ch 28

• Fertilization

  The process in sexual reproduction that involves the union of a sperm and an egg to form a diploid zygote
• Fertilization time window

  • Oocyte is only viable for 24 hours (at most) after ovulation
  • Sperm are most viable 24-48 hours in the female reproductive tract
• Fertilization

  Intercourse must occur no more than 2 days before ovulation and no more than 1 day after ovulation
• Sperm reaching the uterus

  • Millions lost immediately by leaking from vagina
  • Millions more lost by acidic environment of vagina
  • Phagocytes kill off many more
  • Only a few thousand "survivors" reverse peristaltic waves push these sperm toward the uterine tubes

  Sperm have various receptors (chemical, temperature, fluid flow) to ensure they travel in ”correct” direction
• Sperm


  • Have various receptors (chemical, temperature, fluid flow) to ensure they travel in "correct" direction
• Sperm Capacitation

  •Problem: Sperm are incapable of fertilizing the oocyte immediately after entering vagina

  
  

  •Before fertilization can occur sperm must be “capacitated”

  1. Motility is enhanced

  2. Weakening of membranes allows easier release of enzymes that help with penetrating the egg
• Oocyte protective structures

  • Zona pellucida: innermost layer, produced by primary oocyte and follicle, protects secondary oocyte, is necessary for fertilization to occur, but prevents multiple sperm from fertilizing single egg
  • Corona radiata: outer layer, contains cells that allows communication between cells and oocyte, protects egg after it has been ovulated
• Fertilization

  1. Sperm burrow through cells of corona radiata

  2. Acrosomal reaction: release of enzymes from the acrosome of the sperm

  Sperm bind zona pellucida

  Ca2+ levels in sperm rise

  Acrosomal enzymes released digest holes through zona pellucida to the plasma membrane of the oocyte

  2. Sperm plasma membrane binds to sperm-binding receptors on plasma membrane of the oocyte

  Sperm contents enter the oocyte with the two membranes fused
• Polyspermy

  The entry of more than one sperm cell into the oocyte results in incorrect chromosomal number, resulting embryo cannot mitotically divide and dies
• Blocking polyspermy

  1. Oocyte membrane blocks: sperm-binding receptors are shed from oocyte surface, sperm unable to bind oocyte surface and fertilize eggs with these receptors

  2. Cortical reaction: endoplasmic reticulum of oocyte releases Ca2+ in response to binding of single sperm cell, granules release enzymes to destroy binding receptors and causes the zona pellucida to harden
• Completion of Meiosis II & Fertilization

  1. Sperm nucleus travels to the oocyte nucleus, swells in size to form male pronucleus

  2. Surge in Ca2+ from cortical reaction causes release of zinc from oocyte, Meiosis II is completed as zinc leaves cell, forms 2nd polar body & mature ovum

  3. After meiosis II, female pronucleus forms

  4. Pronuclei membranes rupture, chromosomes are released together, Maternal and paternal chromosomes combine to form a diploid zygote
• Cleavage


  Rapid mitotic divisions of the zygote
• ~36 hours after fertilization, zygote has completed first division, contains 2 identical blastomere cells
• By 72 hours after fertilization, morula is formed, contains ~16 cells
• Benefit of cleavage for embryonic development

  Allows rapid division of the zygote into many cells
• •Morula continues to divide forms blastocyst with 100+ cells

  •Composed of two layers:

  
  

  •1) External trophoblast: single layer of flattened cells

  •Aids in embryo implantation, contributes to chorion formation/function, has immunosuppressive effects

  
  

  •2) Internal embryoblast (inner cell mass): cluster of 20-30 rounded cells

  •Will eventually form the embryo proper and extraembryonic membranes
• Implantation of blastocyst in the endometrium

  Endometrium is receptive to implanting embryo due to high estrogen and progesterone levels, cell adhesion molecules on endometrium bind the trophoblast
• Implantation usually occurs high in the uterus
• Benefit of implantation high in the uterus

  Allows the growing embryo more space to develop
• Implantation

  1. Once bound, trophoblast releases digestive enzymes & growth factors on epithelium of endometrium

  2. Growth factor influence: Endometrium thickens as blood vessels enlarge & become leaky, Trophoblast proliferates, forms 2 layers: 1) Cytotrophoblast: innermost layer, mass of cells that anchors growing embryo to uterine tissue & release enzymes to facilitate implantation 2) Syncytiotrophoblast: outermost layer, mass of fused cells that produce & release enzymes to facilitate implantation

