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Abstract

We have generated a zebrafish GFP reporter line for pax7, a known master regulator of muscle stem cell (MSC) physiology. This allowed us to study the dynamics and cellular architecture of the myogenic lineage by non- invasive 4D in vivo imaging during primary and secondary myogenesis. The analysis of overlapping imaging sequences allowed us to obtain the full complement of MPs and showed that they derive from a population of MSCs that originate in the teleost dermomyotome. Clonal analysis combined with anatomical, morphological and functional criteria led to the separation of quiescent and activated MSCs from Pax7+ early myoblasts and concomitantly identified the vertical myoseptum (VM) as the niche of zebrafish MSCs. Furthermore it allowed us to obtain estimates for the cell cycle time of these subpopulations during physiological muscle development and muscle regeneration. By transcriptional profiling we have found that MSCs play a pivotal role in the ontogeny of the VM and hence in the construction of their own niche by secreting a number of non-collagenous ECM components including tenascin w (tnw). Upon morpholino-mediated knockdown of tnw we observe illegitimate activation of the quiescent MSC population, a dramatic decline of the total MSC number as well as structural defects of the VM and muscle fibre disorganization. Currently, we are screening a small set of Wnt, BMP, FGF and TGFb signalling inhibitors for their effect on myogenic lineage dynamics.