Epithelial polarity and morphogenesis.

The adult form of a multicellular organism is shaped by a series of morphogenetic processes that organise the body into tissues and organs. Most of these events involve the deformation of sheets of epithelial cells that are highly polarised along their apical-basal axes and attached to each other by lateral junctions. Here we discuss the role played by modifications in the apical-basal polarity system in driving morphogenesis, with an emphasis on well-characterised events during Drosophila development. Changing the activity of polarity factors can alter the relative sizes of the apical, lateral and basal domains. This can drive transitions between cuboidal, columnar and squamous epithelial morphologies, to increase or decrease the surface area of an epithelial sheet. These changes can also cause epithelial cells to become wedge-shaped, which can drive tissue bending and invagination. In addition, it has recently emerged that the activity of apical-basal polarity factors can also be modulated in a planar polarised manner. By affecting the contractility of the actomyosin cytoskeleton and the stability of adherens junctions, changes within the plane of the epithelium can cause cell rearrangements that contribute to convergence and extension movements, boundary formation and cell alignment.