Pulsatile secretion of luteinizing hormone releasing hormone (LHRH) by the hypothalamus stimulates the production and secretion of the gonadotrophins luteinizing hormone (LH) and follicle-stimulating hormone (FSH) by the pituitary gland. These gonadotrophins circulate in the blood to reach the testis. LH stimulates the secretion of testosterone and oestradiol by the interstitial cells of Leydig. Very high concentrations of testosterone surround the seminiferous tubules and these are required for spermatogenesis. Testosterone in blood induces puberty and virilization, and exerts feedback inhibition of LHRH and LH secretion, after aromatization and 5-alpha reduction at the hypothalamo-pituitary level. FSH binds to Sertoli cells, stimulating the production and secretion of enzymes and substances that support spermatogenesis. Depending on the intensity of spermatogenesis, the Sertoli cells secrete inhibin B into the blood, which exerts feedback inhibition of FSH secretion by the pituitary. Optimal spermatogenesis depends on adequate functioning of all aspects of the hypothalamo- pituitary-testicular axis, but can be deregulated by many internal and external factors.
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