Pregnancy and Human Development

Pregnancy and Human Development
28 Pregnancy and Human Development

Pregnancy - events that occur from fertilization until infant born
Conceptus - developing offspring Gestation period - time from last menstrual period until birth (~280 days) Embryo - conceptus from fertilization through week 8 Fetus - conceptus from week 9 through birth © 2013 Pearson Education, Inc.

Embryo Fertilization 1-week conceptus 3-week embryo (3 mm) 5-week
Figure Diagrams showing the approximate size of a human conceptus from fertilization to the early fetal stage. Embryo Fertilization 1-week conceptus 3-week embryo (3 mm) 5-week embryo (10 mm) 8-week embryo (22 mm) 12-week fetus (90 mm) © 2013 Pearson Education, Inc.

Oocyte viable for 12 to 24 hours
From Egg to Zygote Oocyte viable for 12 to 24 hours Sperm viable 24 to 48 hours after ejaculation © 2013 Pearson Education, Inc.

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 - sperm's chromosomes combine with those of secondary oocyte to form fertilized egg (zygote) © 2013 Pearson Education, Inc.

Accomplishing Fertilization
Ejaculated sperm Leak out of vagina immediately after deposition Destroyed by acidic vaginal environment Fail to make it through cervix Dispersed in uterine cavity or destroyed by phagocytes Few (100 to a few thousand) reach uterine tubes © 2013 Pearson Education, Inc.

Accomplishing Fertilization
Sperm must become motile Sperm must be capacitated before they can penetrate oocyte Motility must be enhanced; membranes must become fragile to release hydrolytic enzymes Secretions of female tract weaken acrosome membrane Sperm follow "olfactory trail" to reach oocyte © 2013 Pearson Education, Inc.

Acrosomal Reaction and Sperm Penetration
Sperm must breach oocyte coverings Corona radiata and zona pellucida Sperm weaves through corona radiata, then binds to zona pellucida and undergoes acrosomal reaction Enzymes released to digest holes in zona pellucida Hundreds of acrosomes release enzymes to digest zona pellucida © 2013 Pearson Education, Inc.

Figure 28.2 Sperm Penetration and the Cortical Reaction.
Slide 1 Fusion. The sperm and oocyte plasma membranes fuse, allowing sperm contents to enter the oocyte. 4 5 Block of polyspermy. Entry of the sperm’s contents causes Ca2+ levels in the oocyte’s cytoplasm to rise, triggering the cortical reaction (exocytosis of cortical granules). As a result, the zona pellucida hardens and the sperm receptors are clipped off (slow block to polyspermy). 2 Acrosomal reaction. Binding of the sperm to sperm-binding receptors in the zona pellucida causes the Ca2+ levels within the sperm to rise, triggering the acrosomal reaction. Acrosomal enzymes from many sperm digest holes through the zona pellucida, clearing a path to the oocyte membrane. 3 Binding. The sperm’s membrane binds to the oocyte’s Sperm-binding receptors. Sperm, delivered to the vagina and capacitated in the female reproductive tract, stream toward a secondary oocyte. 1 Approach. Aided by enzymes on its surface, a sperm cell weaves its way past granulosa cells of the corona radiata. Extracellular space Sperm Sperm Zona pellucida Polar body Granulosa cells of corona radiata Oocyte nucleus arrested in meiotic metaphase II Oocyte sperm- binding membrane receptors Microtubules from sperm flagellum Cortical granules Zona pellucida Sperm-binding receptors Mitochondria Sperm nucleus Zona pellucida Extracellular space Oocyte plasma membrane © 2013 Pearson Education, Inc.

Acrosomal Reaction and Sperm Penetration
Sperm head approaches oocyte Rear portion of acrosomal membrane binds to oocyte plasma membrane  Oocyte and sperm membranes fuse Gametes fuse as sperm's cytoplasmic contents enter oocyte Only one sperm allowed to penetrate oocyte (monospermy) © 2013 Pearson Education, Inc.

Upon entry of sperm, Ca2+ surge from ER causes cortical reaction
Block to Polyspermy Upon entry of sperm, Ca2+ surge from ER causes 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) © 2013 Pearson Education, Inc.

