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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 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 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 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 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 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 Reproductive system
in animals and man, the male gonad, which produces spermatozoa. The testes are usually a paired organ.
In invertebrates the simplest testes are found in coelenterates and consist of a mass of sexual cells. Sperm is released to the outside in hydroids through a rupture in the body wall, and in scyphozoans, anthozoans, and ctenophorans through the gastrovascular cavity and then through the mouth opening. Platyhelminths have numerous testes, each with their own ducts; some trematodes have 30 to 200 testes, and tapeworms have as many as 1,000 testes in every body segment. The numerous testes of nemertines do not have permanent ducts and are arranged along the sides of the intestine. In nematodes the tubular testis enlarges into a sperm duct, which opens near the posterior end. In annelids many body segments have one pair of testes (polychaetes) or two or, less commonly, four pairs of testes (oligochaetes); sperm is excreted through special canals, or coelomoducts, which open into the coelom and do not communicate with the testes.
In arthropods the paired testes, which vary in shape and structure, have ducts. The testes are paired in bivalves and unpaired in other mollusks. The testes of echinoderms usually are located in the interradii; for example, a starfish has ten testes, with two in each interradius. In enteropneusts numerous paired testes open on the dorsal side of the trunk. Among the tunicates, appendicularians have unpaired and ductless testes, Salpae have two testes with ducts, and ascidians have from one to 150 pairs of testes with ducts. Acraniates, for example, lance-lets, have approximately 25 pairs of ductless testes.
In most vertebrates the testes are paired organs symmetrically arranged in the abdominal cavity. They form during the gestation period as thickenings in the abdominal wall. (In cyclostomes both rudimentary testes fuse together.) In most mammals, with the exception of monotremes, many insectivores and edentates, proboscidians, hyracoids, sirenians, pinnipeds, and cetaceans, the testes are located in the scrotum, a saccular process in the body wall. The testes descend into the scrotum from the abdominal cavity through the inguinal canal, usually during embryonic development. In some mammals, for example, in many rodents, the testes descend into the scrotum only during the breeding period, after which they are withdrawn into the abdominal cavity.
Human testes are also called testicles.
(also testicle), either of the paired male gonads. The testes are organs of reproduction and internal secretion. They are responsible for the formation of male sex hormones, chiefly testosterone, in humans. The egg-shaped organs are situated in the scrotum and measure on the average 4 cm in length and 3 cm in diameter. The parenchyma consists of seminiferous tubules.
Congenital disorders of the testis include cryptorchidism and anorchism. Injury, such as contusion or rupture, may cause sharp pain, swelling of the scrotum, and hemorrhaging. For contusion, it is necessary to prescribe rest, application of cold to the scrotum, the use of an athletic supporter, and antibiotics. Open injuries require surgery.
The most frequent inflammatory disease of the testis is orchitis. Testicular tuberculosis usually results from tuberculosis of the prostate or appendix testis. The testis becomes thickened and tuberous and adheres to the scrotum, and purulent fistulas are observed. Treatment is surgical, involving complete or partial removal of the testis. Injury from syphilis or brucellosis is less common. Diagnosis is based on the results of special tests and reactions. Treatment is by medication. Testicular tumors mainly occur early in life. The most common type is seminoma, although teratoblastoma and choriocarcinoma also occur. Treatment is chiefly surgical.
A. L. SHABAD