The male testes have the dual responsibilities for the production and release of the germ cells, the spermatozoa, and for the biosynthesis and secretion of the key androgenic steroid hormone, testosterone. This steroid hormone and others produced from it play a dominant role in the differentiation, growth, and maintenance of the sexual reproductive tissues necessary for continuation of the species, the growth and maintenance of secondary sex characteristics, and the anabolic effects of androgens on skeletal muscle growth and skeletal growth. The testis and the internal and external genitalia with which it is closely associated provide the venue in which these two important functions, hormonogenesis and spermatogenesis, are carried out and integrated.
The male reproductive system comprises the paired units consisting of the testis, epididymis and vas deferens, the accessory glands (prostate, seminal vesicles, bulbourethral glands), and the penis and scrotum. These are illustrated in Figure 1.
Fig1. Anatomy of the male reproductive system. A midsagittal view of the organs, glands, and ducts involved in human male reproduction.
In mammals, the testes are contained in the scrotum, which is suspended outside the abdominal cavity between the thighs. Spermatogenesis does not take place at the internal body temperature of 35–37°C, but occurs optimally at the scrotal temperature of 33°C. Descent of the testes from the abdominal cavity into the scrotum occurs just prior to birth in a process that is dependent on androgens and a testis-specific hormone INSL3 (insulin-like hormone 3). Failure of one or both testes to descend into the scrotum is termed cryptorchidism and, if bilateral and not corrected, results in sterility.
The spermatic cord, the wall of which is an extension of the abdominal wall, is the conduit between the testis and the abdominal cavity and contains the vas deferens, blood vessels, and some nerves and lymphatics.
Each adult human testis is a 4–5 cm long ellipsoid weighing 35–45 g. As shown in Figure 2, the tunica vaginalis encapsulates the entire surface of the testis except the posterior region which lies in close proximity to the epididymis. More interiorly, the testis is sur rounded by the tunica albuginea, a thick capsule that also divides the parenchymal seminiferous tubules into discrete lobules. The seminiferous tubules, which are the site of production of spermatozoa, have a total length of ~500 m. The seminiferous tubules are organized in a highly convoluted irregular pattern, with some blind pouches, ultimately leading to the rete testis and the epididymal duct. The duct system, composed of the epididymis, ductus (vas) deferens, and ejaculatory duct, functions to transport the mature spermatozoa during ejaculation.
Fig2. The testis and epididymis. In the scrotal sac, the epididymis lies posterior to the testis, which is covered with the thick tunica albuginea. The spermatic cord is the conduit for blood vessels, nerves, and lymphatics. The sperm produced by the seminiferous tubules are collected in the rete testis and travel through the epididymis to the ductus (vas) deferens.
The epididymis is attached to the posterior surface of the testis. Approximately 12 days are required for the transit of the spermatozoa through the epididymis to the vas deferens; during this interval they undergo the morphological and functional changes necessary to provide them with the capacity to fertilize an ovum. The vas deferens is a 7- to 8-cm tubule connecting the epididymis with the ejaculatory duct (see Figure 1). The passage of the mature sperm through the vas deferens is accomplished by peristaltic contractions of smooth muscle in the duct wall. The ejaculatory duct is located at the merger of the vas deferens and the seminal vesicles and passes the prostate to empty into the urethra.
The accessory structures include the seminal vesicles, prostate gland, and bulbourethral glands. In adult males, the two seminal vesicles are 3–5 cm in length and lie distal to the ejaculatory duct. Under the actions of androgens they produce the mucoid secretions, including high concentrations of fructose, that constitute the major fluid volume of the ejaculate. The prostate gland lies at the junction of the ejaculatory duct with the urethra. The prostatic secretions, which are highly responsive to androgens, consist of simple sugars and some proteolytic enzymes. The two bulbourethral glands, sometimes referred to as Cowper’s glands, are 2–3 mm in diameter and secrete prostaglandins and an alkaline mucoid solution to neutralize and coat the urethral lining immediately prior to the arrival of the ejaculate.
The penis carries the urethra to the exterior of the body. Its erectile tissue is highly vascularized, so that when the penis is filled with blood it is in an erect and firm state, facilitating deposition of the spermatozoa in the female vaginal canal.
