Hox gene expression, AV-1 antigen expression, and cartilage pattern formation in chick recombinant limb buds

A recombinant limb bud composed of dissociated and reaggregated mesenchyme and an ectodermal jacket develops a limblike structure with bifurcated and segmented cartilage. We compared the cartilage structure formed from recombinants that were composed of mesenchymal cells derived from the limb bud progress zone at various stages. In the case of recombinants containing distal mesenchyme of early-stage limb buds (stage 18 or 20), long and thick cartilage structures were formed in the proximal region, and segmented digitlike structures were formed in the distal region. On the contrary, in the case of recombinants containing distal mesenchyme of late-stage limb buds (stage 25 or 27), only poorly developed cartilage structures were formed. Next, we analyzed expression patterns of the position-specific genes HoxA11, A13 and D12 and the position-specific antigen AV-1 protein in recombinants containing distal mesenchyme of stage 20 limb buds (stage 20 recombinants) or stage 25 limb buds (stage 25 recombinants). In stage 20 recombinants, HoxA11 was expressed throughout the mesenchyme, but HoxA13 was expressed only in the distal half of the mesenchyme. In stage 25 recombinants, HoxA13 was expressed throughout the mesenchyme, but HoxA11 was only faintly expressed. The expression pattern of HoxD12 was similar to that of HoxA13 in both stage 20 and stage 25 recombinants, and no asymmetric expression pattern, which is observed in normal limb buds, was detected. AV-1 antigen was expressed in the core region of stage 20 recombinants, and anteroposterior asymmetry, which is observed in the anterior-ventral-distal region of normal limb buds, was not found. No AV-1 expression was observed in stage 25 recombinants. These results suggest that the mesenchyme in recombinants shows spatially controlled gene/protein expressions along the proximodistal axis, and that these differences in gene/protein expressions may affect cartilage pattern formation in recombinants.