ABSTRACT
SUMMARY During development, distinct tissues arise by differentiation and morphogenesis from initially uniform cells. We will here concentrate on the discussion of the morphogenesis of epithelia, which form an astounding array of different structures, e.g. hollow cysts, single or multi-layered plates or branched tubules. Signals for differentiation and morphogenesis of epithelia are frequently given by surrounding mesenchymal tissuses which grow and differentiate concomitantly. Tyrosine kinase receptors and their ligands can constitute paracrine signaling systems which provide a molecular basis for signal exchange between different cellular compartments like mesenchyme and epithelia. Scatter factor/hepatocyte growth factor and the c-met receptor, neuregulins and the family of c-erbB receptors, FGFs and their receptors as well as GDNF and cret constitute such paracrine signaling systems, which control growth, morphogenesis and differentiation of epithelia. Recent experiments with cultured epithelial cells have shown that different tyrosine kinase receptors can elicit different morphogenic responses. The important roles of tyrosine kinase receptors and their ligands in the development of epithelia were substantiated by the analysis of mice that carry targeted mutations or transdominant receptors. Together, these results demonstrate an essential role of such paracrine acting signaling systems in the control of the development of epithelial cells types.