Figure 28.2 Sperm Penetration and the Cortical Reaction.
Slide 7 Fusion. The sperm and oocyte plasma membranes fuse, allowing sperm contents to enter the oocyte. 4 5 Block of polyspermy. Entry of the sperm’s contents causes Ca2+ levels in the oocyte’s cytoplasm to rise, triggering the cortical reaction (exocytosis of cortical granules). As a result, the zona pellucida hardens and the sperm receptors are clipped off (slow block to polyspermy). 2 Acrosomal reaction. Binding of the sperm to sperm-binding receptors in the zona pellucida causes the Ca2+ levels within the sperm to rise, triggering the acrosomal reaction. Acrosomal enzymes from many sperm digest holes through the zona pellucida, clearing a path to the oocyte membrane. 3 Binding. The sperm’s membrane binds to the oocyte’s Sperm-binding receptors. Sperm, delivered to the vagina and capacitated in the female reproductive tract, stream toward a secondary oocyte. 1 Approach. Aided by enzymes on its surface, a sperm cell weaves its way past granulosa cells of the corona radiata. Extracellular space Sperm Sperm Zona pellucida Polar body Granulosa cells of corona radiata Oocyte nucleus arrested in meiotic metaphase II Oocyte sperm- binding membrane receptors Microtubules from sperm flagellum Cortical granules Zona pellucida Sperm-binding receptors Mitochondria Sperm nucleus Zona pellucida Extracellular space Oocyte plasma membrane © 2013 Pearson Education, Inc.

Completion of Meiosis II and Fertilization
As sperm nucleus moves toward oocyte nucleus it swells to form male pronucleus The Ca2+ surge triggers completion of meiosis II  ovum + second polar body Ovum nucleus swells to become female pronucleus Fertilization – moment when membranes of two pronuclei rupture and chromosomes combine © 2013 Pearson Education, Inc.

Figure 28.3a Events of fertilization.
Sperm nucleus Slide 1 Extracellular space 1 After the sperm penetrates the secondary oocyte, the oocyte completes meiosis II, forming the ovum and second polar body. Corona radiata Zona pellucida Second meiotic division of oocyte Second meiotic division of first polar body Male pronucleus Female pro-nucleus (swollen ovum nucleus) 2 Sperm and ovum nuclei swell, forming pronuclei. Polar bodies Male pronucleus 3 Pronuclei approach each other and mitotic spindle forms between them. Mitotic spindle Centriole Female pronucleus 4 Chromoomes of the pronuclei intermix. Fertilization is accomplished. Then, the DNA replicates in preparation for the first cleavage division. Zygote © 2013 Pearson Education, Inc.

Figure 28.3b Events of fertilization.
Male and female pronuclei Polar bodies © 2013 Pearson Education, Inc.

Events of Embryonic Development: Zygote to Blastocyst Implantation
Cleavage Occurs while zygote moves toward uterus Mitotic divisions of zygote First cleavage at 36 hours  two daughter cells (blastomeres) At 72 hours  morula (16 or more cells) At day 4 or 5, blastocyst (embryo of ~100 cells) reaches uterus © 2013 Pearson Education, Inc.

Embryonic Development
Blastocyst - fluid-filled hollow sphere composed of Trophoblast cells Display immunosuppressive factors Participate in placenta formation Inner cell mass Becomes embryonic disc ( embryo and three of embryonic membranes) © 2013 Pearson Education, Inc.

Figure 28.4 Cleavage: From zygote to blastocyst.
(fertilized egg) 4-cell stage 2 days Morula (a solid ball of blastomeres). 3 days Early blastocyst (Morula hollows out, fills with fluid, and “hatches” from the zona pellucida). 4 days Zona pellucida Degenerating zona pellucida Implanting blastocyst (Consists of a sphere of trophoblast cells and an eccentric cell cluster called the inner cell mass). 7 days Sperm Blastocyst cavity Uterine tube Fertilization (sperm meets and enters egg) Ovary Oocyte (egg) Trophoblast Uterus Blastocyst cavity Ovulation Endometrium Inner cell mass Cavity of uterus © 2013 Pearson Education, Inc.

Blastocyst floats for 2–3 days
Implantation Blastocyst floats for 2–3 days Nourished by uterine secretions Implantation begins 6–7 days after ovulation Trophoblast cells adhere to site with proper receptors and chemical signals Inflammatory-like response occurs in endometrium Uterine blood vessels more permeable and leaky; inflammatory cells invade area © 2013 Pearson Education, Inc.

Figure 28.5a Implantation of the blastocyst.
Endometrium Uterine endometrial epithelium Inner cell mass Trophoblast Blastocyst cavity Lumen of uterus © 2013 Pearson Education, Inc.

