Cell aggregation precedes the onset of Sox9-expressing preSertoli cells in the genital ridge of mouse

Abstract

Sox9 is expressed at the onset of the genital ridge formation in both sexes. It is assumed that Sry, the testis determining gene, turns Sox9 on in male embryos because it is turned off in female embryos. Spatial expression of Sry follows a cranio-caudal pattern. Here, we asked if Sox9 is expressed in the same cell lineage and with a similar pattern as Sry. A correlative study between the structural changes in the genital ridge and the immunocytochemical localization of Sox9-positive cells was undertaken. We used a transgenic strain expressing the green fluorescent protein (GFP) that considerably enhanced the cell context where the first Sox9-positive cells appear. Although Sox9-positive cells are located among loose mesenchymal cells by stages of 8–14 tail somites (ts) in both sexes, they are absent in the thickening coelomic epithelium of females. At 15 ts the first Sox9-positive cells appear within the core of the condensed cells only in male genital ridges. At 17 ts, a gradient of Sox9-positive cells in males is apparent, closely following the cranio-caudal pattern of cell aggregation seen in genital ridges of both sexes. Hence, our results suggest that Sox9 is expressed only in loose mesenchymal cells in both sexes and that expression of Sox9 in males requires the prior aggregation of cells in the genital ridges. The correspondence of Sox9 and Sry pattern of expression supports that both genes are expressed in the preSertoli cell lineage in the core of the genital ridges.