Abnormal dentin and enamel formation are known to occur under various pathological conditions. In the case of enamel, hypoplasia is manifested in diabetes mellitus. Current studies are investigating the effect of insulin on enamel synthesis and secretion in an in vitro molar tooth germ system. We are also looking at the effect of insulin on the utilization of glucose by the enamel organ. The main purpose of these studies is to determine whether the hypoplasia seen during diabetes mellitus is the result of an underproduction of enamel by the enamel-producing cells (ameloblasts), or the result of a change in the composition or mineralization of the teeth thereby rendering them more susceptible to attrition. In this respect, the insulin effect on calcium utilization is being studied.
Complete occlusion of the pulp chamber by abnormal dentin production is known to occur in patients with dentinogenesis imperfecta, and in patients with osteogenesis imperfecta. We have shown that the pulp chamber of the incisors of adriamycin-treated rats eventually becomes occluded by abnormal dentin production. We are interested in the cells that are involved in this abnormal dentin production, the mode of action of adriamycin on these cells, and the nature of the abnormal dentin matrix. These studies are being pursued with radioautography, electron microscopy, immunocytochemistry, and tissue culturing.
With regard to the enamel hypoplasia, clinically visible disturbances of mineralization, in the form of symmetrical external hypoplasia of the enamel, was found to be more common in the offspring of diabetic mothers (28%) than in the control (3%). Whether this effect is due to the lack of insulin, the high glucose concentration in the body, or to changes in other physiological processes is still uncertain. Experimentally, we have looked at the alloxan diabetic rat model (Karim, 1983a) and the streptozotocin diabetic rat model (Karim, 1983b). Although these studies did show changes in enamel formation, the involvement of other physiological parameters could not be eliminated. Recently, we began utilizing an in vitro hamster molar system to study the effect of several parameters in the diabetic condition on enamel formation. Our hypothesis is that the enamel hypoplasia seen in diabetes may not be directly related to abnormalities in glucose concentration, but to changes in calcium uptake and, therefore, may involve both enamel formation and mineralization.
At present our studies are directed towards: (1) determining whether the enamel organ of hamster molar tooth germs in culture incorporates 3H-glucose; (2) determining the relationship between tooth development and 3H-glucose incorporation; (3) determining the effect of insulin on 3H-glucose incorporation into hamster molars in vitro, and (4) determining the effect of insulin on 3H-glucose incorporation into hamster molars in vitro, and (4) determining the effect of insulin on tooth development. After completing these studies we intend to investigate how the above parameters influence calcium concentration and uptake. The reason for studying calcium is as follows: Assuming that the insulin binding sites in the zone of maturation of the enamel organ (Martineau-Doizee et al., 1986) are related to calcium transport, a phenomenon that also influences enamel matrix protein secretion (Wöltgens et al., 1987), then it appears that calcium may be the determining factor where changes in its concentration (and, or uptake) may influence the development of the enamel hypoplasia.
Therefore, in order to elucidate the etiology of the enamel hypoplasia seen in diabetics, it is necessary not only to determine whether the abnormalities seen in the enamel organ (Karim, 1983a; 1983b) are due to the lack of insulin or to the glucose concentration, but also to determine what effects these parameters have on calcium concentration in the enamel organ, and what influence this may have on enamel formation and mineralization.
This in vitro model therefore provides a good system for studying the factors involved in enamel formation. Also, elucidating the underlying mechanism involved in the development of the hypoplastic enamel seen in diabetics could be better pursued, because the involvement of different parameters could be better controlled. More important, however, it is hoped that these studies will help assess the response of this enamel lesion to insulin therapy.
Our laboratory has been investigating also the effects of adriamycin, an antineoplastic drug, on rodent incisors. We are especially interested in the ability of this drug to stimulate pulp cells to form a mineralized matrix in situ (Karim and Eddy, 1984). Studies from our laboratory have shown that adriamycin destroys immature pulp cells (Karim and Pylypas, 1985), and precursors of preodontoblasts (Karim, 1985a). It also stimulates a certain population of surviving pulp cells to produce osteodentin (Karim, 1985b; Karim and Pylypas, 1986b). In addition, to differentiating into mineralized tissue producing cells, these adriamycin-affected pulp cells show a higher alkaline phosphatase activity than non-treated cells (Daeninck and Karim, 1987). Electron microscopy has revealed this activity on the plasma membrane, and also along the secretory pathway.