TYLER, Seth; SMITH, Julian P.S., III: Evolution of the metazoan integument

The seminal step that set the stage for complexly structured metazoans–namely, those with tissues and organs–was the origin of epithelia. Once cells were able to organize themselves into sheets that could physiologically isolate compartments, particularly the outside environment from an internal one, they provided capacity for organisms to control the environments in which their cells functioned and allowed differentiation of specialized tissues and organs. The origin of epithelia hinged on the development of cell junctions, including sealing junctions between the cells and a variety of junctional complexes to extracellular matrix along the basal and apical ends of the cells (to basal lamina and cuticle). Junctions providing for communication among epithelial cells, including gap junctions and other plasmatic connections, were also essential for turning these collections of cells into functional units. We compare these special features of epithelial cells in the integuments of cnidarians and various phyla of lower bilaterians and propose ways in which such characters may have evolved from the more plesiomorphic conditions in sponges. Parsing the elements in the complicated interplay among epithelial cells and between them and the extracellular matrices to which they are bound provides new insights into the evolution of integument.