The origin and fate of pioneer myotomal cells in the avian embryo

The ontogeny of the myotome was investigated using [³H]thymidine or Brdu treatment in conjunction with 1,1', di-octadecyl-3, 3, 3', 3',- tetramethylindo-carbocyanine perchlorate (DiI) labeling and expression of specific markers. We have identified a subset of early post-mitotic cells that is present in the dorsomedial aspect of epithelial somites and is homogeneously distributed along their entire rostrocaudal extent. The post- mitotic quality of this cell subset enabled us to trace their fate in time- course experiments. Following initial somite dissociation, this epithelial post-mitotic layer bends underneath the medial portion of the nascent dermomyotome. Then, these cells progressively lose epithelial arrangement and migrate in a rostral direction where they accumulate temporarily. Subsequently, these early post-mitotic precursors extend processes that reach both rostral and caudal edges of each segment. Medial somite-derived myofibers also fill the entire mediolateral extent of the segment and reach the dorsomedial lip of the dermomyotome, thus forming the primary myotome. During this process, their large nuclei localize to a narrow stripe in the middle of the nascent myotome. Consistent with the proliferation studies, DiI labeling of the medial epithelial somite cells gave rise to a primary myotomal structure, and continuous pulsing of the DiI-injected embryos with radioactive thymidine revealed that these fibers indeed developed from post- mitotic progenitors. As these early post-mitotic cells that arise prior to somite dissociation are the first wave of progenitors that constitutes the myotome, we have termed them avian muscle pioneers. We propose that the primary myotome formed by the muscle pioneers constitutes a longitudinal scaffold that serves as a substrate for the addition of subsequent waves of myotomal cells.