Growth and Development 2: Placentation and Hormones

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How does the placenta form?

The placenta forms from the embryonic trophoblastic/mesoderm tissues, as well as the maternal endometrial tissues. Vessels develop in the embryonic tissues which then supply nutrients to the foetus from the maternal blood supply.

The chorion develops fingerlike villi which have become vascularised and extend to the embryo as umbilical arteries and veins. The embryo, amnion and yolk sac float in a fluid-filled chamber attached to the placenta by a body stalk.

The placenta grows and begins to develop into organised regions, bulging into the uterine cavity as it grows. It becomes fully developed by the end of the third month (end of first trimester), where the foetus moves farther from the placenta connected by the umbilical cord. By 12 weeks, the placenta is providing nutrients and removing wastes, meaning it takes on the role of oestrogen and progesterone which used to be released from the corpus luteum.
  • Decidua capsularis: endometrium surrounding the uterine cavity face of the implanted embryo
  • Decidua basalis: endometrium lying between the chronic villi and the stratum basalis
  • Decidua parietalis: remaining uterine endometrium not in contact with the chorion
NOTE: the placenta slowly increases the amount of oestrogen produced which aids in labour

Hormonal involvement in pregnancy

Human chorionic gonadotrophin (hCG)
  • From trophoblasts
  • Corpus luteum to secrete progesterone and oestrogen until placenta secretes these itself (prevents menses)
Human placental lactogen (hPL)
  • Prepares mammary glands for milk production
  • Assists hormones: placental prolactin, oestrogen, progesterone
  • Functions as a growth hormone for maternal tissues, thereby ensuring nutrients are supplied to the foetus
Relaxin
  • Increases flexibility of pubic symphysis for expansion
  • Causes cervical dilation during delivery
  • Inhibits uterine contractions by delaying oxytocin release

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