LAUSR

Cumulus expansion is necessary for the release of a fertilizable oocyte from the ovary, which is critical for the normal fertilization of mammals. Cumulus expansion requires cooperation between epidermal growth factor (EGF)‐like growth factors and oocyte paracrine factors. Growth differentiation factor 9 (GDF9) and bone morphogenetic protein 15 (BMP15) are well‐known paracrine factors secreted by oocytes. In addition, transforming growth factor‐β2 (TGFB2) was primarily expressed in oocytes and its membrane receptors type 1 receptor (TGFBR1) and type 2 receptor (TGFBR2) were located in cumulus cells. In our present study, TGFB2 induced expansion of oocytectomized (OOX) complexes and increased the expression of expansion‐related genes in the presence of EGF, suggesting that TGFB2 enables cumulus expansion. Inhibition of TGF‐β signaling with SD208 blocked TGFB2‐promoted cumulus expansion. Furthermore, in the culture of OOX complexes from mice of Tgfbr2‐specific depletion in granulosa cells, TGFB2‐promoted cumulus expansion and the expression of expansion‐related genes were impaired. These results suggest that TGFB2 could induce cumulus expansion through TGFBR‐SMAD2/3 signaling. Tgfb2‐specific depletion in oocytes using Zp3‐Cre mice had no effect on cumulus expansion in vivo, possibly due to the compensatory effect of other cumulus expansion‐enabling factors. Taken together, TGFB2 is involved in expansion‐related gene expression and consequent cumulus expansion.