The recombinant limb as a model for the study of limb patterning, and its application to muscle development
Morphogenetic competence (MC) exists in embryonic limb tissue once thought to have lost this property as a consequence of cytodifferentiation. By stage 25 of chick embryonic development, cells in the proximal core of the limb have committed to the cartilage phenotype and are producing their characteristic extracellular matrix. Recombinant limb-bud grafts constructed using isolated fragments of this tissue produce outgrowths with a limb-like skeletal pattern. Inclusion of proximal peripheral tissue in the grafts (with or without the polarizing tissue) inhibits outgrowth and skeletal morphogenesis, explaining the failure of earlier studies to reveal the MC of the proximal core (chondrogenic) cells. Since definitive chondroblasts express MC in more permissive surroundings, it appears that Zwilling's assertion, that the onset of cytodifferentiation causes the loss of MC, is an oversimplification and that complex tissue interactions are probably involved.