In human males, after birth, the HPG axis is active until approximately 6 months of life, with gonadotropin and sex steroids concentrations peaking between 4 and 12 weeks of life. After this period, GnRH pulsatility declines and the HPG axis becomes quiescent throughout childhood.
Onset of puberty is characterized by pulsatile GnRH release from the hypothalamus that stimulates pituitary LH secretion, which, in turn, drives testosterone production by testicular Leydig cells. GnRH also increases FSH secretion, which promotes maturation of the seminiferous tubules and spermatogonia (a type of developing sperm).
In human females, during childhood the ovarian cycle, controlled by HPG axis, is quiescent. A growth spurt occurs at 12 years of age, and subsequently, the first menstrual cycle occurs (menarche). Menarche, induced by follicle growth and estradiol production, occurs with the initial shedding of the endometrium with subsequent bleeding. Female puberty ends when women ovulate and repeated ovulatory cycles ensure reproductive competence.
Low GnRH secretion is characterized by delayed or absent sexual development and infertility associated with inappropriately low gonadotropins (LH and FSH) and sex steroids (testosterone or estradiol). Low gonadotropins are not sufficient to trigger the production of sperms, or ovulation. Thus without reproductive cells, the pregnancy is not possible.