Definition
- Placenta is an endocrine organ, secreting steroid and protein hormones throughout pregnancy.
Classification of Placental Hormones
Hormones secreted from the placenta
Category
Steroid hormones
- Hormones
- Estriol, 17-β estradiol, estrone, progesterone
Protein hormones
- Hormones
- hCG, hCS, relaxin, prolactin, CRH, GnRH, β-endorphin, α-MSH, dynorphin-A, insulin-like growth factors, ACTH
Human Chorionic Gonadotropin (hCG)
- hCG is a glycoprotein hormone secreted by the syncytiotrophoblast, from about the 6th day after fertilization.
- hCG binds LH receptors in the corpus luteum (luteotropic activity), stimulating progesterone production from the corpus luteum.
- Plasma hCG peaks at about 5 mg/mL by 10 to 12 weeks of gestation, then falls to 75% by 25 weeks and stays there till term.
- hCG comprises an α subunit (92 amino acids) and a β subunit (145 amino acids).
- As hCG declines after its peak, the placenta begins secreting large amounts of estrogen and progesterone.
- The α subunit of hCG is structurally similar to the α subunit of LH, FSH and TSH.
- hCG also contributes to placental steroidogenesis and to sexual differentiation in the male fetus.