Trophoblasts proliferate and form two distinct layers
Implantation Trophoblasts proliferate and form two distinct layers Cytotrophoblast (cellular trophoblast) - inner layer of cells Syncytiotrophoblast (syncytial trophoblast) - cells in outer layer lose plasma membranes, invade and digest endometrium Blastocyst burrows into lining surrounded by pool of leaked blood; endometrial cells cover and seal off implanted blastocyst © 2013 Pearson Education, Inc.

Figure 28.5c Implantation of the blastocyst.
Endometrial stroma with blood vessels and glands Syncytiotrophoblast Cytotrophoblast Blastocyst cavity Lumen of uterus © 2013 Pearson Education, Inc.

Implantation completed by twelfth day after ovulation
Menstruation must be prevented Corpus luteum maintained by hormone human chorionic gonadotropin (hCG) © 2013 Pearson Education, Inc.

Figure 28.5d Implantation of the blastocyst.
Endometrial stroma with blood vessels and glands Syncytiotrophoblast Cytotrophoblast Lumen of uterus © 2013 Pearson Education, Inc.

Hormonal Changes During Pregnancy
Human chorionic gonadotropin (hCG) Secreted by trophoblast cells; later chorion Prompts corpus luteum to continue secretion of progesterone and estrogen Promotes placental development via its autocrine growth factor activity hCG levels rise until end of second month, then decline as placenta begins to secrete progesterone and estrogen; low values at 4 months and rest of pregnancy © 2013 Pearson Education, Inc.

Figure 28.6 Hormonal changes during pregnancy.
Human chorionic gonadotropin Estrogens Relative blood levels Progesterone 4 8 12 16 20 24 28 32 36 Gestation (weeks) Ovulation and fertilization Birth © 2013 Pearson Education, Inc.

Formation of placenta from embryonic and maternal tissues
Placentation Formation of placenta from embryonic and maternal tissues Temporary organ Embryonic tissues Mesoderm cells develop from inner cell mass; line trophoblast Together these form chorion and chorionic villi © 2013 Pearson Education, Inc.

Villi lie in intervillous spaces, immersed in maternal blood
Placentation Cores of chorionic villi invaded by new blood vessels; extend to embryo as umbilical arteries and vein Erosion  blood-filled lacunae (intervillous spaces) in stratum functionalis Villi lie in intervillous spaces, immersed in maternal blood © 2013 Pearson Education, Inc.

Maternal portion of placenta
Placentation Maternal portion of placenta Decidua basalis (stratum functionalis between chorionic villi and stratum basalis of endometrium) Fetal portion of placenta Chorionic villi © 2013 Pearson Education, Inc.

Figure 28.7a–c Events of placentation, early embryonic development, and extraembryonic membrane formation. Endometrium Amniotic cavity Lacuna (intervillous space) containing maternal blood Primary germ layers Chorionic villus • Ectoderm Maternal blood vessels Chorion • Mesoderm Proliferating syncytiotrophoblast Amnion • Endoderm Forming umbilical cord Cytotrophoblast Amniotic cavity Yolk sac Allantois Bilayered embryonic disc • Epiblast • Hypoblast Extraembryonic mesoderm Endometrial epithelium Lumen of uterus Chorion being formed Extraembryonic coelom 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. 12-day blastocyst. Implantation is complete. Extraembryonic mesoderm is forming a discrete layer beneath the cytotrophoblast. 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. © 2013 Pearson Education, Inc.

Placenta fully formed and functional by end of third month
Placentation Decidua capsularis - part of endometrium at uterine cavity face of implanted embryo Placenta fully formed and functional by end of third month Nutritive, respiratory, excretory, endocrine functions Placenta also secretes human placental lactogen, human chorionic thyrotropin, and relaxin © 2013 Pearson Education, Inc.

vessels that connect it (through the umbilical cord) to the placenta.
Figure 28.7d Events of placentation, early embryonic development, and extraembryonic membrane formation. 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 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 nourished via the umbilical vessels that connect it (through the umbilical cord) to the placenta. © 2013 Pearson Education, Inc.

Placenta Decidua basalis Chorionic villi Yolk sac Amnion Amniotic
Figure 28.7e Events of placentation, early embryonic development, and extraembryonic membrane formation. Placenta Decidua basalis Chorionic villi Yolk sac Amnion Amniotic cavity Umbilical cord Uterus Decidua capsularis Lumen of uterus Extraembryonic coelom 13-week fetus. © 2013 Pearson Education, Inc.

If placental hormones inadequate, pregnancy aborted
Placentation If placental hormones inadequate, pregnancy aborted Throughout pregnancy blood levels of estrogens and progesterone increase Prepare mammary glands for lactation © 2013 Pearson Education, Inc.

