Formation of extra digits in the interdigital spaces of the chick leg bud is not preceded by changes in the expression of the Msx and Hoxd genes.

By in situ hybridization we studied the expression patterns of Msx and Hoxd genes during the late development of the chick leg autopodium (foot) and compared them to patterns during the experimental development of interdigital extra digits. Extra digits are induced in the third interdigital space after various experimental manipulations, such as transient isolation of the interdigit, or removal of the interdigital marginal ectoderm and mesoderm. Msx1 and Msx2 are normally expressed in the interdigital tissue programmed to die. Our experiment changes the fate of the interdigital tissue from cell death to chondrogenesis and provides a good model for studying Msx involvement in defining areas of programmed cell death. Among the proposed roles of Hoxd genes is their involvement in the specification of digit identity early in development. The induction of extra digits allows us to examine whether this new morphogenetic commitment of the interdigital tissues involves changes in the domains of expression of Hoxd genes. Our results show that extra digits develop without a previous modification of the normal pattern of expression of Msx or Hoxd genes. This observation does not support the correlation between the expression of Msx genes and programmed cell death and suggests a role for these genes in maintaining the interdigital tissue in an undifferentiated state. Our results show that an increased number of digits can be formed without modifications in the pattern of expression of the 5'-located Hoxd genes and suggest the existence of latent or residual digit organization mechanisms past the time when digits are normally determined, independent of Hoxd gene expression.