A cellular lineage analysis of the chick limb bud.

The chick limb bud has been used as a model system for studying pattern formation and tissue development for more than 50 years. However, the lineal relationships among the different cell types and the migrational boundaries of individual cells within the limb mesenchyme have not been explored. We have used a retroviral lineage analysis system to track the fate of single limb bud mesenchymal cells at different times in early limb development. We find that progenitor cells labeled at stage 19-22 can give rise to multiple cell types including clones containing cells of all five of the major lateral plate mesoderm-derived tissues (cartilage, perichondrium, tendon, muscle connective tissue, and dermis). There is a bias, however, such that clones are more likely to contain the cell types of spatially adjacent tissues such as cartilage/perichondrium and tendon/muscle connective tissue. It has been recently proposed that distinct proximodistal segments are established early in limb development; however our analysis suggests that there is not a strict barrier to cellular migration along the proximodistal axis in the early stage 19-22 limb buds. Finally, our data indicate the presence of a dorsal/ventral boundary established by stage 16 that is inhibitory to cellular mixing. This boundary is demarcated by the expression of the LIM-homeodomain factor lmx1b.