Fig. 8. Expansion of Sf1 expression in gonads and mesonephroi of Pod1 KO mice. (A) In situ hybridization analyses and immunohistochemistry of transverse sections and whole-mount genital ridges of XY gonads (go) at 11.5 dpc are shown. Gonads were labeled with a riboprobe against Pod1, or double-labeled with antibodies to Sf1 (green) and PECAM (red). Pod1 expression was concentrated in the coelomic epithelium (arrows) and the boundary of the gonad and mesonephros (arrowheads). Sf1-positive cells were evenly scattered in the interior of the control XY gonads. In the Pod1 KO XY gonads, Sf1 expression was increased in both the coelomic epithelium (arrow) and the interior of the gonad. A large population of Sf1-positive cells was seen at the boundary region between the gonad and the mesonephros (arrowhead). Note that the coelomic epithelium and the boundary between the gonad and the mesonephros are domains where Pod1 is expressed. (B) Confocal images of 13 dpc gonads double-labeled with antibodies to Sf1 (green) and ß-gal (red). In 13 dpc XX and XY control gonads, Sf1 is not expressed in cells that express the Pod1/lacZ reporter gene. By contrast, Sf1 is co-expressed in mutant Pod1/lacZ-expressing cells (arrowheads, yellow). (C) Model for Pod1 function in XY gonad development. Pod1 is proposed to repress Sf1 expression in a pluripotent interstitial cell precursor (in the mesonephros and/or coelomic epithelium, ce), thereby permitting differentiation of several interstitial cell lineages, including fetal Leydig cells (Le), peritubular myoid cells (PMC) and pericytes (Pe). Loss of Pod1 leads to ectopic expression of Sf1, and prematurely commits the progenitor cells to a steroidogenic cell lineage.