The extracellular matrix in epithelial biology: shared molecules and common themes in distant phyla.

Epithelia are polarized structures, asymmetric by definition because they represent the boundary between what is topologically inside and what is outside an organism. Epithelial cells provide a physical barrier with the outside world and regulate bidirectional chemical exchange, as in absorption of nutrients or oxygen and secretion of wastes or other materials. Such cells are extremely varied and ancient, being found in multiple forms within most organs and in all animal taxa. Epithelia form specialized three-dimensional structures suited to their physiological role, such as the highly branched structures seen in many secretory organs, the flattened bilayer of cells in the wings of a fly, or the stratified layers of cells in vertebrate skin. These tissues are distinctive not just in the forms the cells assume, but also in the nature and structure of the extracellular matrix (ECM) deposited asymmetrically adjacent to the epithelial sheet. Epithelial histogenesis, the development of these elaborate structures, requires cells to regulate their pattern of gene expression, their cell cycle status and rate of apoptosis, their shape, and their motility. They must also respond to soluble factors such as cytokines and interact with other cell types, both epithelial and mesenchymal. In the course of these structural and functional changes, cells interconvert between epithelial and mesenchymal phenotypes, a transition that is seen repeatedly, starting in early development.