Tooth germs of first molars from four‐ to six‐day old mice were transplanted heterotopically into the connective tissue under the dorsal skin of adult syngeneic recipients. The rate of the transplants was studied by histologic staining, vital staining with procion dyes, and autoradiography. Together these methods provided the following picture of the development of the transplant. Extraction of the tooth germ severs its vascular connections and disturbs the nutritional environment of the tooth so that many cells undergo necrosis and degeneration. The resulting inflammatory response clears the transplant of necrotic tissue, leaving a fibrous pulp of low cellularity and an extensively disrupted odontoblastic layer; the enamel organ is reduced to only a few epithelial cells. In the second week following transplantation, however, the surviving cells start to proliferate and the tissues of the transplant begin to reorganize. Reorganization starts at the apical foramen and spreads to the rest of the tooth. The central pulp is penetrated by new blood vessels, its cellular content is reestablished, and its morphology returns to normal. Odontoblasts start producing dentin which at first is highly irregular and contains many cells (osteodentin). Later the dentin formation becomes more and more regular and the dentin assumes its normal tubular structure. New enamel is not formed at any time after transplantation. The cells of the enamel organ undergo squamous metaplasia and form epithelial cysts. Abortive root formation is observed but typical roots never develop. Rudiments of periodontal ligament develop rarely. The transplants are enclosed in a capsule of connective tissue. It is concluded that the heterotopically transplanted tooth germs maintain their capability to recover, develop, and differentiate in syngeneic recipients. Although almost all of the developmental processes continue in the transplant after a period of adjustment, some of them are abortive.
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