Structure and organisation of the human testis and seminiferous tubules.

A. Macroscopic appearance. The testis is divided into compartments (lobules) separated by connective tissue. There are an estimated 250 lobules per testis, which vary in size; for clarity only 7 lobules are illustrated here. B. (i) Diagrammatic representation of the highly convoluted organisation of 3 seminiferous tubules (blue, black, grey) within a lobule. (ii) A magnified cross section through the lobule in i reveals how the tubules would appear in thin microscopic sections. Dashed lines (above section) and solid lines (below section) join the contiguous tubules. C. Cross section through an individual seminiferous tubule. Spermatogonia, located at the basal lamina of the seminiferous tubules, comprise a heterogeneous population of diploid germ cells. These can be classified according to their morphologies and correspond to three main maturation stages: A_dark spermatogonia, which are considered to represent the reserve stem cell population; highly proliferating A_pale spermatogonia; and more mature B spermatogonia that give rise to primary spermatocytes. Primary spermatocytes undergo meiosis to form secondary spermatocytes that differentiate to form spermatids, which when fully formed are released into the central lumen as spermatozoa that progress to the rete testis. Spermatogenesis is supported by the presence of the somatic Sertoli cells. The vascular and connective tissue network lie external to the wall of the seminiferous tubule.