Spermatogonial Stem Cells
Spermatogonial stem cells (SSCs) are a subtype of undifferentiated spermatogonia.
During foetal development gonocytes develop from primordial germ cells and following this SSCs develop from gonocytes in the testis. SSCs are the early precursor for spermatozoa and are responsible for the continuation of spermatogenesis in adult mammals. The stem cells are capable of dividing into more SSCs which is vital for maintaining the stem cell pool. Alternatively they go on to differentiate into , spermatids and finally spermatozoa.
One SSC is the precursor for multiple spermatozoa and therefore SSCs are much less numerous in the testes than cells undergoing spermatogenesis.
In Humans
Undifferentiated spermatogonia can be split into 2 groups; A Dark (Ad) and A Pale (Ap)
Ad spermatogonia are reserve stem cells. These cells are capable of dividing to produce more SSCs but usually do not. Ap spermatogonia are actively dividing to maintain the stem cell pool. B1-B4 spermatogonia encompass the differentiating spermatogonia and are no longer considered to be stem cells.
Most research into SSCs has been carried out in rodents. The subtypes of spermatogonia differs between mice and mammals.
In mice
A Single (As) spermatogonia are capable of creating 2 separate daughter SSCs when they divide or the daughter cells can join and form A Paired (Apr) spermatogonia.
Both As and Apr spermatogonia are undifferentiated. Chains of these cells form and are referred to as A Aligned (Aal). Aal spermatogonia differentiate and thus are no longer classed as stem cells. They go on to divide 6 times eventually forming B type spermatogonia.
The most important somatic cells that support regulation of SSCs are Sertoli cells Various other somatic cells in the interstitial tissue support Sertoli cells such as Leydig cells and peritubular myoid cells therefore indirectly influencing SSCs and the location of their niche.
Spermatogonia stem cells in mammals are found between the basal membrane of the seminiferous tubules and the Sertoli cells. They remain here until the meiotic prophase stage of meiosis. Here the spermatocytes pass through the basal membrane via the sertoli cell barrier.
SSCs stay within their niche where they are encouraged to self renew. When they move past the basal membrane they differentiate due to cell signals.
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