Learning Objective:

Understand the hormones controlling testicular function, their cellular sources, and the mechanisms of feedback regulation in spermatogenesis.

The figure below illustrates the source and nature of the hormones controlling testicular function.

NOTE:

• Sertoli cells provide the nourishment required for normal spermatogenesis.
• FSH, along with very high local levels of testosterone from neighboring Leydig cells, produces growth factors necessary for the growth and maturation of sperm.
• FSH and testosterone induce the synthesis of androgen-binding protein, which maintains high local testosterone concentrations.
• Leydig cells express aromatase, converting testosterone into estradiol, important for sperm growth and maturation.
• Sertoli cells secrete inhibin B, producing feedback regulation on FSH.

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Definitions

• Androgen: Any steroid controlling the development and maintenance of masculine characteristics.
• Testosterone: A natural male androgen of testicular origin, regulated by LH.
• Dihydrotestosterone: A more potent form of testosterone made by 5-alpha-reductase; responsible for the development of the penis, prostate, and scrotum in the embryo.
• Methyl testosterone: A synthetic androgen, anabolic steroid sometimes used by athletes.
• Adrenal androgens: Weak androgens of adrenal origin (DHEA and androstenedione), regulated by ACTH.
• Inhibins: Peptide hormones secreted into the blood; inhibit the secretion of FSH by pituitary gonadotrophs.
• Aromatase: Enzyme converting the A-ring of testosterone into estradiol.

Other than spermatogenesis, the physiologic importance of this conversion is not fully understood. Approximately 35% of estradiol in a man’s blood arises from the testes, with the remainder from peripheral conversion of testosterone to estradiol by aromatase in adipose tissue.

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