The mammalian testis contains male germ cells as well as a number of somatic cell types, including supporting cells (such as Sertoli cells) and interstitial cells (such as Leydig cells). Although the origin and differentiation of germ cells has been well-characterized, the developmental course of somatic lineages in the testis is ill-defined. Now, Humphrey Yao and colleagues construct a comprehensive map of somatic cell lineage progression in the mouse testis (p. 3700). Their lineage-tracing studies reveal that both supporting and interstitial cells arise from a population of WT1-expressing progenitors. A sub-population of these, marked by SOX9 expression, then gives rise to Sertoli cells of the testis cords. The researchers demonstrate that the interstitial progenitors further diversify, based on differential Notch and Hedgehog pathway activation, giving rise to foetal steroid-producing Leydig cells and non-steroidogenic progenitors. Finally, the authors report that non-steroidogenic progenitors, which are maintained in an undifferentiated state throughout foetal development, eventually become adult Leydig cells. Together, these findings provide key insights into the lineage progression events that occur during testis development in mammals.
