(1) A membrane containing much organic material—not mentioned in dental literature—is always present at the dentino-enamel junction, and may be added to the list of visible organic dental structures, such as lamellae and enamel spindles. This membrane is visible in ground as well as decalcified specimens; its organic matrix, in ashed sections, is rich in mineral matter. In general, dentinal tubules do not enter it. (2) Available data on the porosity of enamel do not support any conclusions regarding its permeability. (3) According to BOdecker’s theory, lymph is normally present in enamel. Karshan et al. have estimated that enamel contains as much soluble protein as would correspond to 0.001 cc. of blood serum per 500 mgm. of enamel. The author’s polarographic investigation—by far the most sensitive method— failed to detect any soluble protein in enamel. There could not be more than 0.0001 cc. (lymph) per 500 mgm., if any. On the other hand, findings of organic material in aqueous enamel extracts have been confirmed by means of the polarograph. (4) The acid-insoluble protein residue in enamel does not yield cystine by hydrolysis. Therefore, its classification as a keratin is open to question.
(a) The porosity of enamel was investigated by two methods devised by Bechold, with thin slabs of fresh enamel. The first method is based upon measurement of the pressure necessary to drive air through pores. Pressure up to 20 atmospheres failed to drive any air through enamel slabs. The second method considers the amount of water that flows through a unit cross-section in a unit of time, when definite pressure is applied. Pressures up to 20 atmospheres—applied for hours—failed to drive detectable quantities of water through the enamel slabs. Control experiments with porcelain slabs gave positive results with both methods. The negative results with enamel tend to show that it contains either no pores at all, or pores having diameters so small that a pressure of 20 atmospheres (300 lbs. per square inch) fails to drive water through them.
(b) A thin layer of paste made of water and methylene blue was placed upon the enamel surface of intact teeth 5-30 minutes before extraction; each was protected with a rubber-dam from access of saliva. After extraction, the teeth were rinsed with water and crowns sectioned. In all cases there was bluish coloration of the enamel, ending abruptly at the dentino-enamel junction. At first it was concluded that methylene blue had penetrated the enamel to that junction. Lefkowitz (personal communication) objected that the bluish color was due to that portion of the dye that, despite thorough rinsing with water, always remains at the surface of enamel after application of this paste. He claimed that coloration of the cut surface of enamel disappears when the exterior film of dye is removed by grinding with a stone. Further experiments tended not only to invalidate this objection, but also to void the author’s previous conclusions regarding permeability of enamel. Cross sections were made of crowns of intact teeth, near the occlusal surface but in a region containing dentin; the sections were then so placed in an opening in the cover of a cardboard box that the cut surface (enamel surface within, cut surface without) was approximately level with the cover. The box had a circular opening in each of two sides, into which cardboard cylinders were inserted in such a way that they pointed toward the enamel surface of the specimen. Flashlights of fountain-pen type were placed in the cylinders. There was also a device that permitted colored-light filters to be inserted into the path of either light beam. Observations were made in a dark room. It was found that all of the cut surface of the crown could be illuminated in this way by any color of the visible spectrum. Contrary to the dye experiments, the coloration was not restricted to the enamel ; all of the tooth was penetrated by light, and diffused light came out from all of the cut surface, including the dentin, excepting a narrow zone in the region of the dentino-enamel junction. These experiments tend to show that diffusion of light that penetrates enamel in a direction perpendicular to the (observed) cross section of the tooth is not limited to enamel. But such a limitation is the tacit assumption upon which Lefkowitz’s objection is based. Hence the objection appears to be invalidated.
(c) In a second series of experiments, analogous to those in (b), above, lateral light penetrating the enamel was practically excluded by application of rather opaque media over the lateral enamel-surfaces. Papers and metal foils of various colors, including black, blue, yellow, red, and white, were used. In every instance, a bluish color—on the cross section—was strictly limited to the enamel and did not involve the dentin. This color was comparable to that observed in the dye experiments. The only possible conclusion seems to be that the observed color is that of enamel itself in reflected light, and that this color may be submerged by admission of lateral light, diffused through the observed cut surface. The same phenomenon probably takes place when enamel surface is covered by a rather opaque film of dye, as in the first experiments (b). Hence the author’s conclusions with regard to permeability appear to be as erroneous as the objection, and it seems that from the original experiments (a) there can be no conclusions in favor of or against permeability. The bluish color of enamel, shielded from lateral light, may be explained by the assumption that enamel has a particular colloidal structure. The same assumption explains the color of the surface of water, or of the sky. Hence, it appears reasonable to assume that enamel is not homogeneous like a mineral, but made up of at least two phases in most intimate mixture, as in a solid colloidal solution. One of the phases may be organic in nature.
References: J. Am. Den. Assoc., 1938, 1939.