The initiation of osteogenesis in the mandibular mesenchyme of the embryonic chick at 7 days is dependent upon an epithelial induction which occurs in the mandible up to the fourth day in ovo. In the present study, transfilter tissue recombinations were used to study this inductive mechanism. The epithelial and mesenchymal components of the mandibles were separated before the completion of the induction and recombined to form transfilter explants which were either cultured for 9 days or grafted onto the chorioallantoic membrane for host embryos for 7 days.

Control experiments demonstrated that the tissue separation and recombination techniques did not interfere with the normal epithelial induction, and confirmed that mandibular mesenchyme isolated at this stage was incapable of forming bone.

Bone was observed in 86 % of the CAM-grafted intact mandible controls and in 80 % of the cultured intact mandible controls. Bone failed to form in the mesenchyme of transfilter explants when Millipore filters with 0·45 μm pores were used. Bone was observed as frequently as in control explants when the mandibular mesenchyme was separated from its epithelium by 0·8 μm or 0·4 μm porosity Nuclepore filters. Only about 30% of the transfilter explants prepared with 0·1 μm porosity Nuclepore filters formed bone and none of the explants prepared with 0·03 μm porosity Nuclepore filters formed bone. SEM studies demonstrated a distinct correlation between the formation of bone in transfilter explants and the ability of the epithelium and mesenchyme to penetrate the pores of the filters.

Thus, the present study provides evidence that the site of the induction is restricted to the epithelial—mesenchymal interface, and that the induction is not mediated by a diffusible substance. The nature of the inductive mechanism is discussed with respect to this and other recent studies which suggest that the induction may be mediated by a non-diffusible epithelial cell product resident in the epithelial basal lamina.

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