Topics: CNS, pituitary, testis, epididymis, prostate, seminal vesicles, scrotum, penis

The male reproductive system consists of a number of individual organs acting together to produce functional spermatozoa, and to deliver these spermatozoa to the female reproductive tract. The haploid germ cells originate in the testis and continue their maturation as they transit the epididymis. The vas deferens carries the spermatozoa from the epididymis to the ampulla, provides a site for the mixing of seminal vesicle secretions, and continues as the ejaculatory duct through the prostate, emptying into the prostatic urethra (Fig. 1). The germ cells, mixed with ejaculatory secretions from the accessory sex glands (seminal vesicles, prostate, bulbourethral gland), then exit the body through the penile urethra. The entire system is dependent upon neuro-endocrine regulation from the pituitary and hypothalamus. Knowledge of the anatomy and embryological origins of each of the components of the male reproductive tract is important in developing a basic and thorough understanding of the system as a whole. Although the discussion of male reproductive anatomy and embryology in this chapter is confined to the human system, subsequent chapters will show that much of our understanding of reproductive biology has been gained from research using various experimental animal models.

Testis
The testis is central to the male reproductive system. It is the organ which generates the haploid germ cell by the process of spermatogenesis and it is the site of androgen production. The testis arises from the primitive gonad on the medial surface of the embryonic mesonephros. Primitive germ cells, which migrate to this region from the yolk sac, cause the coelomic epithelial cells to proliferate and form the sex cords. Formation of the sex cords gives this region a raised contour that is called the genital ridge. By the seventh week of fetal development, proliferation of the mesenchyme has separated the sex cords from the underlying coelomic epithelium. During the fourth month, the sex cords become U-shaped and their ends anastomose to form the rete testis (Fig. 2A). At this point, the primordial sex cells are referred to as pre-spermatogonia and the epithelial cells of the sex cords as Sertoli cells. The mesenchymal tissue in the interstitial space between the tubules gives rise to the Leydig cells that are the site of androgen production. The rete testis extends into the mesonephric tissue and will anastomose with some of the mesonephric tubules forming the efferent ducts which communicate with the epididymis (discussed below). The sex cords will become the seminiferous tubules, although the tubules do not develop a lumen until after birth.

The testis develops abdominally and successful descent into the scrotum is essential for fertility. The scrotum is formed as coelomic epithelium penetrates the abdominal wall and protrudes into the genital swelling as the processus vaginalis. An outgrowth of each layer of the abdominal wall is carried with this epithelium, giving rise to the fascial layers of the scrotum. The testis then descends behind the processus vaginalis and the layers of fascia covering the testis on each side, with the overlying skin of the genital swelling, fuse to form the scrotum.

Epididymis
The epididymis, vas deferens and seminal vesicles have their origin in the mesonephric duct (or Wolffian duct). Initially formed as the early embryonic excretory system, the mesonephric system is comprised of the longitudinal duct and a series of tubules that branch from the duct toward the developing gonad. Although most will degenerate, several of these tubules persist and anastomose with the confluence of the seminiferous tubules (rete testis), forming the efferent ducts (or ductuli efferentes) through which spermatozoa exit the testis (Fig. 2B). The portion of the mesonephric duct closest to the ductuli efferentes elongates, becomes extensively convoluted, and forms the epididymis (Fig. 2B). Because it arises from a single duct, the epididymis, unlike the testis, consists of a single tubule through which all spermatozoa must pass. The epididymis remains in close contact with the testis and descends with the testis into the scrotum.

Testicular spermatozoa are non-motile and incapable of fertilization. The function of the epididymis is to bring testicular spermatozoa to maturity. How this maturation process is accomplished by the epididymis is currently unknown but is an area of active research in reproductive biology. It is known that the epididymis secretes proteins that become part of the surface architecture of the mature spermatozoa and presumably are important in post-ejaculatory function of the spermatozoa.

Vas Deferens
That portion of the mesonephric duct extending from the caudal end of the epididymis to the seminal vesicles (discussed below) becomes thickened and muscular and forms the vas deferens (or ductus deferens). That portion of the duct which continues distal to the seminal vesicle is known as the ejaculatory duct and is contained entirely within the prostate gland. In its course the vas deferens ascends from the scrotum, with the vessels that vascularize the testis and epididymis, through the inguinal canal, over the pubic ramus, over the superior lateral aspect of the bladder medial to the ureter, and enters the posterior superior aspect of the prostate, just distal to the seminal vesicle (Fig. 1). The primary function of the vas deferens and ejaculatory duct is transport of mature spermatozoa and seminal vesicle secretions to the prostatic urethra.

