The testes are paired ovoid organs located in the scrotal sac outside the body cavity. The extra-abdominal location, coupled with vascular countercurrent heat exchangers and muscular reflexes that retract the testes to the abdomen, permits testicular temperature to be maintained constant at about 2°C below body temperature. For reasons that are not understood, this small reduction in temperature is crucial for normal spermatogenesis (sperm production). Failure of the testes to descend into the scrotum results in failure of spermato-genesis, although production of testosterone may be maintained. The two principal functions of the testis— sperm production and steroid hormone synthesis—are carried out in morphologically distinct compartments. Sperm are formed and develop within seminiferous tubules, which comprise the bulk of testicular mass. Testosterone is produced by the interstitial cells of Leydig, which lie between the seminiferous tubules (Fig. 1). The entire testis is encased in an inelastic fibrous capsule consisting of three layers of dense connective tissue and some smooth muscle.
Blood reaches the testes primarily through paired spermatic arteries and is first cooled by heat exchange with returning venous blood in the pampiniform plexus. This complex tangle of blood vessels is formed by the highly tortuous and convoluted artery intermingling with equally tortuous venous branches that converge to form the spermatic vein. This arrangement provides a large surface area for warm arterial blood to transfer heat to cooler venous blood across thin vascular walls. Rewarmed venous blood returns to the systemic circulation primarily through the internal spermatic veins.
Leydig cells are embedded in loose connective tissue that fills the spaces between seminiferous tubules. They are large polyhedral cells with an extensive smooth endoplasmic reticulum characteristic of steroid-secreting cells. Although extensive at birth, Leydig cells virtually disappear after the first 6 months of postnatal life, only to reappear more than a decade later with the onset of puberty.
Seminiferous tubules are highly convoluted loops that range from about 120 to 300 mm in diameter and from 30 to 70 cm in length. They are arranged in lobules bounded by fibrous connective tissue. Each testis has hundreds of such tubules that are connected at both ends to the rete testis (Fig. 2). It has been estimated that, if laid end to end, the seminiferous tubules of the human testes would extend more than 250 m. The seminiferous epithelium that lines the tubules consists of three types of cells: spermatogonia, which are stem cells; spermatocytes, which are in the process of becoming sperm; and Sertoli cells, which nurture developing sperm and secrete a variety of products into the blood and the lumens of seminiferous tubules. Seminiferous tubules are surrounded by a thin coating of peritubular epithelial cells, which
in some species are contractile and help propel the non-motile sperm through the tubules toward the rete testis.
Spermatogenesis goes on continuously from puberty to senescence along the entire length of the seminiferous tubules. Though a continuous process, spermatogenesis can be divided into three discrete phases: (1) mitotic divisions, which replenish the spermatogonia and provide the cells destined to become mature sperm; (2) meiotic divisions, which reduce the chromosome number and produce a cluster of haploid spermatids; and (3) transformation of spermatids into mature sperm (spermio-genesis), a process involving the loss of most of the cytoplasm and the development of flagella. These events occur along the length of the seminiferous tubules in a definite temporal and spatial pattern. A spermatogenic cycle includes all of the transformations from spermatogonium to spermatozoan and requires about 64 days. As the cycle progresses, germ cells move from the basal portion of the germinal epithelium toward the lumen. Successive cycles begin before the previous one has been completed, so that at any given point along a tubule different stages of the cycle are seen at different depths of the epithelium (Fig. 3). Spermatogenic cycles are synchronized in adjacent groups of cells, but the cycles are slightly advanced in similar groups of cells located immediately upstream, so that cells at any given stage of the spermatogenic cycle are spaced at regular intervals along the length of the tubules. This complex series of events ensures that mature spermatozoa are produced continuously. About 2 million spermatogonia, each giving rise to 64 sperm cells, begin this process in each testis every day. Hundreds of millions of spermatozoa are thus produced daily throughout six or more decades of reproductive life.
Sertoli cells are remarkable polyfunctional cells whose activities are intimately related to many aspects of the formation and maturation of spermatozoa. They extend through the entire thickness of the germinal epithelium from basement membrane to lumen, and in the adult take on exceedingly irregular shapes determined by the changing conformation of the developing sperm cells embedded in their cytoplasm (Fig. 4). Differentiating sperm cells are isolated from the bloodstream and must rely on Sertoli cells for their sustenance. Adjacent Sertoli vas deferens ductuli vas deferens ductuli
cells arch above the clusters of spermatogonia that nestle between them at the level of the basement membrane, and they form a series of tight junctions that limit passage of physiologically relevant molecules into or out of seminiferous tubules. This so-called blood-testis barrier actually has selective permeability that allows rapid entry of testosterone, for example, but virtually completely excludes cholesterol. The physiological significance of the blood-testis barrier has not been established, but it is probably of some importance that spermatogonia are located on the blood side of the barrier, whereas developing spermatids are restricted to the luminal side. In addition to harboring developing sperm, Sertoli cells secrete a watery fluid that transports spermatozoa through the seminiferous tubules and into the epididymis, where 99% of the fluid is reabsorbed.
The remaining portion of the male reproductive tract consists of modified excretory ducts that ultimately deliver sperm to the exterior along with secretions of accessory glands that promote sperm survival and fertility. Sperm leave the testis through multiple ductuli efferentes, whose ciliated epithelium facilitates passage from the rete testis into the highly convoluted and tortuous duct of the epididymis. The epididymis is the primary area for maturation and storage of sperm, which remain viable within its confines for months.
Sperm are advanced through the epididymis, particularly during sexual arousal, by rhythmic contractions of circular smooth muscle surrounding the duct. At ejaculation, sperm are expelled into the vas deferens and ultimately through the urethra. An accessory storage area for sperm lies in the ampulla of the vas deferens, posterior to the seminal vesicles. These elongated, hollow evagina-tions of the deferential ducts secrete a fluid rich in citric acid and fructose that provides nourishment for the sperm after ejaculation. Metabolism of fructose provides the energy for sperm motility. Additional citrate and a variety of enzymes are added to the ejaculate by the prostate, which is the largest of the accessory secretory glands. Sperm and the combined secretions of the accessory glands make up the semen, of which less than 10% is sperm.
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