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Visceral adipose tissue (VAT) depots are associated with the adverse metabolic consequences of obesity, such as insulin resistance. The developmental origin of VAT depots and the identity and regulation of adipocyte progenitor cells have been active areas of investigation. In recent years, a paradigm of mesothelial cells as a source of VAT adipocyte progenitor cells has emerged based on lineage tracing studies using the Wilms' tumor gene, Wt1, as a marker for cells of mesothelial origin. Here, we show that Wt1 expression in adipose tissue is not limited to the mesothelium but is also expressed by a distinct preadipocyte population in mice and humans. We identify keratin 19 (Krt19) as a highly specific marker for the adult mouse mesothelium and demonstrate that Krt19-expressing mesothelial cells do not differentiate into visceral adipocytes. These results contradict the assertion that the VAT mesothelium can serve as a source of adipocytes.