Stem Cells in the Mammalian Gonads

Besides spermatogonial stem cells (SSCs) and ovarian stem cells (OSCs), a novel population of pluripotent stem cells termed very small embryonic-like stem cells (VSELs) has been reported in both adult mouse and human testes and ovaries. VSELs and SSCs/OSCs are developmentally linked to each other. VSELs are relatively quiescent, small-sized stem cells that undergo asymmetrical cell divisions (ACDs) whereby they self-renew and give rise to the slightly bigger SSCs/OSCs which in turn undergo symmetrical cell divisions (SCDs) and clonal expansion to form germ cell chains/nests before further differentiation into gametes. Comparison of VSELs and SSCs/OSCs for their potential to differentiate into sperm/oocytes is irrelevant since VSELs only undergo ACD to give rise to SSCs/OSCs that further differentiate into gametes. Being relatively quiescent, VSELs survive oncotherapy and can be manipulated to regenerate nonfunctional gonads of cancer survivors, and thus there is possibly no need to bank testicular/ovarian tissue prior to oncotherapy. Being developmentally linked to the primordial germ cells (PGCs) which are the natural precursors to the gametes, VSELs differentiate into haploid sperm/oocyte-like structures in vitro when cultured on appropriate feeder support, in the absence of a cocktail of growth factors. VSELs express receptors for pituitary and sex hormones (FSHR, ER) and thus get directly stimulated/affected by their circulating levels. Excessive self-renewal of VSELs in the gonads may initiate testicular and ovarian cancers. To conclude, VSELs can be targeted to regenerate the gonads of patients with gonadal insufficiency including cancer survivors and are excellent candidates to differentiate into gametes in vitro.