The Morphogenetic Toolkit

Abstract

The most dramatic aspect of embryogenesis is the appearance of tissue structure from a simple mass of cells, the process of morphogenesis. In the past we have often had to make do with mere descriptions of these events, but recent progress in the study of the molecular basis of cell behaviour has begun to elucidate the details of the interactions and the forces that underpin the large scale changes in cellular organisation responsible for the formation of organs. It is now clear that the abilities of three sets of geographically distinct molecules control many of these events in both vertebrate and invertebrate embryos: the first is located in the matrix surrounding the cells, the second in cell membranes and the last involves the cytoskeleton of the participating cells. As to the forces responsible for reorganising cells, these are partly generated by this cytoskeleton, partly by growth and partly by the exertion of pressure. The properties that control cell behaviour in embryos include those responsible for generating cell movement and directing it, those that modulate cell adhesion and those that lead to the reorganisation of cell sheets. These properties as a whole can be viewed as a toolkit, with unique subsets being used to form particular tissues. In this review, we will consider the individual tools and how they are responsible for the morphogenesis of a range of tissues that include the movements of the neural crest, the formation of ducted glands and the formation of the tissues of the eye. The review ends by pointing to some unsolved problems.