Formation of prechondrogenic mesenchymal cell aggregates in the regenerating newt limb

Abstract

Development of the pattern of prechondrogenic condensations was documented by quantitating regional cell densities within blastemas of regenerating forelimbs of newts. The pattern of development resembles that of embryonic limb development in that the prechondrogenic pattern develops from proximal to distal, and by the presence of an apical epithelial thickening which is most prominent postaxially during the intermediate phase of mesenchymal cell accumulation. Development differs from that described previously in that an increased central cell density is present in the smallest blastemas analyzed, and the sequence of digit formation is from anterior to posterior. Also, a blister often intervenes between the epithelium and mesenchyme during the early phase of development. Cumulatively, these observations support models which suggest mechanisms for proximodistal development and a stimulatory role for apical epithelium. However, the presence of a central cell density prior to definitive aggregation implies a mechanism other than de novo condensation which is assumed in current models. Additionally, increased mesenchymal cell density subjacent to thickened epithelium is contrary to models which imply that such areas should be less dense. Finally, digital aggregates develop from preaxial to postaxial, which is opposite that of most amniotes, and is not predicted by current models of limb morphogenesis.