Embryonic musculoskeletal development is a tightly regulated process that requires the intimate, yet distinct interactions of two organs. During development, somites, spheres of epithelial cells that form juxtaposed to the neural tube, will subdivide into two main compartments; the dermomyotome and the sclerotome. The dorsally located dermomyotome will further subdivide forming the underlying myotome and provide the progenitors for all skeletal muscle, while the ventrally located sclerotome will give rise to the axial skeleton, including vertebrae and ribs. The myogenic regulatory factors (MRFs), Myf-5, MyoD, Mrf4, and Myogenin, are a family of transcription factors that directly regulates muscle development. Past experiments have demonstrated that MyoD and Myf-5 are primarily responsible for myoblast determination, while Mrf4 and Myogenin are necessary for myofiber differentiation. Here we specifically ablate MyoD+ cells through Cre/loxP mediated diphtheria toxin subunit A (DTA) expression. We demonstrate that loss of MyoD expressing cells results in a systemic loss of skeletal muscle, including all Myf-5+ cells and Pax7+ progenitors. Moreover, dramatic defects in distal rib development were detected. Interestingly, while other skeletal elements develop in close association with skeletal muscle, skeletal defects were restricted to rib development. We demonstrate that distal rib defects result as a loss of surrounding musculature, subsequent failure of Sox9 expression, and activation of the apoptotic pathway. Moreover, our results suggest that an abrogation of FGF4, PDGFα and/or Scleraxis expression may be responsible for rib defects. Furthermore, we demonstrate that variations in DTA ablation kinetics of MyoD+ cells can result in dramatic differences in rib defects. While past experiments demonstrated MyoD loss does not result in rib defects, our experiments demonstrate that MyoD+ cells are required for proper rib development. Our experiments demonstrate that MyoD expressing cells are critical for skeletal myogenesis, and early interactions with surrounding musculature are critical for proper rib development.
