BIOMINERALIZATION DURING EARLY STAGES OF THE DEVELOPING TOOTH IN-VITRO WITH SPECIAL REFERENCE TO SECRETORY STAGE OF AMELOGENESIS

In this survey we summarize data on mineralization of enamel mostly ob tained in organ culture experiments in our laboratory. Historically, t he enzyme alkaline phosphatase has been proposed to stimulate minerali zation by supplying phosphate or by splitting away inorganic pyrophosp hate PPi, a potent inhibitor of mineralization. Localization of alkali ne phosphatase in developing teeth by enzyme histochemistry shows that cells of the stratum intermedium contain extremely high levels of alk aline phosphatase but secretory ameloblasts that are engaged in deposi tion of the matrix and in transport of mineral ions lack alkaline phos phatase. The function therefore must be an indirect one, since no acti vity was seen at the site of enamel mineralization. We propose that th e main function of alkaline phosphatase in the stratum intermedium is to transport phosphate or nutrients from blood vessels near the stratu m intermedium into the enamel organ. Another function of the enzyme in stages of cell differentiation was deduced from inhibition experiment s with the specific alkaline phosphatase inhibitor I- pBTM, showing th at in tooth organ culture the enzyme may be involved in the generation of phosphorylated macromolecules from P ions originating from pyropho sphate. Calcium plays an indispensable role in enamel mineralization i n vitro. Low calcium concentration in the culture medium prevented ini tial dentin mineralization and enamel formation. Moreover, differentia ting ameloblasts did not become secretory, in contrast to odontoblasts that secreted a layer of predentin matrix. Variations in phosphate co ncentration in the culture medium do not seem to affect tooth organ cu ltures adversely during mineralization in vitro. Exposure to F-, howev er, has adverse effects on enamel mineralization depending on concentr ation and exposure time and produces a variety of disturbances. Many o f the fluoride-induced changes in the enamel organ are reversible: you ng ameloblasts recover and resume secretion and mineralization of the fluorotic matrix when fluoride is removed from the medium. This recove ry is enhanced when medium calcium levels are increased. Only the chan ges in the hypermineralized enamel remain irreversible. Thus, we hypot hesize that fluoride induces a local hypocalcemia in the enamel fluid surrounding the enamel crystals by stimulating a hypermineralization o f the pre-existing enamel crystals.