues from which all body organs derive Ectoderm  nervous system and skin epidermis Endoderm  epithelial linings of the digestive, respiratory, and urogenital systems Mesoderm  forms all other tissues Endoderm and ectoderm are considered epithelia

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

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)

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

Unique vascular modifications Umbilical arteries and umbilical vein Three vascular shunts All are occluded at birth

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

Respiratory system Estrogens may cause nasal edema and congestion Tidal volume increases Dyspnea (difficult breathing) may occur later in pregnancy

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

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

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

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