... and differentiation

A crucial event during early development of the testis is the specification of somatic cell lineages such as the Leydig cells. Little is known about the origin of fetal Leydig cells, or of the signals that induce them to differentiate. Now, however, reporting in Genes and Development, Blanche Capel and colleagues provide evidence that the Desert Hedgehog (DHH)/Patched 1 (PTCH1) pathway triggers Leydig cell differentiation.

Fetal Leydig cells are responsible for the initial masculinization of an embryo. They are first identifiable in the interstitium of XY gonads, where they express enzymes that are needed for the production of male sex hormones. There is some evidence that Leydig cell precursors migrate to the gonad from the mesonephros; but whatever their origin, most are present in the gonad by 11.5 days post coitum (dpc).

The PTCH1 protein is expressed in the interstitium of XY gonads at 12.5 dpc, which made Capel and co-workers wonder whether this receptor — and its ligand, DHH — might be involved in the differentiation of Leydig cells. To test this, they first studied the expression patterns of the Ptch1, Dhh and P450 side chain cleavage enzyme ( Scc ) genes (Scc is a marker for fetal Leydig cells). Expression of Dhh began at 11.5 dpc in XY gonads, as has been observed previously. At 12.5 dpc, most of the interstitial cells also expressed Ptch^LacZ — but not Scc. However, by 13.5 dpc, most of the Ptch^LacZ-positive cells were also expressing Scc.

These expression patterns support the idea that DHH signalling is involved in the early development of Leydig cells. So Capel and colleagues next asked what effect the loss of this pathway would have on Leydig cell differentiation. They analysed the expression of Scc in Dhh^+/+, Dhh^+/− and Dhh^−/− XY gonads at 13.5–14.5 dpc. Scc staining was seen at the centre of Dhh^+/+ and Dhh^+/− gonads at 13.5 dpc, yet it was absent from 70% of the Dhh^−/− gonads at this stage. Even by 14.5 dpc, the Dhh^−/− gonads showed only very sparse staining for Scc.

The authors next investigated why the lack of DHH signalling leads to defects in Leydig cell differentiation. One idea was that it might affect the migration of cells from the mesonephros to the gonad. However, two independent experimental approaches showed that this migration process was normal in Dhh^−/− gonads. Another possibility was that DHH signalling is involved in the proliferation or survival of fetal Leydig cell precursors in the interstitium of XY gonads. Again, however, there was no difference in these processes in the presence or absence of DHH signalling.

So how does the DHH pathway affect differentiation? Capel and colleagues think that its main role is in the upregulation of Scc in Leydig precursor cells. Scc is the target of the steroidogenic factor 1, and there is evidence that this, too, is upregulated in Leydig cells. By upregulating these factors, the DHH pathway could trigger the differentiation of precursors into Leydig cells. However, as the authors point out, not all cells that express Ptch1 differentiate as Leydig cells, so other signals probably combine with the DHH pathway to specify Leydig cell fate.