The production and regulation of the testis involve a hormonal axis that includes hypothalamic gonadotropin-releasing hormone (GnRH) neurons and pituitary gonadotropes. These gonadotropes produce luteinizing hormone (LH) and follicle-stimulating hormone (FSH).

The function of Leydig cells is regulated by LH, which interacts with the LH receptor on Leydig cells similarly to how adrenocorticotropic hormone (ACTH) affects zona fasciculata cells in the adrenal cortex. This interaction leads to immediate effects such as the breakdown of cholesterol esters and the expression of StAR protein. Longer-term effects include promoting Leydig cell growth and proliferation through LH. LH also influences the expression of steroidogenic enzymes and LDL.

Testosterone and estradiol, derived from testosterone, provide negative feedback on GnRH neurons indirectly by inhibiting kisspeptin-producing neurons. They also negatively regulate pituitary gonadotropes. Dihydrotestosterone (DHT), another product of testosterone conversion, has minimal impact on LH or FSH levels.

The function of Sertoli cells is stimulated by both testosterone and FSH. FSH not only stimulates the synthesis of proteins involved in the supportive role of Sertoli cells, like androgen-binding protein (ABP), but also induces the production of inhibin, a dimeric protein. Inhibin then negatively feeds back on gonadotropes, selectively inhibiting FSH production.

After leaving the efferent ductules, spermatozoa travel through the male reproductive tract, which includes the epididymis, vas deferens, ejaculatory duct, prostatic urethra, membranous urethra, and penile urethra. Unlike the female reproductive tract, the male tract has a continuous lumen from the seminiferous tubule to the tip of the penile urethra. Additionally, the male reproductive tract connects to the distal urinary tract. Besides transporting sperm, the primary functions of the male reproductive tract include the maturation of sperm, which takes place in the epididymis over approximately a month.
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.