The testis is regulated by an endocrine axis (Fig. 44.10 ) involving parvicellular hypothalamic gonadotropin-releasing hormone (GnRH) neurons and pituitary gonadotropes that produce both luteinizing hormone (LH) and follicle-stimulating hormone (FSH).
Regulation of Leydig Cell Function
The Leydig cell expresses the LH receptor, which acts on Leydig cells much like adrenocorticotropic hormone (ACTH, or corticotropin) does on zona fasciculata cells in the adrenal cortex (see ). Rapid effects include hydrolysis of cholesterol esters and new expression of StAR protein. Less acute effects include an increase in steroidogenic enzyme gene expression and expression of the LDL. Over the long term, LH promotes Leydig cell growth and proliferation.
•Fig. 44.10 Thehypothalamic-pituitary-testicularaxis.Abbreviationsasinotherfigures.Kisspeptin hypothalamic neurons GnRH hypothalamic neurons GnRH Pituitary gonadotropes LH FSH Sertoli cell Sperm cells E2 T DHT Leydig cell T (–) (–) (–)(–) (–) Inhibin B
Testosterone, and estradiol produced from peripheral conversion of testosterone, negatively feed back on GnRH hypothalamic neurons indirectly through inhibition of kisspeptin-producing neurons (see
Fig. 44.10 ). Testosterone and estradiol also negatively feed back on pituitary gonadotropes. DHT, the other major product of peripheral conversion of testosterone, has little effect on LH or FSH levels.
Regulation of Sertoli Cell Function
The Sertoli cell is stimulated by both testosterone and FSH. In addition to stimulating synthesis of proteins involved in the “nurse cell” aspect of Sertoli cell function (e.g., ABP), FSH stimulates synthesis of the dimeric protein inhibin. Inhibin is induced by FSH and negatively feeds back on the gonadotrope to selectively inhibit FSH production (see
Fig. 44.10
Once spermatozoa emerge from the efferent ductules, they leave the gonad and enter the male reproductive tract (see
Fig. 44.1 ). The segments of the tract are the: epididymis (head, body, and tail), vas deferens, ejaculatory duct, prostatic urethra, membranous urethra, and penile urethra. Unlike the female tract, there is a contiguous lumen from the seminiferous tubule to the end of the male tract (i.e., the tip of the penile urethra), and the male reproductive tract connects to the distal urinary tract (i.e., male urethra). In addition to conveying sperm, the primary functions of the male reproductive tract are: 1. Sperm maturation. Sperm spend about a month in the epididymis, where they undergo further maturation.