A PHYSICO-CHEMICAL STUDY OF THE STRUCTURE OF DENTAL ENAMEL

¹ Department of Physiology, School of Medicine, University of Pennsylvania, Philadelphia, Pa.

1. Membrane potentials may be demonstrated across the enamel of the canine teeth of dogs when the structure is made to separate two salt solutions. When the salt-concentration gradient is 0.1 M -0.01 M KCl, the P.D. values observed range between 19 and 27 millivolts, the dilute solution being electropositive. This is less than half the theoretical maximum calculated for an ideal membrane completely impermeable to the ion of one electrical sign. Even in more dilute solutions the P.D. values observed for a 10:1 concentration gradient do not rise above 45 millivolts, whereas the theoretical maximum at 20°C. is 58 millivolts. Such values for enamel indicate that it permits both ions of the salt to penetrate its structure, but very much retards the movement of negatively charged anions.

2. When solutions of CaCl₂ or of the chlorides of other divalent cations are used, instead of solutions of monovalent salts, the polarity of the dilute solution is electronegative. This reversal is fully accomplished only after some hours in these solutions, indicating a slow penetration of the salt into the enamel, the physical condition of which is profoundly modified, so that it now retards the movement of positively charged cations.

3. The nature of this modification is made clear by electroendosmotic determinations. It is found that water can be made to pass through enamel slowly under the influence of an applied P.D. In a 0.2 M solution of KCl the direction of flow indicates that the water behaves as if positively charged; the membrane must therefore bear a negative charge. In a 0.2 M solution of CaCl₂ the water behaves as if negatively charged; the membrane must therefore bear a positive charge, presumably as a result of the adsorption of some of the Ca.

4. When the membrane potential and electroendosmotic readings are considered together, it is evident that the enamel membranes, whether positively or negatively charged, tend to retard the movement into them of ions of the same electrical sign as they themselves bear. It is suggested that this is a simple electrostatic effect, whereby a charge on the surfaces and pore walls of the enamel tends to repel ions of like sign, and hinder their entrance into the membrane, allowing oppositely charged ions to enter more freely.

5. The slow movement of water across enamel as the result of an osmotic gradient is also demonstrated, under conditions free from technical defects in previous work.

6. These observations do not permit the identification of the path-ways through the enamel structure by which water and salts penetrate. It is suggested that the movements probably occur along the organic matrix between the enamel rods described by Malleson and by BÖdecker.

Note:

We wish to express our appreciation to Mr. Samuel Fels of Philadelphia, and to the Philadelphia Academy of Stomatology, for their generous financial support.