Maternal and embryonic blood supplies normally do not intermix
Placentation Maternal and embryonic blood supplies normally do not intermix Embryonic placental barriers include Membranes of chorionic villi Endothelium of embryonic capillaries © 2013 Pearson Education, Inc.

Placenta Decidua Maternal Maternal basalis arteries veins Chorionic
Figure Detailed anatomy of the vascular relationships in the mature decidua basalis. Placenta Decidua basalis Maternal arteries Maternal veins Chorionic villi Umbilical cord Myometrium Stratum basalis of endometrium Uterus Lumen of uterus Maternal portion of placenta (decidua basalis) Decidua capsularis Chorionic villus containing fetal capillaries Maternal blood in lacuna (intervillous space) Fetal portion of placenta (chorion) Fetal arteriole Umbilical arteries Fetal venule Umbilical vein Amnion Connection to yolk sac Umbilical cord © 2013 Pearson Education, Inc.

Events of Embryonic Development: Gastrula to Fetus
Germ Layers During implantation, blastocyst begins conversion to gastrula Inner cell mass develops into embryonic disc (subdivides into epiblast and hypoblast) Three primary germ layers form; extraembryonic membranes develop © 2013 Pearson Education, Inc.

Extraembryonic Membranes
Amnion - epiblast cells form transparent sac filled with amniotic fluid Provides buoyant environment that protects embryo Helps maintain constant homeostatic temperature Allows freedom of movement; prevents parts from fusing together Amniotic fluid comes from maternal blood, and later, fetal urine © 2013 Pearson Education, Inc.

Allantois - small outpocketing at caudal end of yolk sac Structural base for umbilical cord Becomes part of urinary bladder Chorion - helps form placenta Encloses embryonic body and all other membranes © 2013 Pearson Education, Inc.

Figure 28.7a–c Events of placentation, early embryonic development, and extraembryonic membrane formation. Endometrium Amniotic cavity Lacuna (intervillous space) containing maternal blood Primary germ layers Chorionic villus • Ectoderm Maternal blood vessels Chorion • Mesoderm Proliferating syncytiotrophoblast Amnion • Endoderm Forming umbilical cord Cytotrophoblast Amniotic cavity Yolk sac Allantois Bilayered embryonic disc • Epiblast • Hypoblast Extraembryonic mesoderm Endometrial epithelium Lumen of uterus Chorion being formed Extraembryonic coelom 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. 12-day blastocyst. Implantation is complete. Extraembryonic mesoderm is forming a discrete layer beneath the cytotrophoblast. 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. © 2013 Pearson Education, Inc.

vessels that connect it (through the umbilical cord) to the placenta.
Figure 28.7d Events of placentation, early embryonic development, and extraembryonic membrane formation. 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 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 nourished via the umbilical vessels that connect it (through the umbilical cord) to the placenta. © 2013 Pearson Education, Inc.

Embryonic disc  three-layered embryo with primary germ layers present
Gastrulation Occurs in week 3 Embryonic disc  three-layered embryo with primary germ layers present Ectoderm, mesoderm, and endoderm Begins with appearance of primitive streak, raised dorsal groove; establishes longitudinal axis of embryo © 2013 Pearson Education, Inc.

Cells begin to migrate into groove
Gastrulation Cells begin to migrate into groove First cells form endoderm Cells that follow push laterally, forming mesoderm Notochord - rod of mesodermal cells that serves as axial support Cells that remain on embryo's dorsal surface form ectoderm © 2013 Pearson Education, Inc.

Gastrulation Ectoderm, mesoderm, endoderm - primitive tissues from which all body organs derive Epithelia cells Ectoderm  nervous system; skin epidermis Endoderm  epithelial linings of digestive, respiratory, urogenital systems; associated glands Mesenchyme cells Mesoderm  everything else © 2013 Pearson Education, Inc.

Figure 28.9 Formation of the three primary germ layers.
Amnion Bilayered embryonic disc Head end of bilayered embryonic disc Yolk sac Frontal section 3-D view Section view in (e) Primitive streak Head end Epiblast Cut edge of amnion Yolk sac (cut edge) 14-15 days Hypoblast Right Endoderm Left Ectoderm Primitive streak Tail end Bilayered embryonic disc, superior view 16 days Mesoderm Endoderm © 2013 Pearson Education, Inc.

Gastrulation sets stage for organogenesis At eighth week
Formation of body organs and systems At eighth week All organ systems recognizable End of embryonic period © 2013 Pearson Education, Inc.

Embryo begins as flat plate 
Organogenesis Embryo begins as flat plate  Cylindrical body resembling three stacked sheets of paper folding laterally into tube, and at both ends © 2013 Pearson Education, Inc.

