Stem cells are present at various sites in the female and male reproductive tract of adults, which include the gonads, the endometrium, and decidua as well as the placenta. Their characteristics and potencies vary as also do their regulation of stemness and their immunogenicity.
Stem cells possess an enormous potential for novel treatment strategies of a wide spectrum of diseases. Self-renewal and a wide degree of potency are found not only in embryonic stem cells (ESC) but also in different organs of adult individuals in all species. The reproductive tract is an organ system that requires fast and well-regulated reconstitution of a variety of cells and tissues, which indicates the presence of comparatively high proportions of stem cells.
Fig.1 Cell lineages of the developing gonads. (Pask, 2016)
Existing understanding is that the mammalian testes harbor spermatogonial stem cells (SSCs) and SSCs are the only stem cells in the testes. A lot of work has been done to show the presence of ovarian stem cells (OSCs) in adult mammalian ovaries. It has extensively reported the presence of a novel population of pluripotent, very small embryonic-like stem cells (VSELs) in addition to SSCs/OSCs in adult mammalian gonads.
VSELs are the quiescent stem cells that undergo asymmetric cell divisions whereby they self-renew and also give rise to slightly bigger progenitors (SSCs/ OSCs) which in turn undergo symmetric cell divisions and clonal expansion before undergoing meiosis and differentiation into gametes. Thus, VSELs are developmentally linked to SSCs/OSCs in the testes and ovaries. Being quiescent, VSELs survive monotherapy and can be manipulated to restore gonadal function in individuals with gonadal insufficiency including cancer survivors.
During testicular development, primordial germ cells (PGCs) on arriving in the gonadal ridge get converted into gonocytes which get enclosed by the Sertoli cells. They continue to proliferate and then get mitotically arrested as spermatocytes. Mammalian testes harbor two populations of stem cells including relatively quiescent, true stem cells VSELs, and actively dividing progenitor SSCs. It is very likely that VSELs possibly initiate testicular cancer.
More than 90% of ovarian cancers arise in the surface epithelium. Epithelial-mesenchymal transition (EMT) is considered a major process for the conversion of early-stage ovarian tumors to invasive and metastatic malignancies and promoting the aggressiveness of ovarian cancers. It has been reported that it is the stem cells lodged in the OSE that possibly initiate ovarian cancers, whereas EMT of the epithelial cells may lead to the formation of myofibroblasts that exist at the frontal end of cancer during metastasis.
Accumulating literature suggests that the altered biology of VSELs possibly initiates testicular and ovarian cancers. It will be of interest to investigate whether altered stem cells and their niche result in various pathologies like reduced sperm count, infertility, and menopause. Researchers focus on stem cell therapy in:
Reference
For Research Use Only. Not For Clinical Use.
