The expression of villin, an actin-binding protein and major structural component of the brush border of specialized absorptive cells, was studied during mouse embryogenesis. We show that the ontogeny of villin expression is limited to the epithelial cell lineages of the digestive and uro-genital tracts and accounts for the tissue-specific expression observed in adult mice. This spatiotemporal pattern of villin expression is distinctive in sequence, intensity, regional distribution and polarization. During the development of the primitive gut, villin is faintly and discontinuously expressed in the invaginating foregut but it is expressed in every cell bordering the hindgut pocket. Later, villin expression increases along the developing intestine and concentrates in the brush border of the epithelium bordering the villi. In gut derivatives, villin is present in liver and pancreas primordia but only biliary and pancreatic cells maintain a faint villin expression as observed in adults. In the urogenital tract, mesonephric tubules are the first mesodermal derived structures to express villin. This expression is maintained in the ductuli efferentes, paradidymis and epoöphoron. Villin then appears in the proximal metanephric tubules and later increases and concentrates in the brush border of the renal proximal tubular epithelial cells. Thus villin expression can be considered as an early marker of the endodermal cell lineage during the development of the digestive system. Conversely, during the development of the excretory and genital system, villin is only expressed after the mesenchyme/epithelium conversion following the appearance of tubular structures.

These observations emphasize the multiple levels of regulation of villin gene activity that occur during mouse embryogenesis and account for the strict pattern of tissue-specific expression observed in adults. In the future, regulatory elements of the villin gene may be used to target the early expression of oncogenes to the digestive and urogenital tracts of transgenic mice.