Seminal Vesicles
The seminal vesicle develops as an out-pocketing of the mesonephric duct, distal to the epididymis (Fig. 2B). Consequently, this gland shares a common embryological origin with the epididymis and vas deferens. The fully developed seminal vesicle resides immediately above the prostate gland (Fig. 1). It is comprised of a series of tubular alveoli, lined with a very active secretory epithelium. In fact, the seminal vesicle contributes the majority of the fluid volume of the ejaculate. Seminal vesicle secretions are rich in fructose and prostaglandins. While fructose may be an important energy source for spermatozoa, the role of prostaglandins is unknown. The seminal vesicle also produces several androgen-dependent secretory proteins that are involved in the rapid clotting of the ejaculate.

Prostate
The prostate gland is located in the space below the bladder and above the urogenital diaphragm (Fig. 1). It is separated posteriorly from the rectum by the rectovesical (Denonvillier's) fascia. Its location immediately anterior to the rectum allows the prostate to be palpated and biopsied from the rectum. The prostate arises from several distinct sets of tubules that evaginate from the primitive posterior urethra. Each set of tubules develops into a separate lobe: the right and left lateral lobes, which are the largest, the middle lobe, and the very small anterior and posterior lobes. The lobes are composed of alveoli, lined with a secretory epithelium, that drain through a series of converging tubules into the prostatic urethra. Although the lobes arise independently, they are continuous in the adult with no apparent gross or morphologic distinctions. Consequently, a more useful subdivision of the prostate has been recently developed which distinguishes prostatic zones based on morphologic and functional properties (i.e., central, peripheral and transitional zones).

Prostatic secretions contribute to the fluid volume of the ejaculate. These secretions are high in zinc, citric acid and choline. The function of these substances is unknown, although an anti-microbial activity for zinc has been postulated. The prostate also secretes several proteins including acid phosphatase, seminin, plasminogen activator and prostate specific antigen (PSA). The exact roles of most prostatic secretions are unknown, although they are presumed important for the function of spermatozoa during and after ejaculation. For instance, plasminogen activator and seminin are proteases involved in the liquification of coagulated ejaculate. Although the function of PSA is not known, an elevated level of this protein in the blood is often diagnostic of abnormal prostatic growth such as occurs with cancer of the prostate.

Penis
The penis arises from the genital tubercle, a region just cranial to the cloacal folds in the embryo. Under the influence of androgens produced by the fetal testis, the cells of the genital tubercle proliferate causing elongation of the tubercle into the primitive phallus. The penile urethra is formed from the urethral folds as the phallus elongates. In the adult penis the urethra is divided into the membranous portion, which extends through the urogenital diaphragm, and the pendulous portion, which courses through the penis. Lateral to the urethra are the two corporal bodies, the corpus cavernosi, which become engorged with blood to produce the penile erection. The physiology of erection is complex and is subject to a number of clinical disorders. The importance of proper erectile function to sex and reproduction, and the common occurrence of erectile dysfunction (affecting 10-20 million men in the United States), has made erectile dysfunction a primary clinical concern in andrology.

Endocrine and nervous control of the male reproductive tract
The entire male reproductive tract is dependent on hormones for proper function. The pituitary produces the gonadotropins, follicle-stimulating hormone (FSH) and luteinizing hormone (LH), under the control of the hypothalamus. FSH is required for spermatogenesis and LH stimulates androgen production by the testicular Leydig cells. The testis requires testosterone to maintain the process of spermatogenesis and the accessory organs are dependent on androgen for proper secretory function. The production of LH is regulated by feedback inhibition of circulating testosterone on the pituitary and hypothalamus. FSH secretion is regulated by inhibin, a peptide hormone produced by Sertoli cells, and also by circulating testosterone. This endocrine loop is known as the hypothalamic-pituitary-testicular axis. In addition to hormonal control, the reproductive organs are also subject to sympathetic and parasympathetic nervous control. This is particularly true for the erectile function of the penis, which is under parasympathetic control, and for ejaculation, which is under sympathetic control.

Conclusion
This brief introduction to the male reproductive tract demonstrates the integrated nature of the system. The seminiferous tubules are continuous with the penile urethra via the vas deferens, with the accessory organs contributing their secretions along this course. The entire system is maintained by androgens, secreted by the testis under the control of the pituitary and hypothalamus. It is important to remember that many of these structures are embryologically distinct and, consequently, that developmental abnormalities will affect these structures in different ways. Knowing the embryology and anatomy of the tract will help the student of male reproductive medicine to understand the common, and the not-so-common, disorders encountered in the clinic.

Suggested Reading

McNeal JE. The Prostate Gland: Morphology & Pathology. In: Stamey TA, ed. Monographs in Urology, Vol 9. Princeton: Medical Directions Publishing; 1988:36-54.