Localization of the Wilm's tumour protein WT1 in avian embryos.

The Wilms' tumour suppressor gene WT1 encodes a zinc-finger transcription factor which is essential for the development of kidney, gonads, spleen and adrenals. WT1-null embryos lack all of these viscerae and they also show a thin ventricular myocardium and unexpectedly die from cardiac failure between 13 and 15 days post coitum. We studied the localization of the WT1 protein in chick and quail embryos between stages HH18 and HH35. In early embryos, WT1 protein was located in specific areas of the coelomic mesothelium adjacent to the nephric ducts, the myocardium or the primordia of the endodermal organs (gut, liver and lungs). These mesothelial areas also showed localized expression of Slug, a zinc-finger transcription factor involved in epithelial-mesenchymal transitions. WT1+ mesenchymal cells were always found below the immunoreactive mesothelial areas, either forming a narrow band on the surface of the endodermal organs (gut, liver and lungs) or migrating throughout the mesodermal organs (mesonephros, metanephros, gonads, spleen and heart). In the developing heart, the invasion of WTI+ cells started at stage HH26, and all the ventricular myocardium was pervaded by these cells, presumably derived from the epicardium, at HH30. We suggest that WT1 is not required for the epithelial-mesenchymal transition of the coelomic mesothelium, but it might be a marker of the mesothelial-derived cells, where this protein would be acting as a repressor of the differentiation.