The developing mammalian gonad comprises multiple different cell types, each with unique and important roles. Among these are the germ cells, which eventually become gametes and are crucial for generational inheritance, as well as supporting cells and steroidogenic cells, which support and nourish the gametes and provide important hormonal regulation, respectively. Despite the importance of supporting and steroidogenic cell types, the molecular mechanism that leads to their specification remains unclear. In this issue, on p. 44, Fei Gao and colleagues uncover a mechanism by which the expression of Wilms’ tumor 1 (Wt1) directs the lineage specification of supporting cells via the suppression of steroidogenic factor 1 (Sf1). Sf1 usually directs the specification of steroidogenic cells; however, the authors show that Wt1 binds directly to the promoter region of Sf1 in both sexes, supressing its expression. Deletion of Wt1 in the mouse undifferentiated genital ridge somatic cells before sex determination completely blocked the differentiation of the supporting Sertoli (in male) and granulosa (in female) cells, and resulted in the differentiation of steroidogenic cells instead. This study provides novel insight into somatic cell differentiation during gonadal development and provides a molecular mechanism for the specification of the supporting cells in both sexes.

• © 2017. Published by The Company of Biologists Ltd