The control of cell motility during embryogenesis.

Morphogenesis is the establishment during development of the complex organization of tissues and organs that characterizes the adult. In multicellular animals, one of the most important processes is morphogenetic movement, the translocation of individual cells or whole tissue rudiments from one site in the body to another. Active cellular locomotion is important in many situations of morphogenetic movement. Characteristically, cell migration in the embryo displays impressive precision: cells at defined sites in the embryo begin migration at particular stages of development, traverse precisely-characterized pathways during migration, and localize finally at particular sites in the body, in specific association with other tissues. One of the most challenging problems of experimental biology is the definition of the mechanisms that regulate the active migration of embryonic cells and tissues. Recent years have seen gratifying progress in this direction, with the definition and characterization of a number of processes of potential importance. This review describes selected instances of morphogenetic movement and contains a discussion of our current understanding of the problem of regulation of cell motility in the embryo.