Figure 28.10 Folding of the embryonic body, lateral views.
Tail Head Amnion Yolk sac Ectoderm Mesoderm Trilaminar embryonic disc Endoderm Future gut (digestive tube) Lateral fold Somites (seen through ectoderm) Tail fold Head fold Yolk sac Neural tube Notochord Primitive gut Hindgut Yolk sac Foregut © 2013 Pearson Education, Inc.

Specialization of Endoderm
Primitive gut formed from endodermal folding Forms epithelial lining of GI tract Organs of GI tract become apparent, and oral and anal openings perforate Mucosal lining of respiratory tract forms from pharyngeal endoderm (foregut) Glands arise further along tract © 2013 Pearson Education, Inc.

Figure 28.11 Endodermal differentiation.
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 © 2013 Pearson Education, Inc.

Specialization of Ectoderm
Neurulation First major event of organogenesis Gives rise to brain and spinal cord Induced by chemical signals from notochord Ectoderm over notochord thickens, forming neural plate Neural plate folds inward as neural groove with neural folds © 2013 Pearson Education, Inc.

Specialization of Ectoderm
By 22nd day, neural folds fuse into neural tube Anterior end  brain; rest  spinal cord Neural crest cells migrate widely  cranial, spinal, and sympathetic ganglia and nerves; adrenal medulla; pigment cells of skin; contribute to some connective tissues Brain waves recorded by end of second month © 2013 Pearson Education, Inc.

Figure 28.12a Neurulation and early mesodermal differentiation.
Head Amnion Amniotic cavity Left Right Neural plate Cut edge of amnion Primitive streak Ectoderm 17 days. The flat three-layered embryo has completed gastrulation. Notochord and neural plate are present. Tail Mesoderm Notochord Endoderm Yolk sac © 2013 Pearson Education, Inc.

Figure 28.12b Neurulation and early mesodermal differentiation.
Neural groove Somite Neural fold Neural crest Intermediate mesoderm 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). Lateral plate mesoderm Coelom © 2013 Pearson Education, Inc.

Figure 28.12c Neurulation and early mesodermal differentiation.
Surface ectoderm Neural crest 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. Neural tube Somite Notochord © 2013 Pearson Education, Inc.

Figure 28.12d Neurulation and early mesodermal differentiation.
Neural tube (ectoderm) Dermatome Myotome Sclerotome Epidermis (ectoderm) Somite Gut lining (endoderm) 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. Kidney and gonads (intermediate mesoderm) Lateral plate mesoderm Limb bud Smooth muscle of gut Visceral serosa Peritoneal cavity (coelom) Parietal serosa Dermis © 2013 Pearson Education, Inc.

Specialization of Mesoderm
First evidence - appearance of notochord Eventually replaced by vertebral column Three mesoderm aggregates appear lateral to notochord Somites, intermediate mesoderm, and double sheets of lateral plate mesoderm © 2013 Pearson Education, Inc.

Somites (40 pairs) each have three functional parts Sclerotome cells - produce vertebra and rib at each level Dermatome cells - form dermis of skin on dorsal part of body Myotome cells - form skeletal muscles of neck, trunk, and limbs (via limb buds) © 2013 Pearson Education, Inc.

Specialization of the Mesoderm
Somatic mesoderm forms Dermis of skin in ventral region Parietal serosa of ventral body cavity Most tissues of limbs Splanchnic mesoderm forms Heart and blood vessels Most connective tissues of body ~ Entire wall of digestive & respiratory organs © 2013 Pearson Education, Inc.

Specialization of the Mesoderm
At end of embryonic period Bones have begun to ossify; skeletal muscles well formed, contracting; metanephric kidneys developing; gonads formed Lungs, digestive organs attaining final shape and body position Blood delivery to/from placenta constant & efficient Heart and liver bulge on ventral surface © 2013 Pearson Education, Inc.

Figure 28.12a Neurulation and early mesodermal differentiation.
Head Amnion Amniotic cavity Left Right Neural plate Cut edge of amnion Primitive streak Ectoderm 17 days. The flat three-layered embryo has completed gastrulation. Notochord and neural plate are present. Tail Mesoderm Notochord Endoderm Yolk sac © 2013 Pearson Education, Inc.

Figure 28.12b Neurulation and early mesodermal differentiation.
Neural groove Somite Neural fold Neural crest Intermediate mesoderm 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). Lateral plate mesoderm Coelom © 2013 Pearson Education, Inc.

