Spermatogenesis takes place in the seminiferous tubules. This includes mitosis and meosis to form haploid gametes, followed by maturation to form spermatocytes (called spermiogenesis). The stages that a developing spermatogonium passes through as it develops and matures to form a mature spermatozoon are covered in more detail here.

The seminiferous tubules are lined by a complex stratified epithelium containing two distinct populations of cells, spermatogenic cells, that develop into spermatozoa, and Sertoli cells which have a supportive and nutrient function.

Sertoli cells are the epithelial supporting cells of the seminiferous tubules. They are derived from the epithelial sex cords of the developing gonads. They are tall simple columnar cells, which span from the basement membrane to the lumen. They surround the proliferating and differentiating germ cells forming pockets around these cells, providing nutrients, and phagocytosing excess spermatid cytoplasm, not needed in forming the spermatozoa.

They are connected to each other by continuous tight junctions that seal the tubule into two compartments: the basal (close to the basal lamina) and adluminal (towards the lumen) compartment. Large molecules cannot pass between the basal and adluminal compartment - this is called the blood-testis barrier.

This is a schematic diagram of part of a seminiferous tubule, showing the stages in the formation of spermatozoa.

Like all epithelial cells, the Sertoli cells are avascular. Sertoli cells suport the germ cell progenitors and help to transfer nutrients from the nearby capillaries. The developing spermatogonia rely on the Sertoli cells for all of their nourishment. The blood-testis barrier formed by the Sertoli cells effectively isolates the developing spermatogonia, spermatocytes, spermatids and mature spermatozoa from blood. Differentiating spermatozoa nestle in pockets in the peripheral cytoplasm of these cells.

Sertoli cells also produce testicular fluid, including a protein that binds to and concentrates testosterone, which is essential for the development of the spermatozoa. They also help to translocate the differentiating cells to the lumen, and phagocytose degenerating germ cells and surplus cytoplasm remaining from spermiogenesis.

Blood-testis barrier: Importantly, the basal regions of the lateral borders of the Sertoli cells are connected to each other by continuous tight junctions. These divide the tubules into two separate compartments. The mitotic spermatogonia remain in the basal compartment. Differentiating progeny enter the adluminal compartment, and are sealed off from the basal compartment. If novel antigens are expressed on the haploid cells, then it is less likely that they will be detected by the immune system in this sealed off compartment. The developing cells are also in a very protective environment.

This diagram shows a single seminiferous tubule, and cells in the interstitial tissue outside the tubule. In thisconnective tissue between the seminiferous tubules there are large cells with eosinophilllic cytoplasm called Leydig cells.

There are also cells called myoid cells surrounding the basement membrane, which are squamous contractile cells, and generate peristaltic waves in the tubules.

Leydig cells are 'interstitial' cells (as they lie between the tubules). They have pale cytoplasm because they contain many cholesterol-lipid droplets. The Leydig cells make and secrete testosterone, in response to lutenising hormone from the pituitary. The cholesterol is used in the first step of testosterone production.

Testosterone promotes production of spermatozoa, secretion from the accessory sex glands, and acquisition of male secondary characteristics. This process does not start until puberty when LH stimulates the Leydig cells to produce testosterone.

FSH stimulates the Sertoli cells to secrete androgen-binding protein into the lumen of the seminiferous tubules. Binding of testosterone in the lumen provides a local testosterone supply for the developing permatogonia.