Formation of placenta

1
Formation of placenta

• before implantation, conceptus receives nutrition
  from endometrial fluids
• as conceptus implants (days 7-14), trophoblasts
  form placenta
• functions of placenta
• allows implantation
• delivers important substances
• eliminates wastes
• secretes hormones (hCG)
• summary placenta is lungs, digestive system,
  kidneys for the fetus

2
Placenta

• trophoblasts differentiate into 2 layers
  (cytotrophoblasts, syncytiotrophoblasts)
• syn. secrete proteases gt break down endo.
• trophoblasts form villi
• villi secrete enzymes that dissolve walls of
  uterine blood vessels
• result blood-filled intervillus spaces (lacunae)
• maternal blood bathes villi
• mesoderm extends into villi gt forms embryonic
  circulatory portion of placenta
• maternal blood never mixes with fetal blood
• bl. vessels in villi ultimately join umbilical
  arteries, vein present in umbilical cord

3
Placenta

• by day 14, hCG is produced gt rescues corpus
  luteum
• hCG maintains CL in species where length of
  gestation gt luteal phase (primates, large farm
  animals)
• in others (e.g. dogs) luteal phase is same
  duration as pregnancy
• others (rodents) gt coitus extends luteal phase

4
Elaboration, contd.

• by end of 2nd month gt placenta is fully
  functional
• rate of growth
• by 4th week, covers 20 of inner walls
• by 20th week, covers half of uterine walls

5
Placenta

• molecules greater than 500 mol. wt. cant pass
  from maternal to fetal circulation
• protective mechanism gt eliminates passage of most
  maternal hormones
• some proteins (e.g. antibodies) can pass by
  active transport
• some steroid hormones pass but are degraded by
  placental enzymes
• some hormones can pass and affect fetus (e.g.
  thyroxine)
• bacteria are typically too large to cross, but
  viruses can pass barrier

6
Pattern of hCG secretion

• in nonpregnant state, no hCG is produced
  (exception tumors)
• by day 14 of pregnancy, hCG is produced by
  placenta
• high hCG maintains corpus luteum until month 4
• then, placenta produces its own estrogen,
  progesterone
• after month 4, hCG levels in maternal circulation
  plummet (as do E, P)
• women often experience relief from morning
  sickness at this point in time