Fig. 1

Novel strategy to restore gonadal function. Stem cells exist in various tissues including testis and ovary in close association with their niche which controls their fate. Niche factors support gene expression, proliferation and differentiation of stem cells thereby maintaining tissue homeostasis. When the gonads (ovary or testis) are exposed to cytotoxic injury, radio- or chemo-therapy, ‘true’ stem cells survive the insult (due to their quiescent nature) and rather increase in numbers [25,26,27, 31] in an attempt to restore homeostasis. However, the niche undergoes irreversible changes due to the insult. As a result, although stem cells exist; they are unable to differentiate and restore gonadal function. On transplanting niche cells (mesenchymal stromal cells or Sertoli cells), surviving VSELs differentiate and thus restore gonadal function [25, 26]. This strategy of manipulating endogenous stem cells to regenerate non-functional gonads after oncotherapy is far superior to making gametes in vitro. Transplanting mesenchymal stromal cells (Tables 1 and 2) results in positive outcome in animal studies and was confirmed by a recent meta-analysis [141]. A baby girl has been born by transplanting mesenchymal cells in POF ovary [91]. This successful strategy obviates the need to differentiate gametes in vitro and cryo-banking of gonadal tissue prior to oncotherapy. [VSELs: Very small embryonic-like stem cells]