Epithelial cell morphology and cytoskeletal organization

The extracellular matrix (ECM), including the basement membrane, serves as an interface between epithelial cells and the surrounding mesenchyme. Therefore, the inductive interactions that occur between groups of mesenchymal and epithelial cells to regulate organ development are transmitted through the ECM. These inducing signals include both diffusible growth factors and components of the ECM itself. Integrins are heterodimeric cell surface receptors that specifically bind components of the ECM and therefore are obvious candidates to transduce signals that elicit behavioral responses determined by the composition of the surrounding ECM (Hynes, 1992). Behaviors such as cell division, migration and differentiation have either been demonstrated to be directly modulated by integrin:matrix interactions or found to be aberrant in embryos or animals carrying mutations in particular integrin genes (Hynes, 1996). The observation that integrin:matrix interactions result in cytoskeletal rearrangements in cultured cells has fostered intense study (Clark and Brugge, 1995; Haimovich et al., 1993). Focal adhesions have been identified as points of cellular attachment to the underlying matrix, where integrin clustering results in the assembly of an array of cytoskeletal proteins (Burridge and Chrzanowska-Wodnicka, 1996). Stress fibers, composed of actin filaments, converge at focal adhesions, where they are physically linked to actin-binding proteins such as α-actinin, vinculin and talin, some of which are also physically linked to integrins (Burridge and Chrzanowska-Wodnicka, 1996). Therefore, integrins also serve 2925 Journal of Cell Science 112, 2925-2935 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 JCS0405