Evolution and cell physiology. 4. Why invent yet another protein complex to build junctions in epithelial cells?

The formation of the first epithelium was an essential step for animal evolution, since it has allowed coordination of the behavior of a cell layer and creation of a selective barrier between the internal medium and the outside world. The possibility of coupling the cells in a single layer has allowed morphogenetic events, such as tube formation, or gastrulation, to form more complex animal morphologies. The invention of sealed junctions between cells has allowed, on the other hand, creation of an asymmetry of nutrients or salts between the apical and the basal side of the epithelial layer. Creation of an internal medium has led to homeostasis, allowing the evolution of more complex physiological functions and the emergence of sophisticated animal shapes. During evolution, the origins of the first animals coincided with the invention of several protein complexes, including true cadherins and the polarity protein complexes. How these complexes regulate formation of the apicolateral border and the adherens junctions is still not fully understood. This review focuses on the role of these apical polarity complexes and, in particular, the Crumbs complex, which is essential for proper organization of epithelial layers from Drosophila to humans.