testes

testes Each testis (synonym: testicle), enclosed in its fibrous capsule, moves freely within its scrotal sac. Above and behind its oval ‘ball’ is the epididymis — a complex of coiled tubes which convey sperm from their origin in the testis to the vas deferens. The testes have two major functions; to produce sperm and to produce male sex hormones.

Spermatogenesis

The testis is divided into 200–300 lobes, made up mainly of seminiferous tubules where sperm are generated. By the time of puberty there are some 700 coiled tubules, each as long as an arm and as thin as a hair. These tubules are formed from two cell types, the cells which are developing into sperm and the Sertoli cells — named after the Italian physiologist who described them in 1865. Each Sertoli cell spans the thickness of the tubule wall, and all the germinal cell types, in different stages of maturation, lie in between these Sertoli cells. The role of the Sertoli cells is to nourish the developing and maturing germinal cells which are eventually released into the lumen of the tubule as spermatozoa. Each Sertoli cell is closely attached to the adjacent ones near its base. This attachment forms a barrier which protects the maturing sperm cells from potentially harmful substances circulating in the blood (see sperm).

The primitive germinal cells are the spermatogonia, which lie peripherally in the tubule wall, outside the barrier of Sertoli cell junctions. After they have divided and formed primary spermatocytes with the normal complement of 46 chromosomes, these push their way through the barrier. Lying between, and nurtured by, the Sertoli cells, the primary spermatocytes undergo two meiotic divisions (reducing to 23 chromosomes) and cellular remodelling to produce mature sperm.

The production of mature sperm from spermatogonia takes about 10 days and, during this process of development and maturation, the germinal cells move closer and closer to the lumen of the seminiferous tubules. Eventually they are released from the protection of the Sertoli cells. With little motility of their own at this stage, they are helped by fluid secretions to move into the epididymis; traversing the coils of this final tube takes about two weeks and the sperm become fully fertile and mobile along the way. This process of spermatogenesis is normally achieved by the age of 16 in most males. A fully-functioning testis has the capacity to produce 200 million sperm each day.

Hormone secretion

Lying between the seminferous tubules are the interstitial cells, commonly referred to as Leydig cells after the German histologist who described them in 1850. These are the cells which produce the male sex hormones, the androgens. Testosterone is the major product and is the most potent of the androgens. It diffuses into the seminiferous tubules where it is essential for spermatogenesis. It also diffuses into the general circulation and is thus carried to its target tissues, where it is responsible not only for the development of male characteristics, both in fetal life and at puberty, but also in maintaining them after puberty.

The functions of the testis are primarily controlled by two hormones secreted by the pituitary gland. Follicle stimulating hormone (FSH) acts on the Sertoli cells to maintain their nursemaid function in spermatogenesis, whereas luteinizing hormone (LH) stimulates the Leydig cells to produce testosterone. (Both hormones rather oddly retain the names which originally referred to their actions in the ovary in the female.) If the testes are not maintained at a temperature about 2 °C lower than body temperature the process of spermatogenesis is arrested, although male hormone secretion is unaffected. Thus in normal development the testes descend into the scrotal sac between the seventh and eighth months of fetal life, and remain outside the body at a cooler temperature throughout life. Failure of the testes to descend is known as cryptochordism.

Inflammation of the testis is orchitis, and its removal, orchidectomy — from its Greek name, orchis.

Saffron Whitehead

See urogenital system.See also sex hormones; sperm.