Development of the hypothalamic-pituitary-ovarian axis.

The onset of puberty is a centrally driven process, the detailed mechanisms of which are not known. It is translated into an increased activity of the hypothalamic GnRH pulse generator. This in turn is seen as increased pituitary pulsatile secretion of LH and FSH. LH pulses are observed even in midchildhood, particularly after the onset of sleep. Onset of puberty is associated with a greater increase in LH pulse amplitude than frequency and a much greater increase in LH and FSH. A progressive increase in daytime pulsatility occurs, with a gradual reduction of sleep-entrained amplification. Prepubertal FSH concentrations are relatively high in girls, and continous ovarian follicular growth and atresia take place, with estradiol concentrations being higher than in boys. Only after the steep early pubertal increase in LH, ovarian steroidogenesis is activated, with increases in androgen and estrogen secretion. Under further FSH stimulation, follicular growth and maturation proceed. The first menstrual cycles are mostly anovulatory for 1 to 2 years. Luteal phase insufficiency is common the first five years after menarche.