  3. Digestive enzyme influence: Erosion of endometrium around blastocyst, Blastocyst buries itself in the blood-rich lining, Surrounding endometrial cells proliferate & cover the blastocyst
• hCG

  Human chorionic gonadotropin, a hormone released by the embryo that prevents breakdown of the corpus luteum & suppresses female immune system, drops at ~4 months, corpus luteum breaks down
• Placenta


  • A temporary organ originating from embryonic and maternal tissues, functions: maintain pregnancy, exchange respiratory gases, provide nutrients to embryo/fetus, dispose of waste
  • Embryonic contribution: Formation of the chorion membrane that surrounds embryo/fetus, chorion has chorionic villi that contact maternal blood, blood vessels extend from chorionic villi form umbilical vein and arteries
  • Maternal contribution: Functional layer of endometrium develops blood-filled lacunae, chorionic villi immersed in lacunae, endometrium that lies underneath embryo becomes decidua basalis, endometrium that surrounds uterine cavity face of embryo forms decidua capsularis
• •Placenta is fully functional organ by the 4th month of pregnancy

  •Will allow oxygen, nutrient, and waste exchange before this time

  Maternal and fetal blood DO NOT MIX in the placenta
• Extraembryonic membranes

  • Amnion: thin, transparent layer that extends completely around the embryo, filled with amniotic fluid, functions: provides buoyancy & protection, maintains consistent temperature, prevents developing parts of embryo from sticking together/fusing, allows movement
  • Yolk sac: sac structure that hangs from ventral surface of embryo, functions: eventually forms digestive tube, forms earliest blood cells, primordial germ cells move from here to gonads
  • Allantois: structural basis for the umbilical cord, will eventually form part of the urinary bladder
  • Chorion: embryonic contribution to the placenta, contains chorionic villi that contact maternal blood & allow exchange of gases, nutrients, & wastes, encloses all other extraembryonic membranes
• Gastrulation


  Cellular rearrangements and migrations of cells from 2-layered embryonic disc into a 3-layered embryo
• Primary germ layers

  • Endoderm
  • Ectoderm
  • Mesoderm
• Gastrulation


  1. Begins with formation of primitive streak in the embryonic disc

  •is a groove with raised edges that will form the long axis of the embryo

  2. Epiblast cells migrate & enter the primitive streak, form the endoderm (inferior germ layer)

  3. Other cells follow, push between cells of upper and lower layers, form the mesoderm (middle layer)

  Mesodermal cells immediately beneath primitive streak form the notochord

  •The notochord is the first axial support of the embryo

  4. Cells remaining at dorsal surface form the ectoderm
• Notochord

  The first axial support of the embryo, formed from mesodermal cells immediately beneath primitive streak
• Organogenesis


  Formation of body organs and organ systems, begins with gastrulation, all organ systems are recognizable by week 8
• Organogenesis

  1. Early in process, embryo looks like flat, stacked plates, these plates will then begin to fold laterally to form a tube, and will also fold at the head and tail end

  2. Endoderm edges come together & fuse, enclosing part of the yolk sac
• Specialization of the Primary Germ Layers

  • Endoderm: Folding process forms the primitive gut, endoderm becomes epithelial lining of GI tract, respiratory tract forms from outpocketing of foregut, other glands formed from endothelial outpocketings along the tract, oral and anal ends of the gut will open
  • Ectoderm: Neurulation, formation of brain and spinal cord, induced by chemicals released by notochord, results in formation of a neural tube sitting over the notochord, anterior portion becomes brain, remaining becomes spinal cord, by week 4 forebrain, midbrain, & hindbrain evident, by week 8 cerebral hemispheres evident, brain waves can be recorded, remaining ectoderm forms epidermis
  • Mesoderm: Forms mesodermal blocks called somites that hug the notochord on either side
• Organogenesis (continued)

