8

• Steps of fertilization

  1. Chemoattraction of the sperm to the ovum

  2. Adherence of the sperm to the zona pellucida

  3. Penetration of the zona pellucida by the sperm and the acrosomal reaction

  4. Adherence of the sperm head to the cell membrane of the ovum and release of the sperm nucleus into the cytoplasm of the ovum

  
• Zona pellucida

  A membrane surrounding the ovum
• Usually, one sperm fuses to the membrane of the ovum</b>

  
• Fertilin


  A sperm head protein that resembles viral fusion protein and mediates the fusion

  
• Fusion function


  • Provide signals that initiate development (zygote cell division and Blastocyst formation)
  • Prevent polyspermy

  
• Polyspermy

  Fertilization of ovum by more than one sperm
• Onset of polyspermy


  • Initially (transient) by reduction of ovum membrane potential
  • Permanently (long-term) by structural changes in the zona pellucida

  
• Blastocyst

  New developing embryo that forms in the fallopian tube, moves down to the uterus in ~3 days, and implants in the endometrium

  
• Blastocyst during implantation


  • Surrounded by syncytiotrophoblast (many cells forming continuous mass)
  • Surrounded by cytotrophoblast (individual cells)
• Human Chorionic Gonadotropin (hCG)


  A glycoprotein made up of α and β subunits, produced by the syncytiotrophoblast, with LH activity and little FSH activity
• hCG can be detected in the blood as early as 6 days after conception and in the urine as early as 14 days after conception

  
• Human Chorionic Somatomammotropin (hCS)


  A hormone very similar to hCG, produced by the syncytiotrophoblast, with lactogenic and small growth-stimulating activity
• Large quantities of hCS are found in maternal blood, but very little reaches the fetus
• Low hCS levels are a sign of placental insufficiency
• The secretion of GH from the maternal pituitary is not increased during pregnancy and may actually be decreased by hCS

  
• hCS brings about nitrogen, potassium, and calcium retention, lipolysis, and decreased glucose utilization to divert glucose to the fetus

  
• Normal human pregnancy duration is 270 days (9 months, 36 weeks), or 284 days if we add 14 days pre-ovulation period
• Shortly before delivery, the uterus shows frequently increasing irregular uterine contractions, peaked in the last month of pregnancy
• At the time of delivery, the cervix softens and dilates, and the body of the uterus contracts to expel the fetus

  
• Onset of labor

  1. Increase in circulating estrogen

  2. Fetal cortisol that hastens respiratory system maturation

  
• The fetus picks the time of birth by increasing CRH secretion by its hypothalamus and the placenta, which increases ACTH and adrenal cortisol secretion

  
• The number of oxytocin receptors in the myometrium and decidua increases >100-fold during pregnancy, reaching a peak during early labor

  
• Action of oxytocin on the uterus

  1. Directly contracts uterine smooth muscles

  2. Stimulates prostaglandin formation in the decidua

  
• Reflex voluntary abdominal muscle contractions also aid in delivery

  
• Development of the breast (lobuloalveolar development)

  1. Estrogen: development of mammary ducts

  2. Progesterone: development of lobules

  3. Prolactin: Stimulate milk formation and secretion into the ducts

  
• Human milk contains water, lactose, protein (casein), fatty acids, minerals, iron, and vitamins A, D, B1, B2, B7, and C
• During lactation, estrogen, progesterone, and prolactin levels all increase in the plasma
• Milk ejection

  Oxytocin causes contraction of myoepithelial cells and milk ejection

  
• After expulsion of the placenta, the abrupt decline in estrogen and progesterone levels initiates lactation

  
• Estrogen antagonizes prolactin action on the breast, and can be administered to stop lactation

  
• Milk secretion begins in the 5th month of pregnancy and takes 1-3 days after delivery to start in women (within an hour in most animals)

  
• Effect of suckling


  1. Evokes reflex release of oxytocin and milk ejection

  2. Stimulates prolactin secretion and maintains/augments milk secretion