Figure 28.12c Neurulation and early mesodermal differentiation.
Surface ectoderm Neural crest 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. Neural tube Somite Notochord © 2013 Pearson Education, Inc.

Figure 28.12d Neurulation and early mesodermal differentiation.
Neural tube (ectoderm) Dermatome Myotome Sclerotome Epidermis (ectoderm) Somite Gut lining (endoderm) 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. Kidney and gonads (intermediate mesoderm) Lateral plate mesoderm Limb bud Smooth muscle of gut Visceral serosa Peritoneal cavity (coelom) Parietal serosa Dermis © 2013 Pearson Education, Inc.

Figure 28.13 Flowchart showing major derivatives of the embryonic germ layers.
Epiblast ECTODERM MESODERM ENDODERM Notochord Somite Intermediate mesoderm Lateral plate mesoderm Somatic mesoderm Splanchnic mesoderm • Epidermis, hair, nails, glands of skin Nucleus pulposus of intervertebral discs • Sclerotome: vertebrae and ribs • Kidneys • Parietal serosa • Wall of digestive and respiratory tracts (except epithelial lining) Epithelial lining and glands of digestive and respiratory tracts • Gonads • Dermis of ventral body region • Brain and spinal cord • Dermatome: dermis of dorsal body region • Connective tissues of limbs (bones, joints, and ligaments) • Neural crest and derivatives (e.g., cranial, spinal, and sympathetic ganglia and associated nerves; chromaffin cells of the adrenal medulla; pigment cells of the skin) • Visceral serosa • Myotome: trunk and limb musculature • Heart • Blood vessels © 2013 Pearson Education, Inc.

Development of Fetal Circulation
First blood cells arise in yolk sac By end of third week Embryo has system of paired vessels Two vessels forming heart have fused; bent into "S" shape Heart beats by 3½ weeks © 2013 Pearson Education, Inc.

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) © 2013 Pearson Education, Inc.

Figure 28.14a Circulation in fetus and 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 Common iliac artery Umbilical arteries Medial umbilical ligaments Urinary bladder Umbilical cord Placenta High oxygenation Moderate oxygenation Low oxygenation © 2013 Pearson Education, Inc. Very low oxygenation

Figure 28.14b Circulation in fetus and newborn.
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 Common iliac artery Umbilical arteries Medial umbilical ligaments Urinary bladder High oxygenation Moderate oxygenation Low oxygenation Very low oxygenation © 2013 Pearson Education, Inc.

Events of Fetal Development
Fetal period - weeks 9 through 38 Time of rapid growth of body structures established in embryo © 2013 Pearson Education, Inc.

Figure 28.15a Photographs of a developing fetus.
Amniotic sac Umbilical cord Umbilical vein Chorionic villi Yolk sac Cut edge of chorion Embryo at week 7, about 17 mm long. © 2013 Pearson Education, Inc.

Figure 28.15b Photographs of a developing fetus.
Fetus in month 3, about 6 cm long. © 2013 Pearson Education, Inc.

Figure 28.15c Photographs of a developing fetus.
Fetus late in month 5, about 19 cm long. © 2013 Pearson Education, Inc.

Table 28.1 Developmental Events of the Fetal Period (1 of 3)
© 2013 Pearson Education, Inc.

Effects of Pregnancy on the Mother: Anatomical Changes
Reproductive organs become engorged with blood Chadwick's sign - vagina develops purplish hue Breasts enlarge and areolae darken Pigmentation of facial skin many increase (chloasma) © 2013 Pearson Education, Inc.

Effects of Pregnancy: Anatomical Changes
Uterus expands, occupying most of abdominal cavity Ribs flare  thorax widens Lordosis occurs with change in center of gravity Relaxin causes pelvic ligaments and pubic symphysis to relax to ease birth passage Weight gain of ~13 kg (28 lb) © 2013 Pearson Education, Inc.

Effects of Pregnancy: Anatomical Changes
Good nutrition vital 300 additional daily calories Multivitamins with folic acid reduce fetal risk of neurological problems, e.g., spina bifida, anencephaly, and spontaneous preterm birth © 2013 Pearson Education, Inc.

(Uterus the size of a fist and resides in the pelvis.) 4 months
Figure Relative size of the uterus before conception and during pregnancy. Before conception (Uterus the size of a fist and resides in the pelvis.) 4 months (Fundus of the uterus is halfway between the pubic symphysis and the umbilicus.) 7 months (Fundus is well above the umbilicus.) 9 months (Fundus reaches the xiphoid process.) © 2013 Pearson Education, Inc.

