Cells of multicellular organisms are semi-fluid creatures. Even when they form specific cell-cell adhesions, they cannot create a defined shape or a tissue-specific architecture. Cartilaginous organs, such as ears and noses, exemplify the fact that form is imprinted in the extracellular matrix (ECM), which leads to the conclusion that cells must have the ability to shape the ECM in which they reside. This seems to be true for most tissues. The role of the ECM as an integrator of cells into functional assemblies with defined architecture is unique to multicellular organisms. The evolution of multicellularity became possible as a consequence of cells acquiring two new properties: first, cell surface macromolecular complexes that function in cell-cell binding; and, second, an ECM that integrates cells into three-dimensional structures. These two new properties allowed the evolution of the two basic types of cells—epithelial and mesenchymal. The appearance of the latter, a fibroblast-like cell with abundant filopodia, enabled the sculpturing of the ECM and the formation of complex tissue-specific architectures.


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pp. 339-356
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