Tissue patterning mechanisms of facial prominences in chicken embryo

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Local patterning events occur within the frontonasal mass, one of the facial prominences that surrounds the embryonic oral cavity. In the chicken embryo, the upper beak is primarily formed by the frontonasal mass, the prominence that lies between the nasal slits. In the present study we dissociate mesenchyme from the lateral and central thirds of the frontonasal mass and compare the ability to form a single cartilage rod. We find that central third cell pellets combined with facial epithelium have a tendency to form a straight cartilage rod whereas the cells from the lateral third produce more branched outgrowths. In the reconstituted cell pellets with lateral and central cells mixed together, the central cells aggregate in the cartilage condensations while lateral cells are distributed throughout the graft. The central cell pellets contain greater numbers of potential chondrogenic cells than lateral cell pellets.To see whether medio-lateral patterning is re-established in the cell pellets, we used Msx-2 as a marker for the lateral mesenchyme at stage 24. Central cell pellet recombinations do not express the gene or turn on bilateral expression. Grafts of lateral cells maintain and up-regulate Msx-2 expression 48 hours after grafting, but are not able to establish bilateral symmetry. The cell pellets that are mixtures of central and lateral third mesenchyme often express three patches of Msx-2 transcripts. Thus the central region of the frontonasal mass probably contains the majority of prechondrogenic cells at stage 24 and these cells have a tendency to segregate from the surrounding non-chondrogenic mesenchyme. Msx-2 may help to set up the differences between central and lateral mesenchyme prior to cell differentiation.