Under certain genetic conditions, the mouse gonad can develop as an ovotestis - displaying both male and female cell types. Where this occurs, it has generally been observed that there is a spatial bias to the organisation of male versus female tissue, with the centre of the gonad containing male Sertoli cells, and the poles containing female granulosa cells. It was thought that this reflects a centre-to-pole bias in the timing of expression of the genes that control Sertoli cell fate. However, Blanche Capel and colleagues now provide evidence that the centre bias in Sertoli cell fate arises from a centre bias in the ingression of precursor cells. Although this biased ingression happens in both sexes, the male pathway is dependent on a cell density-dependent signal, leading to a centre-to-pole gradient in the repression of granulosa fate and commitment to the Sertoli fate. Given that there is a limited time window for repression of granulosa fate, this can explain the spatial patterning of the ovotestis - delays in ingression of cells at the poles during this critical time period would lead to these cells adopting the default granulosa fate. Although the mechanisms controlling the spatial patterning of ingression have yet to be uncovered, this work demonstrates the importance of both spatial and temporal control of cell behaviours for the appropriate patterning of a tissue.
