A morphometric and biochemical study of the pre-eruptive development of hamster molars in vivo.

Surface area measurements of cross-sections of M1 and M2 at 1-11 days were related to biochemical parameters for cell proliferation ([3H]-thymidine incorporation) and mineralization (45Ca uptake) and with increase in dry weight. The pre-eruptive development of molar tooth germs was divided into 4 phases which partly reflect the life cycle stages of the ameloblast. In phase 1 (proliferation phase, duration in M2 about 5 days) mitotic activity and increase in total area were great; ameloblasts were undifferentiated or in a pre-secretory state. In phase 2 (differentiation phase, duration 2-3 days) the number of secretory ameloblasts increased and 45Ca uptake started and increased rapidly. At the end of phase 2, crown morphogenesis was completed and cell proliferation ([3H]-thymidine incorporation) became less. In phase 3 (secretion phase, duration 0.5-1 days) all ameloblasts throughout the tooth germ had differentiated into secretory ameloblasts and enamel matrix surface area increased about twice as fast as that of dentine. At the end of phase 3, the surface area of the enamel matrix almost attained its final value. In phase 4 (maturation phase, duration in M1 till first eruption about 2-3 days), post-secretory ameloblasts increased in number and, in contrast to the surface area of the enamel matrix, that of the dentine continued to increase. The fast, linearly increasing total uptake of 45Ca during phase 4 which was attributed to the mineralization of both newly formed dentine and existing maturing enamel was the main cause of the rapid increase in dry weight of the whole tooth germ.