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

to one another by intercellular bridges, forming a synchronized syncytium that migrates toward the lumen of the seminiferous tubule as a unit. The three phases of spermatogenesis are spermatocytogenesis, meiosis, and spermiogenesis.

•Spermatocytogenesis is a process involving mitosis, in which pale type A spermatogonia divide to form two types of spermatogonia, more pale type A as well as type B spermatogonia, both of which are diploid.

Dark type A spermatogonia represent a reserve population of cells that normally do not undergo cell division, but when they do, they form pale type A spermatogonia.

Type B spermatogonia divide via mitosis to form diploid primary spermatocytes. All spermatogonia are located in the basal compartment, whereas primary spermatocytes migrate into the adluminal compartment.

•Meiosis phase starts when primary spermatocytes (4CDNA content) undergo the first meiotic division, forming two short-lived secondary spermatocytes

(2CDNA content).

Secondary spermatocytes do not replicate their DNA but immediately start the second meiotic division, and each forms two haploid (N) spermatids.

•Spermiogenesis (Graphic 18-2) is the process of cytodifferentiation of the spermatids into spermatozoa and involves no cell division.

The spermatid loses much of its cytoplasm (which is phagocytosed by Sertoli cells), forms an acrosomal granule, a long cilium, and associated outer dense fibers and a coarse fibrous sheath.

The spermatozoon that is formed and released into the lumen of the seminiferous tubule is nonmotile and is incapable of fertilizing an ovum.

The spermatozoa remain immotile until just before they leave the epididymis. They become capable of fertilizing once they have been capacitated in the female reproductive system.

GENITAL DUCTS

A system of genital ducts conveys the spermatozoa and the fluid component of the semen to the outside.

•The seminiferous tubules are connected by short, straight tubules, the tubuli recti, to the rete testis, which is composed of labyrinthine spaces located in the mediastinum testis.

•From the rete testis spermatozoa enter the first part of the epididymis, the 15 to 20 ductuli efferentes that lead into the ductus epididymis.

The head of the epididymis is composed of the ductuli efferentes,

The body and tail of the epididymis are 5 m long and are highly folded and form the ductus epididymis (Graphic 18-1).

The wall of the epididymis is composed of a smooth muscle coat surrounding a loose connective tissue and a pseudostratified stereociliated epithelium that lines the lumen, where the epithelium is separated from the connective tissue by a basement membrane.

The epithelium is composed of short basal cells and tall principal cells.

The basal cells are regenerative cells.

The principal cells sport stereocilia (long, nonmotile microvilli) that phagocytose cytoplasmic remnants from spermiogenesis, phagocytose luminal fluid, and synthesize and release surface-activated decapacitation factor (glycerophosphocholine).

Spermatozoa become motile near the end of the body of the epididymis.

The head of spermatozoa pick up surface-activated decapacitation factor from the fluid present in the lumen of the epididymis, which prevents them from being able to fertilize an ovum until that factor is removed from their plasma membrane in the female genital tract.

The thick, muscular ductus deferens, the continuation of the tail of the epididymis, passes through the inguinal canal, as a part of the spermatic cord, to gain access to the abdominal cavity.

•Just prior to reaching the prostate gland, the seminal vesicle empties its secretions into the ductus deferens, which terminates at this point.

•The continuation of the ductus deferens, known as the ejaculatory duct, enters the prostate gland, which delivers its secretory product into the ejaculatory duct.

The right and left ejaculatory ducts empty into the urethra, which conveys both urine and semen to the outside.

•The urethra, which passes through the length of the penis, has three regions: prostatic, membranous, and cavernous (spongy) portions.

ACCESSORY GLANDS

The three accessory glands of the male reproductive system, which supply the fluid component of semen, are the two seminal vesicles and the prostate gland. Additionally, a pair of small bulbourethral glands deliver their viscous secretions into the cavernous (spongy) urethra that lubricates the urethra.