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Spermatogonial Stem Cells in the Testis
Spermatogonial stem cells (SSC’s) are a subtype of undifferentiated spermatogonia.

During foetal development gonocytes develop from primordial germ cells and following this SSC’s develop from gonocytes in the testis. SSC’s 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 SSC’s which is vital for maintaining the stem cell pool. Alternatively they go on to differentiate into spermatocytes, spermatids and finally spermatozoa.

One SSC is the precursor for multiple spermatozoa and therefore SSC’s are much less numerous in the testes than cells undergoing spermatogenesis.

Nomenclature

In Humans

Undifferentiated spermatogonia can be split into 2 groups; A Dark and A Pale.

A Dark spermatogonia are reserve stem cells. These cells are capable of dividing to produce more SSC’s but usually do not. A Pale 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 SSC’s 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 SSC’s when they divide or the daughter cells can join and form A Paired (Apr) spermatogonia.

Both As and Ap 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.

SSC Niche.

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 SSC’s 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.[3] 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.