The role of cell mixing in branchial arch development.

Compartmental structures are the basis of a number of developing systems, including parts of the vertebrate head. One of the characteristics of a series of compartments is that mixing between cells in adjacent units is restricted. This is a consequence of differential chemoaffinity between neighbouring cells in adjacent compartments. We set out to determine whether mesenchymal cells in the branchial arches and their precursors show cell-mixing properties consistent with a compartmental organisation. In chimaeric avian embryos we found no evidence of preferential association or segregation of neural crest cells when surrounded by cells derived from a different axial level. In reassociation assays using mesenchymal cells isolated from chick branchial arches at stage 18, cells reformed into clusters without exhibiting a preferential affinity for cells derived from the same branchial arch. We find no evidence for differential chemoaffinity in vivo or in vitro between mesenchymal cells in different branchial arches. Our findings suggest that branchial arch mesenchyme is not organised into a series of compartments.