Male Reproductive Endocrinology

The hypothalamus produces gonadotropin-releasing hormone which is released in a pulsatile fashion every 60 to 120 minutes. Its target organ, the anterior pituitary gland, responds to each pulse of gonadotropin-releasing hormone by producing a corresponding pulse of luteinizing hormone (LH) and, to a lesser degree, follicle-stimulating hormone (FSH). If the gonadotropin-releasing hormone pulses do not occur with the proper amplitude, frequency, and diurnal variation, hypogonadism may result of idiopathic hypogonadotropic hypogonadism. Continuous  stimulation by gonadotropin-releasing hormone agonists actually suppresses pituitary release of LH and FSH and thus testosterone production

Testosterone is synthesized from cholesterol through several intermediate compounds, including dehydroepiandrosterone (DHEA) and androstenedione. Circulating testosterone is mostly protein bound, about 40% avidly bound to sex hormone binding globulin and 58% loosely bound to albumin. Thus, only about 2% of circulating testosterone is bioavailable as free testosterone. This bioactive component of total testosterone is responsible for male characteristics, libido, bone and muscle mass.

Testosterone, DHT, and estradiol provide negative feedback on the hypothalamic-pituitary axis. In males, estradiol is the main inhibitor of LH production, whereas both estradiol and inhibin B, a peptide produced by Sertoli cells of the testes, inhibit production of FSH. In the presence of testosterone, FSH stimulates the Sertoli cells and induces spermatogenesis. In spermatogenesis, each germinal cell located adjacent to the Sertoli cells, undergoes differentiation into 16 primary spermatocytes, each of which generates 4 spermatids. Each spermatid matures into a spermatozoon. Spermatogenesis takes 72 to 74 days and yields about 100 million new spermatozoa each day. Upon maturation, spermatozoa are released into the rete testis, where they migrate to the epididymis and eventually to the vas deferens. Migration requires an additional 14 days. During ejaculation, spermatozoa are mixed with secretions from the seminal vesicles, prostate, and bulbourethral glands.

  • The hypothalamic-pituitary-testicular (HPT) axis
  • The GH, insulin-like growth factor (IGF)-1, and PRL effects
  • The inhibin/activin-follistatin system
  • Spermatogenesis
  • Estrogen and testicular function
  • Paracrine/autocrine regulation of testicular function
  • Testicular apoptosis
  • Aging and male endocrine function

 

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