4D imaging reveals stage dependent random and directed cell motion during somite morphogenesis

Somites are paired embryonic segments that form in a regular sequence from unsegmented mesoderm during vertebrate development. Of fundamental importance, they are transient structures that generate cell lineages of the musculoskeletal system in the trunk such as cartilage, tendon, bone, endothelial cells and skeletal muscle. Surprisingly, very little is known about the morphological transition and cellular dynamics during somite differentiation. Here, we address this by examining cellular rearrangements and morphogenesis in differentiating somites using live multi photon imaging of GFP-transgenic chick embryos. We specifically focussed on the dynamic changes in two principle regions within the somite (the medial and lateral domains) to investigate extensive morphological changes. Furthermore, by using quantitative analysis and cell tracking, we were able to capture for the first time a progenitor cell bulk movement towards the rostral-medial domain of the myotome, where skeletal muscle formation first initiates.