Award for Research in Biological Mineralization

¹ School of Dentistry, Center for Health Sciences, University of California, Los Angeles, California

The beauty of pearls, shells, corals and ivory and the pain of gallstones, kidney stones, and pulp stones are but a few illustrations of the wide variety of mineral deposition in living matter (Calcification in Biological Systems, A.A.A.S. Publ. No. 64, Washington D.C., 1960).

In this presentation, principal attention was directed toward a comparison between bone and the most highly specialized of the dental hard tissues, namely, the dental enamel, specifically with respect to the unique reactivity of enamel at various stages of mineralization, and especially in regard to the most remarkably reactive of all pertinent elements, namely fluoride.

There are three newer sources of information pertinent to this discussion (Science 150:989, 1965). First, it is now evident that, compared to the 1 ppm F required for protection of dental enamel against caries, the intake of fluoride required for skeletal protection against certain metabolic bone diseases needs to be exceedingly high. Second, the blood plasma values of fluoride stay remarkably low (0.2 to 2.0 ppm F) not-withstanding very high elevations in fluoride intake (20 to 200 mg. F per day). Third, the weight of evidence now suggests that cells and organs grown in tissue culture can tolerate fluoride concentrations in the substrate appreciably above those which can be brought into the systemic circulation of man even with very high fluoride intake.

As a result of basic biological differences in the behavior of bones and teeth, two somewhat different approaches to the control of bone and tooth destruction appear to emerge, at least as far as fluoride protection is concerned.

Because of the lifelong skeletal remodeling, a continuous contact with fluoride seems necessary to protect bone. Indeed, it may well be that adult man may need to ingest more, rather than less daily fluoride than children in order to protect aging bones and blood vessels.

In dental structures the situation is somewhat different, not only with respect to cellular and chemical interaction, but also in regard to preventive capabilities.

Just as teeth tend to retain formative defects, they also maintain special chemical advantages gained both during and after primary mineralization. In contrast to bone the secondary increment of fluoride in the enamel is largely there to stay for continuous protection without being subject to subsequent removal through cellular remodeling.

Why is the dental enamel so extremely reactive as evidenced by occasional mottling even with a fluoride ingestion only slightly above 1 ppm, equivalent to 0.5 to 1 mg. F per day? It is proposed that this may be due to (a) some special properties of the enamel organ leading to a concentration of electrolytes in juxtaposition to the mineralizing enamel, and (b) an unusually high reactivity of enamel itself during its process of mineralization.

It has been suggested that there is a striking morphologic similarity between the microchannels within the salt gland of marine turtles and the ultrastructure of the enamel organ, suggesting an electrolyte-concentrating function (J Ultrastructure Res 14:518, 1966). But at this time we have no direct information regarding the relative fluoride levels within the different cell layers of ameloblasts and other strata in this complex glandlike organ.

Radioisotope studies have clearly indicated the extremely high reactivity between enamel and its adjacent tissue fluid in unerupted teeth and its adjacent saliva in erupting teeth (Amer J Physiol 180: 408, 1955). More recently there have been observed two additional "tissue fluids" whose compositions and potential interactions with the enamel surface need to be explored further, namely (a) the "gingival fluid" (Acta Odont Scand 17:11, 1959) and (b) the "enamel fluid" (Arck oral Biol 11:943, 1966). Nothing is known about the fluoride content of these fluids at this time.

In terms of practical applications, the following approaches merit further study:

1. It may be of value to reach the young, cervical enamel with a fluoride solution stronger than that of the gingival fluid; possibly by a waterpick attached to a fluoridated mouthwash.

2. Something may be gained by flushing fluoride solutions below the mucosal flap overlying partially erupted teeth, especially onto the occlusal fissures.

3. It would appear that we should take a closer look at the permanent teeth underlying shedded deciduous teeth, especially under prematurely decayed and extracted teeth, with a view to administer a "subtopical" chemical treatment to the crowns of the still unerupted permanent successor, when the enamel is much more reactive than after eruption.

4. At some time it would be desirable to consider the question of preventive infusion of fluoride around the crowns of deeply buried teeth, because they are then demonstrably extremely reactive.

5. Lastly, it may be necessary to consider systemic vascular fluoride administration in the therapy of metabolic bone disease, because it may be the only way to raise blood plasma fluoride levels significantly without nauseatingly high oral ingestions of fluoride.