Morphogenesis of mesenchyme from regeneration blastemas in the absence of digit formation in ambystoma mexicanum.

1. Undifferentiated fore limb blastemas were denuded of their epidermis and grafted to the flank musculature; each transplant consisted of four such blastemas. Outgrowth of the mesenchyme and subsequent digit formation were prevented by covering the transplants with whole flank skin with its dermis intact. 2. Notwithstanding the absence of digits, 26 out of the 36 differentiated transplants formed one or two oblong, stout cartilages resembling proximal skeletal elements of the limb. Considering that four blastemas had been grafted in random orientation, this indicates considerable intrinsic morphogenetic capacities of the blastemal mesenchyme in the absence of organizing influences of the epidermis. 3. Thus, although permanent contact with wound epidermis has previously been shown to be necessary for blastemal mesenchyme to form distal limb structures, such contact is not required for the formation of proximal skeletal elements. The implications of this finding for regional organization in the regenerating limb are discussed.