Schematic representation of the presence of stem cells in the mammalian gonads. VSELs are present in both ovary and testis. In the ovary the stem cells are lodged in the ovary surface epithelium whereas they are located in the basal epithelial layer in the testicular seminiferous tubules. VSELs (1–4 um) are relatively quiescent, undergo asymmetric cell division and give rise to the progenitors (5-10 um) which divide rapidly, undergo clonal expansion and symmetric cell divisions to form cysts and eventually differentiate into oocytes and sperm respectively in the ovary and testis. Presence of VSELs in the gonads needs to be acknowledged in addition to the recent understanding of gonadal stem cell biology proposed by Woods and Tilly .