The organ of sperm production. In addition, the testis (testicle) is an organ of endocrine secretion in which male hormones (androgens) are elaborated. In the higher vertebrates (reptiles, birds, and mammals), the testes are paired and either ovoid or elongated in shape. In mammals, the testes are usually ovoid or round. In many species (for example, humans) they are suspended in a pouch (scrotum) outside the main body cavity; in other species they are found in such a pouch only at the reproductive season; in still others the testicles are permanently located in the abdomen (for example, in whales and bats).
Within a firm and thick capsule of connective tissue, the tunica albuginea, the testis contains a varying number of thin but very long seminiferous tubules which are the sites of sperm formation. Essentially, these tubules are simple loops which open with both their limbs into a network of fine, slitlike canals, the rete testis. From this the sperm drains through a few, narrow ducts, the ductuli efferentes, into the epididymis, where sperm mature and are stored.
The seminiferous tubules comprise most of the testis, and in different species vary greatly in complexity. Each tubule is surrounded by a layer of thin cells which is contractile and enables the tubules to wriggle slowly. The spaces between tubules are filled with connective tissue, blood vessels, an extensive network of very thin-walled lymph vessels, and secretory cells, the interstitial cells or cells of Leydig, which secrete male hormone.
The sperm cells, spermatozoa, develop in the wall of the seminiferous tubules, either periodically, as in most vertebrates, or continually, as in humans. Most of the cells in the tubules are potential spermatozoa (spermatogenic or germ cells). Nursing cells (Sertoli cells) are interspersed at regular intervals between them. The Sertoli cells support and surround the developing spermatogenic cells and provide a specialized environment, which is absolutely necessary for normal sperm development. See also Sperm cell.
Spermatogenesis in the testis is the result of a balance between proliferation and differentiation, and cell degeneration or apoptosis. Apoptosis of the spermatogenic cells is largely hormonally controlled, and specifically directed apoptosis occurs in conditions of testicular damage due to environmental insults such as heat, radiation, or chemical toxicants. Recovery of spermatogenesis is possible provided the stem cells are not depleted by these processes. See also Spermatogenesis.
The functions of the testis are dependent on the secretion of gonadotropic hormones, the release of which from the pituitary gland is in turn regulated by the central nervous system. In mammals, male-hormone production resides in the Leydig cells, located in the intertubular tissue of the testes.
The principal androgenic hormone released by the testis into the bloodstream is testosterone. The testis is able to form cholesterol and to convert this via a number of pathways to testosterone. Testosterone may be further metabolized into estrogens in the testis. The production of estrogens in the male varies quite widely among species, from relatively low in humans to very high, for example in stallions and boars. Estrogens are important in the development and proper function of the ducts which drain the testis (the rete testis and ductuli efferentes), even in species with relatively low levels of estrogens. See also Androgen.
Testosterone synthesis is normally limited by the rate of pituitary gonadotrophin secretion: administration of the luteinizing hormone or of chorionic gonadotrophin results in increased testosterone synthesis and release within minutes. These hormones also stimulate growth and multiplication of Leydig cells. Hypophysectomy leads to cessation of androgen formation. See also Adenohypophysis hormone; Pituitary gland.
At the ambisexual stage of embryonic development, the testis promotes the growth of the paired Wolffian ducts and their differentiation into the epididymis, vasa deferentia, and seminal vesicles; the fetal testis also causes masculinization of the urogenital sinus, fusion of the labioscrotal folds in the midline, and development of the genital tubercle into a phallus. See also Embryology.
Toward puberty, increased secretion of testosterone stimulates the growth of the penis, scrotum, and male accessory glands responsible for the formation of the seminal plasma, for example, the prostate and seminal vesicles. The hormone brings about the appearance of secondary sex characters, such as the male-type distribution of hair and body fat and lowered pitch of voice in man, the growth of the comb and wattles in birds, the clasping pads of amphibians, or the dorsal spine of certain fishes.
Unlike the ovary, the testis remains functional throughout life, with ongoing spermatogenic development. However, the efficiency of spermatogenesis falls away, and androgen levels begin to fall due to a declining Leydig cell activity. These events can lead to reduced fertility, and androgen insufficiency problems in later life in some men. See also Reproductive system.
