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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.ydbio.2007.02.030 Byline: Nitza Kahane, Raz Ben-Yair, Chaya Kalcheim Keywords: Avian embryo; BMP; Cell delamination; Desmin; Dermomyotome; Myoblast migration; MyoD; Myf5; Myotome; Pioneer myoblasts; Noggin; Somite Abstract: The first wave of myoblasts which constitutes the post-mitotic myotome stems from the medial epithelial somite. Whereas medial pioneers extend throughout the entire mediolateral myotome at cervical and limb levels, at flank regions they are complemented laterally by a population of early myoblasts emerging from the lateral epithelial somite. These myoblasts delaminate underneath the nascent dermomyotome and become post-mitotic. They are Myf5-positive but express MyoD and desmin only a day later while differentiating into fibers. Overexpression of Noggin in the lateral somite triggers their premature differentiation suggesting that lateral plate-BMP4 maintains them in an undifferentiated state. Moreover, directly accelerating their differentiation by MyoD overexpression prior to arrival of medial fibers, generates a severely mispatterned lateral myotome. This is in contrast to medial pioneers that have the capacity for self-organization. Furthermore, inhibiting differentiation of medial pioneers with dominant-negative MyoD also disrupts lateral myoblast patterning and differentiation. Thus, we propose that medial pioneers are needed for proper morphogenesis of the lateral population which is kept as undifferentiated mesenchyme by BMP4 until their arrival. In addition, medial pioneers also organize dermomyotome lip-derived fibers suggesting that they have a general role in patterning myotome development. Author Affiliation: Department of Anatomy and Cell Biology, Hebrew University-Hadassah Medical School, Jerusalem 91120, PO Box 12272, Israel Article History: Received 7 November 2006; Revised 11 February 2007; Accepted 21 February 2007