The cellular basis of epithelial morphogenesis.

Epithelial tissues are ubiquitous in metazoan organisms, performing many different functions and assuming a variety of shapes. This diversity of form and function is ultimately dependent on the behavior of the cells within the epithelia. For example, it is intercellular adhesion and the control of paracellular permeability by cell junctions that permit epithelia to form barriers and act as selective filters. It is cellular polarity that enables absorptive epithelia to extract materials from a particular side of the sheet; it is the collective contributions of cell proliferation, cellular translocation, and changes in cell shape that sculpt epithelia from simple sheets into folds, pouches and tubes. Clearly, a complete understanding of epithelial morphogenesis is inextricably entwined with questions of cell behavior in general, such as how any cell adheres, moves, and maintains its shape. The study of epithelial systems has lent considerable insight into these problems and should continue to do so, just as examination of the behavior and architecture of nonepithelial cells will undoubtedly clarify many aspects of the cellular events underlying epithelial morphogenesis. Although the action of individual cells ultimately shapes epithelial, coordination of that action is necessary for the development of a coherent tissue. Attention must therefore be given to integrative mechanisms in epithelial morphogenesis. How do the many cells in an epithelial sheet act in virtual unison during folding? What defines the boundaries of epithelial invaginations? How does an individual cell detect its position within, and thereby know its role in the morphogenesis of, the epithelial whole of which it is a part? At the most elementary level, epithelial cells interact via their physical attachments to one other. Even such rudimentary communication affects cell shape, movement, and possibly proliferation and also plays a part in the maintenance of epithelial polarity. Additional signals pass among epithelial cells by a number of other mechanisms as well, most notably electrical coupling. However, many questions remain regarding the quality and quantity of what is communicated between epithelial cells. Accordingly, elucidating the means by which supracellular order is maintained in epithelial tissues may still be regarded as the major problem in the study of epithelial morphogenesis.