Neurulation and the cortical tractor model for epithelial folding.

We present here a new model for epithelial morphogenesis, which we call the 'cortical tractor model'. This model assumes that the motile activities of epithelial cells are similar to those of mesenchymal cells, with the added constraint that the cells in an epithelial sheet remain attached at their apical circumference. In particular, we assert that there is a time-averaged motion of cortical cytoplasm which flows from the basal and lateral surfaces to the apical region. This cortical flow carries with it membrane and adhesive structures that are inserted basally and resorbed apically. Thus the apical seal that characterizes epithelial sheets is a dynamic structure: it is continuously created by the cortical flow which piles up components near where they are recycled in the apical region. By use of mechanical analyses and computer simulations we demonstrate that the cortical tractor motion can reproduce a variety of epithelial motions, including columnarization (placode formation), invagination and rolling. It also provides a mechanism for driving active cell rearrangements within an epithelial sheet, while maintaining the integrity of the apical seal. Active repacking of epithelial cells appears to drive a number of morphogenetic processes. Neurulation in amphibians provides an example of a process in which all four of the above morphogenetic movements appear to play a role. Here we reexamine the process of neurulation in amphibians in light of the cortical tractor model, and find that it provides an integrated view of this important morphogenetic process.