Apical constriction and invagination: a very self-reliant couple.

A fundamental phenomenon in development is the capacity of sheets of cells to bend inward, thereby positioning some of the cells from these sheets below the surface on which they were originally placed. This process, known as invagination, generates folds or depressions in a previously uniform surface. Eventually, these groups of cells separate from their original neighbors and generate new internal organs. Invagination is at the origin of the formation of germ layers, such as the mesoderm and endoderm (Shook and Keller, 2008 and Solnica-Krezel, 2005), which have allowed the morphological complexity of the animal kingdom. In more general terms, cell invagination allows the formation of three-dimensional structures out of a cell monolayer. The collective phenomenon of invagination is very often accompanied by an individual cell behavior known as apical constriction, by which cells of columnar morphology shrink their apical surface and perimeter and become wedge-shaped. Indeed, it is widely accepted that the changes in shape produced by apical constriction, or the forces associated with this process, drive the collective invagination process. Along these lines, simulation models in collaboration with mathematicians and physicists have been put forward to account for the coupling of apical constriction and invagination and have even been used to study whether a causality linkage could lead apical constriction to invagination. However, several experimental evidences indicate that there is neither a causal relation nor a strict requirement between these two phenomena. In this context, here we discuss the relationship between cell apical constriction and cell invagination and more precisely on how the former contributes to collective cell invagination and why these two processes are so often together.