The male reproductive system (see Graphic 18-1) consists of the two testes (the male gonads), a system of genital ducts, accessory glands, and the penis.

The male reproductive system functions in the formation of spermatozoa, the elaboration of male sex hormones, and the delivery of male gametes into the female reproductive tract.

TESTES

Each testis is an oval structure housed in its separate compartment within the scrotum. The tunica albuginea, the fibromuscular connective tissue capsule of the testis, is thickened at the mediastinum testis, from which septa are derived to subdivide the testis into approximately 250 small, incomplete compartments, known as the lobuli testis.

•Each lobule houses one to four highly tortuous seminiferous tubules that function in the production of spermatozoa.

•The highly vascular connective tissue surrounding the seminiferous tubules houses Leydig’s cells (interstitial cells of Leydig).

•The wall of the seminiferous tubule is composed of the seminiferous epithelium lining its lumen and a slender connective tissue tunica propria.

The seminiferous epithelium is several cell layers thick and is separated from the tunica propria by a basement membrane.

The basal cells of this epithelium, composed of Sertoli cells and three types of spermatogonia, dark type A, pale type A, and type B spermatogonia, sit on the basement membrane.

-Sertoli cells (see Table 18-1) are supporting cells that form tight junctions with each other

TABLE 18-1 • Functions of Sertoli Cells

M A L E R E P R O D U C T I V E S Y S T E M 435

subdividing the lumen of the seminiferous tubule into a basal compartment and an adluminal compartment, thus establishing a bloodtestis barrier that protects the developing germ cells and spermatozoa from an autoimmune response.

-Spermatogonia are responsible for spermatogenesis.

The cells of the adluminal compartment are primary spermatocytes, secondary spermatocytes, spermatids, and spermatozoa.

The tunica propria of the seminiferous tubule of humans is composed of slender type I collagen fibers interspersed with fibroblasts and, perhaps, some myocytes.

Spermatogenesis

Spermatogenesis, the process of producing haploid male gametes, is dependent on several hormones that are released at puberty from the adenohypophysis, including luteinizing hormone (LH), prolactin, and folliclestimulating hormone (FSH) (see Graphic 18-2). In the mature male, approximately 300 million spermatozoa are produced daily.

•Prolactin induces the interstitial cells of Leydig to express LH receptors.

LH binds to its receptors on the Leydig cells prompting these cells to secrete testosterone.

•FSH causes Sertoli cells to produce adenylate cyclase, which, via a cAMP intermediary, stimulates the production of androgen binding protein (ABP).

ABP binds with and maintains a high enough concentration of testosterone and dihydrotestosterone

(a transformation product of testosterone by the enzyme 5a reductase) in the seminiferous epithelium for spermatogenesis to occur.

•Testosterone acts as a negative feedback for LH release, and inhibin, produced by Sertoli cells, inhibits the release of FSH,

•Activin, also produced by Sertoli cells, enhances FSH release.

•For spermatogenesis to proceed normally, the testes must be maintained at 35°C, the temperature inside the scrotum, a level that is slightly below normal body temperature.

Spermatogenesis takes 74 days to be completed, and it occurs in a cyclic but asynchronous fashion along the length of the seminiferous tubule. These cycles of the seminiferous epithelium consist of repeated aggregates of cells in varying stages of development. Each aggregate is composed of groups of cells that are connected