Located in the testicles, the seminiferous tubules are tiny tubes in which spermatozoa or sperm cells are formed. They are lined with Sertoli cells, specialized cells that serve as a sort of "nursery" for growing sperm cells. Spermatogenic cells line the layer of Sertoli cells, and they mature into full-fledged sperm through a complex process called meiosis. Seminiferous tubules are found in all mammals, as well as in birds and some fish.
The interior of the testicle is divided into lobules, or separate sections, by septa. Septa are formed from connective tissue and serve as a sort of wall between each section. Each section contains several seminiferous tubules, where sperm cells form and from which they are secreted. Surrounding these tubules are Leydig cells that excrete testosterone and other hormones. These cells are formed in utero but remain dormant until puberty, when hormones are released and the process of meiosis begins.
Each tubule contains a convoluted section and a straight section. The convoluted section folds in and over itself, allowing a greater length to function in a smaller space, and the straight section allows mature sperm to exit the testicles. It is within the convoluted section that meiosis occurs. Meiosis is a process by which special cells are created that contain only half the number of chromosomes of other cells. These special cells are called gametes, a term that can refer to either the male sperm or the female ovum.
Seminiferous tubules are structured in such a way as to prevent large molecules from entering. Sertoli cells lining the tubules contribute to this barrier because they are tightly connected to each other. It is generally believed that this barrier is to prevent the body's natural immune reactions from contacting the sperm, which the immune system could interpret as a threat.
Malformation or absence of the seminiferous tubules can result in infertility. One such condition is Sertoli-cell-only syndrome, in which the body has produced Sertoli cells, but the tubules are absent. In such cases, production of sperm cells is impossible. Some genetically linked diseases appear to occur because of a malfunction in meiosis, meaning that such conditions are coded into the sperm before it even leaves the testicles. These conditions usually are caused by a malformed gene or chromosome, and they include Huntington's Disease and other disorders.