One system within the endocrine system, known as the male HPG axis, is responsible for a variety of different physiological functions such as spermatogenesis and the development of sex organs. HPG axis stands for hypothalamus-pituitary-gonad axis and is characterized by neurons in the hypothalamus region of the brain releasing hormones that signal receptors in the anterior pituitary gland. The pituitary gland then releases its own hormones that signal receptors in the gonads and leydig cells, initiating its physiological functions. The hypothalamus releases a hormone called GnRH, or gonadotropin releasing hormone. Hormones released from the hypothalamus set for the pituitary travel through a specialized pathways called the portal system. Once the GnRH has bound itself to receptors in the pituitary, two more hormones are released from the gland: LH and FSH. LH, or luteinizing hormone, will leave the pituitary and enter the bloodstream, traveling to leydig cells. This will stimulate the leydig cells to release testosterone, initiating spermatogenesis and reducing the secretion of LH from the pituitary. FSH, or follicle stimulating hormone, however, takes on a different role. FSH goes straight to the testis to upregulate the production of sperm.
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