Separation of the myogenic and chondrogenic progenitor cells of undifferentiated limb mesenchyme.

Undifferentiated limb bud mesenchyme consists of at least two separate, possibly predetermined, populations of progenitor cells, one derived from somitic mesoderm that gives rise exclusively to skeletal muscle and one derived from somatopleural mesoderm that gives rise to the cartilage and connective tissue of the limb. In the present study, we demonstrate that the inherent migratory capacity of myogenic precursor cells can be used to physically separate the myogenic and chondrogenic progenitor cells of the undifferentiated limb mesenchyme at the earliest stages of limb development. When the undifferentiated mesenchyme of stage 18/19 chick embryo wing buds or from the distal subridge region of stage 22 wing buds is placed intact upon the surface of fibronectin (FN)-coated petri dishes, a large population of cells emigrates out of the explants onto the FN substrates and differentiates into an extensive interlacing network of bipolar spindle-shaped myoblasts and multinucleated myotubes that stain with monoclonal antibody against muscle-specific fast myosin light chain. In contrast, the cells of the explants that remain in place and do not migrate away undergo extensive cartilage differentiation. Significantly, there is no emigration of myogenic cells out of explants of stage 25 distal subridge mesenchyme, which lacks myogenic progenitor cells. Myogenic precursor cells stream out of mesenchyme explants in one or occasionally two discrete locations, suggesting they are spatially segregated in discrete regions of tissue at the time of its explantation. There are subtle overall differences in the morphologies of the myogenic cells that form in stage 18/19 and stage 22 distal subridge mesenchyme explants. Finally, groups of nonmyogenic nonfibroblastic cells which are fusiform-shaped and oriented in distinct parallel arrays characteristically are found along the periphery of stage 18/19 wing mesenchyme explants. Our observations provide support for the concept that undifferentiated limb mesenchyme consists of independent subpopulations of committed precursor cells and provides a system for studying the early determinative and regulatory events involved in myogenesis or chondrogenesis.