4D imaging reveals stage dependent random and directed cell motion during somite morphogenesis (original) (raw)

Somites are paired embryonic segments that form in a regular sequence from unsegmented mesoderm during vertebrate development. Although transient structures they are of fundamental importance as they 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 cellular dynamics underlying the morphological transitions during somite differentiation. Here, we address this by examining cellular rearrangements and morphogenesis in differentiating somites using live multiphoton imaging of transgenic chick embryos, where all cells express a membrane-bound GFP. We specifically focussed on the dynamic cellular changes in two principle regions within the somite, the medial and lateral domains, to investigate extensive morphological transformations. Furthermore, by using quantitative analysis and cell tracking, we capture for the first time a directed movement of dermomyotomal progenitor cells towards the rostro-medial domain of the dermomyotome, where skeletal muscle formation initiates. Embryonic morphogenesis involves dramatic tissue deformation and growth, which often occurs rapidly over short timescales. It is implicit that tissue deformations are driven by local cellular activities, including cell proliferation, changes in morphology and/or size, and cell rearrangements. However, it has been challenging to image, capture and quantify these processes in live tissues. Somites are transient, epithelial, near spherical structures that form during vertebrate development from the presomitic mesoderm (PSM) in a regular sequence and with a rostro-caudal progression 1. Somites can be staged based on morphological landmarks and age of development, using roman numerals 2. Newly formed somites consist of a ball of epithelial cells surrounding a central cavity, the somitocoel, which is filled with mesenchymal cells (stages I-III). As they differentiate, these paired body segments dissociate ventrally (from stage IV) and epithelial-to-mesenchymal transition (EMT) leads to formation of the sclerotome, the source of the axial skeleton. The dorsal somite remains epithelial and produces the dermomyotome and myotome, the source of all trunk and limb skeletal muscles 2,3. Signalling and genetic control of cell lineage specification is well characterised 4-6. For example, expression of the first myogenic marker, the transcription factor Myf5, is first detectable in the medial wall of epithelial somites 7. However, surprisingly very little is known about how individual cell dynamics and cellular rearrangements drive morphogenesis within the somite during its differentiation, for example during the emergence of the myotome. An improved and greater understanding of these processes may also benefit the derivation of musculoskeletal lineages from pluripotent stem cells 8. Along the rostro-caudal axis, each individual somite is flanked by neighbouring somites; other adjacent tissues on the medial, lateral, dorsal and ventral sides are the neural tube (future spinal cord), the intermediate and lateral plate mesoderm, the surface ectoderm and the endoderm respectively. Signalling molecules derived from many of these tissues govern the specification of somite cells towards particular fates 9-20. In addition, these flanking tissues