Somitic contribution to the zebrafish pectoral fin

Abstract

The pectoral fin muscle is generally understood to be derived from the adjacent somites. The detailed migratory processes that lead to the population of somite derived muscle progenitors within the pectoral fin are not well understood. Here I provide a detailed analysis of pectoral fin muscle progenitors from the somitic compartment into the zebrafish pectoral fin. I find somites 4-6 contribute to the pectoral fin and somite 7 contributes to the Posterior Hypaxial Muscle, both likely through the mechanism of 2D sheet migration. This detailed analysis of muscle progenitors revealed a somitically derived group of cells present within the Apical Ectodermal Ridge. The somitic origin of these cells has been validated using a range of lineage tracing methods, including Cre/Lox, photo-conversions, time-lapse analysis and somite transplantations. Both the timing of this event shortly before the Apical Ectodermal Ridge to Apical Fold transition, and the specific location of these mesoderm derived cells within the Apical Fold, are highly suggestive that these cells are actinotrichia forming cells. As such they can indeed play a crucial role in fin fold formation. Considering the loss of actinotrichia has been considered to be a crucial event in fin the limb transition, these results provide new insight in how fin to limb transition may have occurred.