CELL FATE IN THE CHICK LIMB BUD AND RELATIONSHIP TO GENE-EXPRESSION

We have produced detailed fate maps for mesenchyme and apical ridge of a stage 20 chick wing bud, The fate maps of the mesenchyme show that most of the wing arises from the posterior half of the bud. Subapical mesenchyme gives rise to digits, Cell populations beneath the ridge in the mid apical region fan out into the anterior tip of the handplate, while posterior cell populations extend right along the posterior mar gin, Subapical mesenchyme of the leg bud behaves similarly, The absenc e of anterior bending of posterior cell populations has implications w hen considering models of vertebrate limb evolution, The fate maps of the apical ridge show that there is also a marked anterior expansion a nd cells that were in anterior apical ridge later become incorporated into non-ridge ectoderm along the margin of the bud, Mesenchyme and ap ical ridge do not expand in concert - the apical ridge extends more an teriorly. We used the fate maps to investigate the relationship betwee n cell lineage and elaboration of Hoxd-13 and Fgf-4 domains. Hoxd-13 a nd Fgf-4 are initially expressed posteriorly until about the mid-point of the early wing bud in mesenchyme and apical ridge respectively, La ter in development, the genes come to be expressed throughout most of the handplate and apical ridge respectively, We found that at the prox imal edge of the Hoxd-13 domain, cell populations stopped expressing t he gene as development proceeded and found no evidence that the change s in extent of the domains were due to initiation of gene expression i n anterior cells, Instead the changes in extent of expression fit with the fate maps and can be attributed to expansion and fanning out of c ell populations initially expressing the genes.