Spermatogonia are cells found in the gonads of male animals. In those species with seminiferous tubules, spermatogonia are attached to the basal lamina of the tubules. Spermatogonia are those male germ cells before entering meiosis. There are several types: undifferentiated or type A spermatogonia, and differentiating spermatogonia that can be types A1, A2, A3, A4, ln and B. In undifferentiated spermatogonia, there is a population of stem cells (type As) that can divide and produce two new stem cells (auto-renewing), and other population that just divide by mitosis to increase the number and starting differentiation (types Apair to Aal-16). Types A1 to B spermatogonias may divide by mitosis, but they are committed to entering meiosis and produce spermatozoa (Figure 1). As every other stem cell, type As spermatogonia are scarce, up to 0.03 % of the total germ cells in mice gonads.
The cellular path before beginning meiosis is complex and highly regulated. Spermatogonia differentiation process simultaneously occurs in groups of cells because cytokinesis is uncompleted and cell cytoplasms remain connected and sharing key molecules involved in differentiation. In this way, there are cohorts of clones that divide and differentiate coordinately.
The different stages of spermatogonia differentiation can be observed in one seminiferous tubule at the same time, in transversal view. This is because the differentiation happens in precise temporary waves that are initiated at different places and times in the seminiferous tubule, resulting in an asynchronous production of spermatozoa along the seminiferous tube, and therefore in the whole gonad.
In the embryo
During development, the formation of somatic cells of gonads and germ cells follow independent paths. The expression of the Sry gen in the embryo of males drives the differentiation of somatic cells of the gonadal primordia (or gonadal ridges) into Sertoly cells. Then, gonads becomes testis. If the Sry gen is not expressed, somatic cells become granulosa cells that constitute follicles, and therefore the gonad will be an ovary.
Primordial germ cells appear very early during embryo development. In mice, they are found in the epiblasts layer (E6 developmental stage, that is, 6 days of embryo development), much earlier than the formation of the gonad primordial. Then, primordial germ cells leave the embryo and stay in the allantois, and enter again the embryo to populate the already present gonad primordial, at E11. About 3000 cells enter the early gonad in mice. In males, primordial germ cells interact with Sertoli cells, and this interaction leads to the differentiation of Leydig and myoid cells from interstitial cells. At E12.5, the male fate of the gonad is established, and germ cells and Sertoli cells associated to form seminiferous cords that eventually become the seminiferous tubes.
Primordial germ cells initially undergo amplification by mitosis that enormously increases their number. Thus, they become the gonadocytes or pre-spermatogonia. They populate the seminiferous cords formed by Sertoli and myoid cells. Pre-spermatogonia remain quiescent untill birth in the center of the gonadal cords. After birth, they migrate to the periphery and become germ stem cells (As) of seminiferous tubules.
Phillips BT, Gassei K, Orwig KE. 2010. Spermatogonial stem cell regulation and spermatogenesis. Philosophical Transaction Royal Society B. 365: 1663–1678.