The Cellular Basis of Epithelial Morphogenesis

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Abstract
The definitive trait of an epithelium is that its cells form cohesive cell sheets with little acellular material interposed between the cells. Nature has employed this relatively simple arrangement to fashion tissues of enormous functional and morphological diversity. In adult organisms, epithelia cover and protect the body, line the surfaces that absorb nutrients and metabolites from the environment (as in the lung and intestine), and form the tubes, ducts, and acini of glandular organs. They engage in transport, filtration, endocrine and exocrine synthetic functions, and the maintenance of chemical and electrical differentials between body compartments. Epithelia also play the lead roles in morphogenesis. During development, epithelial movement in gastrulation is responsible for much of the reorganization that lays down the overall body plan, and the folding, spreading, budding, cavitation, and delamination of epithelia produce the basic structure of most organs. In the course of morphogenesis, epithelia can form clusters, cords, or tongues of cells. They can also disperse, giving rise to mesenchymal cells, which in turn either form nonepithelial structures or reaggregate to form new epithelia. The means by which epithelial cells produce this wealth of form and function is the subject of this chapter.