Fertilization and pregnancy:

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Fertilization and pregnancy
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Fertilization
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Blocks to polyspermy

• If more than one sperm were to fertilize the egg,
  then the genetic complement would be 3n.
• In order to prevent multiple sperm penetrations,
  two responses have evolved in the egg.
• First, as soon as the first sperm head penetrates
  the egg, it triggers a massive influx of Na.
• This influx depolarizes the egg, making it
  positive inside. This repels the positively
  charged sperm, inhibiting penetration of more
  sperm.

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• Second, the depolarization triggers an influx of
  Ca 2 . This Ca 2 facilitates the exocytosis of
  a number of secretory vesicles, known as cortical
  vesicles.
• The contents of these vesicles surrounds the egg,
  swells with water and gels, pushing other sperm
  away from the egg and blocking their entry.

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Implantation
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Chorionic villi
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Placental hormones

• During early pregnancy, HCG is secreted by the
  syncitial trophoblasts.
• Later, the placenta secretes estradiol,
  progesterone, relaxin and somatomammotropin.

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Function of placental hormones

• HCG is similar to LH and maintains the corpus
  luteum in a functional state for 3-4 months.
• This keeps progesterone levels high and they
  maintain the functional endometrium.
• Relaxin increases flexibility in the pelvic
  joints, as well as suppressing release of
  oxytocin.
• Placental progesterone keeps the uterine wall
  intact.
• Somatomammotropin acts like prolactin and
  triggers the mammary glands to develop.
• Estrogen increases the sensitivity of the
  myometrium to mechanical irritation, as well as
  oxytocin stimulation.

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Labour

• Towards the end of pregnancy, relaxin secretion
  falls off, thus, the uterus becomes more
  sensitive to oxytocin.
• Initially, the fetus secretes oxytocin into the
  maternal circulation.
• The oxytocin stimulates contractions, which push
  the head down against the cervix.

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• This pressure on the cervix stimulates the
  release of oxytocin from the maternal pituitary
  gland.
• The maternal oxytocin causes more contractions of
  the uterus, forcing the head of the fetus against
  the cervix even harder.
• This is a positive feedback system.

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Labour and delivery

• As the head of the fetus is pressed down against
  the cervix, it thins and then starts to dilate.
• This stage is known as the Dilation Stage and can
  last several hours, or days (usually around 8
  hours).
• Once the cervix has dilated, the fetus starts
  moving through the birth canal. Contractions are
  maximal and come about 2-3 minutes.
• This is known as the Expulsion Stage.
• If the vaginal wall has not stretched enough,
  tearing may occur.

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Labour and delivery cont.

• There is also a chance that the fetus will get
  stuck in the birth canal (usually caused by
  insufficient molding of the head.
• In these cases, a cesarean section is performed.
• Finally, after expulsion of the fetus, the
  placenta detaches from the uterine wall and is
  delivered through the birth canal.
• This is known as the Placental Stage.

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Nursing

• Two hormones are involved, PRL and oxytocin.
• PRL stimulates milk production, while oxytocin is
  required for the expression of milk from the
  breast.

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Fertility issues