Mesenchymal cells are generally non-polarized, loosely associated and surrounded by extracellular matrix. Fibroblasts in culture can be considered representative of such mesenchymal cells: their adhesive contacts to the matrix are mediated primarily by cell-substrate adhesion molecules. For instance, fibroblasts express various integrin receptors, of which some (eg a5 pl, a fibronectin receptor) are concentrated in the so-called focal contactd4). Fibroblasts also produce various ligands for epithelial-specific receptor tyrosine kinases, eg SF/HGF, keratinocyte growth factor, and neu differentiation factor (neuregulin), that are important for signal exchange between mesenchmal and epithelial cell compartments (').

Morphologically, epithelial cells are entirely different: they form continuous cell layers, and they are generally polar and much less mobile. In single-layered epithelia (eg the mature intestine), apical and basolateral cell surfaces are separated by tight junctions. An example of a multilayered epithelium is the skin, where basal cells (stem cells) are cov-