Overview

 

Reproductive systems of both males and females appear to remain dormant during childhood. However, the hypothalamic-pituitary unit is at least partially functional as evidenced by the fact that gonadectomy which drastically lowers steroid hormone levels in plasma results in an increase in plasma gonadotropins due to the release of the hypothalamic pituitary unit from the negative feedback effects of gonadol steroids.

Administration of small amounts of steroids further suppresses the already low levels of gonadotropin seen during childhood.

 

Gonadotropins

 

FSH and LH in the fetal circulation of both males and females increase markedly around the middle of gestation and then decline. FSH and LH are secreted by the fetal pituitary in response to GnRH secreted by the fetal hypothalamus. The bell-shaped time course of gonadotropin secretion during gestation reflects the activity of the fetal hypothalamic pituitary unit before the hypothalamic pituitary unit become sensitive to the negative feedback effects of sex steroids.

Gonadotropin levels in infantile males and females are elevated by the sudden absence of the inhibitory effect of placental steroids. During childhood, gonadotropin levels remain low but the pituitary is responsive to exogenous GnRH. Prepubertal males and females secrete gonadotropins in response to GnRH although females are more sensitive. Males and females at about age 10 begin to undergo puberty the transition from juvenile to adult.

Puberty usually lasts 2 to 4 years.

 

Sex Hormones

 

During puberty sex hormones cause transformation of the sexually immature child into a sexually mature adult. Puberty is characterized by a growth spurt, maturation of sexual organs, development of secondary sexual characteristics, and psychological changes. At puberty the hypothalamic pituitary unit begins to function like that of an adult. The hypothalamus begins to secrete increased amounts of gonadotropin-releasing hormone or GnRH in a pulsatile manner and the pituitary responds by secreting increased amounts of gonadotropins, FSH and LH in a pulsatile manner.

The gonads respond to gonadotropins by secreting increased amounts of sex steroids estradiol and progesterone in the female, testosterone in the male. The initial response of the gonads is termed gonadarche. However the 1st recognizable hormonal change is an increase in adrenal androgen production termed adrenarche. Transformation from an immature to a mature state results from maturation of the neural mechanisms in the brain which regulate the pulsatile secretion of gonadotropin releasing hormone by the hypothalamus.

The neural mechanism is termed the GnRH pulse generator. The signal or signals responsible for maturation of GnRH pulse generator have not been identified.