  1. Endoderm edges come together and fuse, enclosing part of the yolk sac

  2. Folding process forms the primitive gut, endoderm becomes epithelial lining of GI tract

  3. Respiratory tract forms from outpocketing of foregut

  4. Other glands formed from endothelial outpocketings along the tract

  5. Oral and anal ends of the gut will open
• Organogenesis: Specialization of the Primary Germ Layers (continued)

  1. Neurulation: formation of brain and spinal cord

  2. Induced by chemicals released by notochord

  3. Results in formation of a neural tube sitting over the notochord

  4. Anterior portion becomes brain, remaining becomes spinal cord

  5. By week 4 forebrain, midbrain, & hindbrain evident

  6. By week 8 cerebral hemispheres evident, brain waves can be recorded

  7. Remaining ectoderm forms epidermis
• Organogenesis: Specialization of the Primary Germ Layers (continued)

  1. Mesoderm forms mesodermal blocks called somites that hug the notochord on either side

  2. 3 functional parts of somites: Sclerotome produces vertebrae and rib at each associated levels, Dermatome forms dermis in dorsal part of body, Myotome forms skeletal muscle of neck, body trunk, and limbs

  3. Remaining mesoderm forms kidneys & gonads, connective tissues of limbs, heart & blood vessels, dermis of ventral body region, etc. etc. etc.

  4. If it's not formed by the endoderm or ectoderm, it is formed by the mesoderm
• Fetal circulation and adult circulation


  Not identical
• Fetal circulation: Modifications Seen Only During Development


  1. Umbilical arteries and vein: Although blood is transported via these structures, gas exchange (O2 and CO2), waste elimination, & various other processes occur in placenta

  2. Vascular shunts: redistributes blood to parts of body that need it most, (mostly) bypasses organs not yet used by developing fetus

  3. Ductus venosus: shunt that bypasses the liver

  4. Foramen ovale: shunts blood from right atrium to left atrium, causes blood to bypass lungs

  5. Ductus arteriosus: shunts blood from pulmonary trunk to aorta, causes blood to bypass lungs
• Fetal Circulation: Modifications Seen Only During Development (continued)

  1. All modifications are closed after birth

  2. Umbilical blood vessels constrict & fibrose

  3. Umbilical artery gives rise to: A) one portion of artery supplies the bladder, B) one portion of artery also becomes median umbilical ligaments

  4. Umbilical vein remnant become round ligament of liver

  5. Ductus venosus collapses & is converted to ligamentum venosum

  6. Foramen ovale closes as pulmonary circulation becomes functional, edges fuse to septal wall of heart forms the fossa ovalis

  7. Ductus arteriosus constricts & closes off, forms the ligamentum arteriosum
• Anatomical changes during pregnancy

  • Uterine enlargement: At beginning of pregnancy uterus is about the size of a pear, At end of pregnancy top of uterus is even with xiphoid process
  • Uterus (& fetus) push all other organs up & out
  • Pelvis widens from release of relaxin
  • Weight gain: ~28 lbs, depends on weight at beginning of pregnancy, ~300 extra calories/day necessary to maintain healthy pregnancy
• Metabolic Changes during pregnancy

  • Placental growth hormone: replaces growth hormone in pregnant women, stimulates lipolysis and glucose production for growing fetus
  • Human placental lactogen (hPL): stimulates maturation of breasts for lactation, promotes fetal growth, and is glucose-sparing
  • Corticotropin-releasing hormone (CRH): rises only toward end of pregnancy, as CRH rises, parent's ACTH and cortisol levels rise, helps during birth, Cortisol leads to maturation of fetal organs especially the lungs
• Physiological changes during gestation by system

  • GI System: Surge in hCG, estrogen, and progesterone can result in nausea/vomiting, "Morning" sickness, Uterus invading abdomen, pressing on other organs causes heartburn, constipation
  • Urinary System: Increased metabolic rate, higher blood volume, & need to dispose of extra waste from fetus = more urine produced, Uterus pushes on bladder increases frequency of urination
  • Respiratory System: Respiratory rate is unaffected, Increase in tidal volume, decrease in residual volume, Uterus pushes on diaphragm can make breathing difficult by end of pregnancy
  • Cardiovascular System: Total blood volume increases by at least 25%, may be as high as 40% by end of 32nd week, Blood pressure remains the same!!, Cardiac output increases 35-40%, Uterus pushes on blood vessels as it grows blocks blood flow, forming varicose veins and edema (especially in the legs)