Viability and proliferation of epithelia and the initiation of osteogenesis within mandibular ectomesenchyme in the embryonic chick.

Ectomesenchyme, a derivative of the embryonic neural crest, forms the membrane bones of the mandibular skeleton, but will only do so after undergoing an inductive interaction with mandibular epithelium. Previously, non-mandibular epithelia have been shown to act as effective substitutes for the mandibular epithelium in this interaction. The role of epithelial viability was examined by enzymatically separating the mandibular epithelium from its ectomesenchyme, killing the epithelium, recombining the epithelium with vital ectomesenchyme and either organ culturing the recombinant or grafting it to the chorioallantoic membrane of a host embryo. Epithelia killed by distilled water, air drying, 80% ethanol, freeze-thawing or with 2500 rad of gamma irradiation did not elicit osteogenesis from the ectomesenchyme while vital epithelia did. Exposure of epithelia to gamma irradiation at doses between zero and 2000 rad resulted in a progressive reduction in the incidence of osteogenesis in ectomesenchme. However, the incidence of osteogenesis progressively increased after irradiation of the mandibular epithelium with 3000 to 5000 rad, only to decrease again after 10000 rad. [3H]thymidine autoradiography was used to show that this pattern of induction of bone by irradiated epithelia could be correlated with the proliferative activity of the epithelia. A similar pattern of induced osteogenesis and epithelia proliferation was seen after epithelia were treated with colchicine. It was concluded that the ability of the mandibular epithelium to permit osteogenesis within mandibular ectomesenchyme was correlated with some property of epithelial proliferation. Several possibilities are discussed and related to other instances of induction of heterotopic bone by epithelia.