Effects of Pregnancy: Metabolic Changes
Placental hormones Human placental lactogen (hPL) (human chorionic somatomammotropin (hCS))  maturation of breasts, fetal growth, and glucose sparing in mother (reserving glucose for fetus) Parathyroid hormone and vitamin D levels high throughout pregnancy  adequate calcium for fetal bone mineralization © 2013 Pearson Education, Inc.

Effects of Pregnancy: Physiological Changes
GI tract Morning sickness believed due to elevated levels of hCG, estrogen and progesterone Heartburn and constipation are common Urinary system  Urine production due to  maternal metabolism and fetal wastes Frequent, urgent urination; stress incontinence may occur as bladder compressed © 2013 Pearson Education, Inc.

Cardiovascular system Blood volume increases 25–40% Safeguards against blood loss during childbirth Cardiac output rises as much as 35-40% Propels greater volume around body Venous return from lower limbs may be impaired, resulting in varicose veins © 2013 Pearson Education, Inc.

Homeostatic Imbalance
Preeclampsia Insufficient placental blood supply  fetus starved of oxygen Woman  edematous, hypertensive, proteinuria May be due to immunological abnormalities Correlated with number of fetal cells that enter maternal circulation © 2013 Pearson Education, Inc.

Parturition Giving birth to baby Labor
Events that expel infant from uterus Increased production of surfactant protein A (SP-A) in weeks before delivery may trigger inflammatory response in cervix  softening in preparation for labor © 2013 Pearson Education, Inc.

Fetus determines own birth date During last few weeks of pregnancy
Initiation of Labor Fetus determines own birth date During last few weeks of pregnancy Fetal secretion of cortisol stimulates placenta to secrete more estrogen Causes production of oxytocin receptors by myometrium Causes formation of gap junctions between uterine smooth muscle cells Antagonizes calming effects of progesterone, leading to Braxton Hicks contractions in uterus © 2013 Pearson Education, Inc.

Fetal oxytocin causes placenta to produce prostaglandins
Initiation of Labor Fetal oxytocin causes placenta to produce prostaglandins Oxytocin and prostaglandins - powerful uterine muscle stimulants Due especially to prostaglandins, contractions  more frequent and vigorous Anti-prostaglandins contraindicated during labor © 2013 Pearson Education, Inc.

Increasing cervical distension
Initiation of Labor Increasing cervical distension Activates hypothalamus, causing oxytocin release from posterior pituitary Positive feedback mechanism occurs Greater distension of cervix  more oxytocin release  greater contractile force  greater distension of cervix  etc. © 2013 Pearson Education, Inc.

Figure 28.17 Hormonal induction of labor.
Start Estrogen Oxytocin (+) from placenta from fetus and mother's posterior pituitary Induces oxytocin receptors on uterus Positive feedback Stimulates uterus to contract Stimulates placenta to release (+) Prostaglandins Stimulate more vigorous contractions of uterus © 2013 Pearson Education, Inc.

Stages of Labor: Dilation Stage
From labor's onset to fully dilated cervix (10 cm) 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 true pelvis © 2013 Pearson Education, Inc.

Umbilical cord Placenta Uterus Cervix Vagina Sacrum Perineum Uterus
Figure Parturition. Umbilical cord Slide 1 1a Placenta Early dilation. Baby’s head engaged; widest dimension Is along left-right axis. Uterus Cervix Vagina 1b Late dilation. Baby’s head rotates so widest dimension is in anteroposterior axis (of pelvic outlet). Dilation nearly complete Pubic symphysis Sacrum 2 Expulsion. Baby’s head extends as it is delivered Perineum 3 Placental stage. After baby is delivered, the placenta detaches and is removed. Uterus Placenta (detaching) Umbilical cord © 2013 Pearson Education, Inc.

Stages of Labor: Expulsion Stage
From full dilation to delivery of infant Strong contractions every 2–3 minutes, about 1 minute long Urge to push increases (in absence of local anesthesia) Crowning occurs when largest dimension of head distends vulva Episiotomy may be done to reduce tearing © 2013 Pearson Education, Inc.

Stages of Labor: Expulsion Stage
Vertex position – head-first Skull dilates cervix; early suctioning allows breathing prior to complete delivery Breech position – buttock-first Delivery more difficult; often forceps required, or C-section (delivery through abdominal and uterine wall incision) © 2013 Pearson Education, Inc.

Stages of Labor: Placental Stage
Strong contractions continue, causing detachment of placenta and compression of uterine blood vessels Limit bleeding; cause placental detachment Delivery of afterbirth (placenta and membranes) occurs ~30 minutes after birth All placenta fragments must be removed to prevent postpartum bleeding © 2013 Pearson Education, Inc.

