The development of the spermatozoa takes place in the male gonads, the testes. In vertebrates and insects, the testes are composite organs consisting of numerous seminiferous tubules, or seminiferous lobules, converging toward common ducts which lead the mature sperm to the exterior.
Spermatogenesis is a continuous process, and various stages of development of the sperm may be observed in the seminiferous tubules at the same time. In the tubule there is, however, an orderly arrangement of cells undergoing different phases of development.
In insects, the proximal ends of the tubules contain the spermatogonia— the cells undergoing proliferation by mitosis. Further down the tubule, the cells in the growth and maturation stages (the spermatocytes) are found. The ripe spermatozoa fill the most distal parts of the tubules.
In vertebrates, the arrangement of the cells in the seminiferous tubules is somewhat different – all stages, from spermatogonia to mature spermatozoa, may be found at the same level of the tubule, but while the early stages (the spermatogonia) are located at the outer surface of the tubule in an epithelium-like arrangement next to the basement membrane, the later stages of differentiation, including ripe spermatozoa, lie nearer to the lumen of the tubule.
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A special feature in the testes of vertebrates is the presence in the seminiferous tubules of somatic cells, which assist the developing spermatozoa by anchoring the differentiating cells and possibly nourishing them during the latter part of sperm development.
These cells, known as Sertoli cells, are tall columnar cells attached proximally to the basement membrane and reaching distally to the lumen of the tubule. The Sertoli cells have large pale nuclei with conspicuous nucleoli, thus differing from the rather dense chromatin-rich nuclei of spermatogonia and spermatocytes.
The cells differentiating into spermatozoa become partially embedded in the cytoplasm of the Sertoli cells, the future heads of the spermatozoa pointing toward the base of the Sertoli cells and the tails growing out toward the lumen of the seminiferous tubule. There are no Sertoli cells in the seminiferous tubules of insects, but similar cells are found in the testes of molluscs.
The spermatogonia, as already stated, are found in vertebrates next to the basal membranes of the seminiferous tubules, and in microscopic preparations many of these may be seen to be in mitosis. While part of the spermatogonia remain in this condition and form a source of new sex cells throughout the reproductive life of the animal, some of the cells which are produced move toward the lumen of the tubule and enter the next phase of spermatogenesis: the phase of growth.
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The cells in this stage are called primary spermatocytes. The growth of the spermatocytes is actually very limited, though as a result they become perceptibly larger in volume than the spermatogonia (roughly by a factor of 2).
However, the main feature of these cells is that they enter into the prophase of meiotic divisions which are of the greatest importance in the reproductive cycle of all organisms, but which can be dealt with only in their essentials here.