GENE-EXPRESSION, POLARIZING ACTIVITY AND SKELETAL PATTERNING IN REAGGREGATED HIND-LIMB MESENCHYME

The developing chick limb has two major signalling centres; the apical ectodermal ridge maintains expression of several important genes and outgrowth of the limb, and the polarising region specifies the pattern of skeletal elements along the anteroposterior axis. We have used rea ggregated leg grafts (mesenchyme dissociated into single cells, placed in an ectodermal jacket and grafted to a host) to study patterning in a system where the developmental axes are severely disrupted, Reaggre gates from different regions of leg mesenchyme developed corresponding ly different digits, giving a system in which skeletal phenotype could be compared with the expression of genes thought to be important in p atterning. We found that posterior third and whole leg reaggregates ga ve rise to different digits, yet expressed the same combination of Hox D, Bmp-2 and shh genes throughout their development, Anterior thirds i nitially only express the 3' end of the HoxD cluster but activate the more 5' members of the cluster sequentially over a period of 48 hours, a period during which Bmp-2 is activated but no shh or Fgf-4 expressi on could be detected, Our results suggest that there are two independe nt mechanisms for activating the HoxD complex, one polarising region-d ependent and one independent, and that shh expression may not be neces sary to maintain outgrowth and patterning once a ridge has been establ ished.