The hypothalamic–pituitary–gonadal (hPg) axis

The hypothalamic–pituitary–gonadal axis regulates reproduction. In primates, neurons scattered all through the hypothalamus synthesize a decapeptide, gonadotrophin-releasing hormone (GnRH, as well known as luteinizing hormone discharging hormone, LHRH) from a large predecessor. GnRH is secreted from axon terminals in the median eminence into the portal system. The GnRH stimulates gonadotrophs of the anterior pituitary to secrete two gonadotrophins, follicle stimulating hormone (FSH) & luteinizing hormone (LH).

The gonadotrophins are big glycoproteins each consist of two peptides, α chain and a β chain. The α chains of FSH and LH are alike (and very identical to the α chain of TSH) though the β chains are different. Gonadotrophins stimulate the gonads to generate sex steroids and have effect on gamete development. The Gonadotrophin secretion is cyclical in females although not in males. The emission of gonadotrophins is pulsatile, as with new anterior lobe hormones, and is drive by bursts of GnRH from the hypothalamus. GnRH neurons have an inherent rhythmicity though this is altered by steroid hormones. In men, the pulses are regular, spaced around 3 hours apart, though in women the period varies between 1 to 12 hours based on the phase of her reproductive cycle. Experimentally substituting pulsatile with continuous GnRH delivery in female rhesus monkeys abolishes gonadotrophin secretion, presenting that pulsatile GnRH output is necessary for accurate HPG axis function.