The genesis of membrane bone in the embryonic chick maxilla: epithelial-mesenchymal tissue recombination studies.

In the present study, the question of whether a relatively non-specific epithelial requirement exists for membrane bone formation within the maxillary mesenchyme was investigted. Organ rudiments from embryonic chicks of three to five days of incubation (HH 18-25) were enzymatically separated into the epithelial and mesenchymal components. Maxillary mesenchyme (from embryos HH 18-19) which in the absence of epithelium will not form bone was recombined with epithelium from maxillae of similarly aged embryos (homotypic-homochronic recombination) and of older embryos (HH 25) (homotypic-heterochronic recombination). Heterotypic recombinations were made between maxillary mesenchyme (HH 18-19) and the epithelium from wing and hind-limb buds (HH 19-22). Recombinants were grown as grafts on the chorioallantoic membranes of host chick embryos. Grafts of intact macillae, isolated maxillary mesenchyme, and isolated epithelia from the maxilla, wing-, and hind-limb buds were grown as controls. The histodifferentiation of grafted intact maxillae was similar to that in vivo; both cartilage and membrane bone differentiated within the mesenchyme. Grafts of maxillary mesenchyme (from embryos HH 18-19) grown in the absence of epithelium formed cartilage but did not form membrane bone. Grafts of maxillary mesenchyme (from embryos HH 18-19) recombined with epithelium in homotypic-homochronic, homotypic-heterochronic, and heterotypic tissue combinations formed membrane bone in addition to cartilage. These results indicate that maxillary mesenchyme requires the presence of epithelium to promote osteogenesis and that this epithelial requirement is relatively non-specific in terms of type and age of epithelium.