INTRO

movem regulat underst origina 1977). Cells at the lateral edge of the somite at the limb level migrate into the limb bud. This phenomenon provides a simple system to investigate the mechanism of cell migration, since (1) the migrating cells are a homogeneous population with a predetermined developmental fate, (2) the migration is completed within a short period of time (from stage 15 until stage 18 at chick wing level, about 10 hours) and (3) the pathway is simple and unique: from somite to adjacent somato-pleure. These characteristics are in contrast with the migration of neural crest cells. Neural crest cells are the precursors of several cell types (fibroblasts, chondrocytes, neural cells, pigment cells), their migration lasts for a long period (from stage 12 until stage 22 at wing level of chick embryo, for example, about 36 hours) and proceeds along several pathways (dorsoventral pathway and ventral pathway in the trunk region). In spite of the simplicity of the process, myogenic cell migration into the limb bud has not been well investigated. One of the reasons for this is that there had been no molecular markers for the migrating myogenic cells untill the recent e