Adjustments of the Infant to Extrauterine Life
Neonatal period - four-week period immediately after birth Physical status assessed 1–5 minutes after birth Apgar score - 0–2 points each for Heart rate • Muscle tone Respiration • Reflexes Color Score of 8–10 - healthy © 2013 Pearson Education, Inc.

Respiratory rate ~45 per minute first two weeks, then declines
First Breath  CO2  central acidosis  stimulates respiratory control centers to trigger first inspiration Requires tremendous effort – airways tiny; lungs collapsed Surfactant in alveolar fluid helps reduce surface tension Respiratory rate ~45 per minute first two weeks, then declines © 2013 Pearson Education, Inc.

First Breath Keeping lungs inflated difficult for premature infant (< 2500 g, or 5.5 pounds, at birth) Surfactant production in last months of prenatal life Preemies usually on respiratory assistance until lungs mature © 2013 Pearson Education, Inc.

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 Baby gags frequently as regurgitates mucus and debris Stabilizes with waking periods occurring every 3–4 hours © 2013 Pearson Education, Inc.

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 © 2013 Pearson Education, Inc.

Occlusion of Fetal Blood Vessels
Umbilical vein becomes round ligament of liver (ligamentum teres) Ductus venosus  ligamentum venosum about 30 minutes after birth Pressure changes from infant breathing cause pulmonary shunts to close Foramen ovale  fossa ovalis up to a year after birth Ductus arteriosus  ligamentum arteriosum about 30 minutes after birth © 2013 Pearson Education, Inc.

Production of milk by mammary glands Toward end of pregnancy
Lactation Production of milk by mammary glands Toward end of pregnancy Placental estrogens, progesterone, and human placental lactogen stimulate hypothalamus to release prolactin-releasing factors (PRFs)  Anterior pituitary releases prolactin 2-3 days later true milk production begins © 2013 Pearson Education, Inc.

Lactation Colostrum Less lactose but more protein, vitamin A, minerals than true milk; almost no fat Yellowish secretion rich in IgA antibodies IgA resistant to digestion; may protect infant against bacterial infection; absorbed into bloodstream for immunity Released first 2–3 days Followed by true milk production © 2013 Pearson Education, Inc.

Prolactin release wanes after birth
Lactation Prolactin release wanes after birth Lactation sustained by mechanical stimulation of nipples - suckling Suckling causes afferent impulses to hypothalamus  prolactin  stimulates milk production for next feeding Hypothalamus also  oxytocin from posterior pituitary  let-down reflex © 2013 Pearson Education, Inc.

Hypothalamus releases prolactin releasing factors (PRF)
Figure Milk production and the positive feedback mechanism of the milk let-down reflex. 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 mammary glands of breasts. Positive feedback Milk production Let-down reflex. Milk is ejected through ducts of nipples. © 2013 Pearson Education, Inc.

Advantages of Breast Milk
Fats and iron better absorbed; amino acids more easily metabolized, compared with cow's milk Beneficial chemicals IgA, complement, lysozyme, interferon, and lactoperoxidase (protect from infections) Interleukins and prostaglandins prevent overzealous inflammatory responses Glycoprotein deters ulcer-causing bacterium from attaching to stomach mucosa © 2013 Pearson Education, Inc.

Advantages of Breast Milk
Natural laxative effect helps eliminate bile-rich meconium, helping to prevent physiological jaundice Encourages bacterial colonization of large intestine Women nursing 6 months lose bone calcium; replaced after weaning if healthy diet Women may ovulate when nursing despite inhibition of GnRH and gonadotropins © 2013 Pearson Education, Inc.

Assisted Reproductive Technology
Surgical removal of oocytes following hormone stimulation Fertilization of oocytes Return of fertilized oocytes to woman's body Disadvantages Costly, emotionally draining, painful for oocyte donor © 2013 Pearson Education, Inc.

In vitro fertilization (IVF) Oocytes and sperm incubated in culture dishes for several days Embryos (two-cell to blastocyst stage) transferred to uterus for possible implantation © 2013 Pearson Education, Inc.

Zygote intrafallopian transfer (ZIFT) Fertilized oocytes transferred to uterine tubes Gamete intrafallopian transfer (GIFT) Sperm and harvested oocytes are transferred together into the uterine tubes Cloning Legal, moral, ethical, political roadblocks © 2013 Pearson Education, Inc.

Pregnancy and Human Development

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