It has been shown through earlier electron microscopy that the crystalline apatite which is the major component of mature dental enamel is overlaid upon a framework of very fine organic fibrils (1,2). There has been difficulty, however, in observing both the inorganic and organic constituents in the same preparation. The former has been brought out best in replicas of ground sections or cleaved fragments, and the latter in thin sections of demineralized enamel. In replicas and pseudoreplicas of untreated or acid-etched specimens the crystalline elements have previously appeared flat and ribbon-like, as shown in Fig. 1, and lengths varying from 400 to 10,000 A have been reported (1,2, 3, 4, 5). Definite and uniform cross-markings at roughly 300 A intervals have also been seen on these structures (2, 4, 5). This segmented appearance, illustrated in Fig. 2, has been of interest because of the possibility that it might reflect the presence of organic material within the larger crystal-like objects. Evidence that this may in fact be the case has now been derived from the examination of pseudoreplicas of enamel from which organic substance has been selectively removed. This has been accomplished through treatment of the surfaces of polished ground sections with ethylene diamine, an agent which has previously been used in the preparation of anorganic bone (6). In the first experiments, the samples were treated in a Soxhlet extractor, which was modified so that condensed solvent did not drip directly on to the surfaces (7). A complete extraction cycle took 15 min, and specimens were exposed over the time required for one to six cycles. Identical results were later obtained by simply immersing the ground sections in warm (80° C) ethylene diamine for 15 min to 2 hr. After gentle washing and drying in air, positive collodion-carbon pseudoreplicas were made by a previously described method (8). The ground sections treated for over 30 min were affected to such a depth that it was often necessary to remove some of the loosened crystalline material by stripping several primary films in succession, before a suitably thin pseudoreplica could be made. The finished replicas were shadowed with tungsten oxide, which is amorphous, so that the same areas that were photographed could be subjected to limited area electron diffraction.
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