Crystal morphology and decalcification patterns compared in rat and human enamel and synthetic hydroxyapatite.

The purpose of this investigation was to compare morphology and dissolution patterns by ultrastructural examination of rat and human enamel crystals as well as synthetic apatite crystals. Mature enamel crystals were of particular interest, since crystal maturation appears to be inhibited in amelogenesis imperfecta. Specimens were isolated from developing and mature rat incisor enamel. Rat enamel, mature human enamel, and synthetic apatite were thin-sectioned without decalcification and examined by transmission electron microscopy. Some sections were exposed to acid, and selected synthetic apatite sections were further treated for removal of embedding plastic, followed by vacuum-shadow-coating with carbon. Results showed that cross-sections of rat, human, and synthetic crystals had a distortion in the flattened hexagonal outline in regions where the growth of one crystal impinged on another. Crystal dissolution occurred preferentially along the c-axis, producing a central defect or hole in the crystals. Preliminary studies with weak acid on mature human enamel indicate that the relatively soluble crystal core is quickly dissolved, while the outer shell remains intact over a much longer period of time. In the mature rat and human enamel, this crystal hole formation had a consistent dimension of approximately 10-nm thickness. The crystal hole dimension was the same size as crystals that are formed during the early secretory phase in rat amelogenesis. Acid-treated synthetic apatite also showed dissolution of the crystal core along the c-axis, but dimensions of the hole were not consistent.(ABSTRACT TRUNCATED AT 250 WORDS)

Ultrastructure of fluoride-induced cysts in the rat molar enamel organ.

Earlier studies have shown that a single high dose of fluoride induces subameloblastic cysts separated by morphologically unaltered ameloblasts in the developing rat molar. In the present investigation, the ultrastructure of both the ameloblasts forming the cystic wall, and the cells within the cystic lumina were studied by means of transmission electron microscopy. It was shown that 24 h after fluoride injection the ameloblasts of the cystic wall showed various degrees of cytoplasmic and nuclear alterations. Some cells displayed signs of necrosis as indicated by condensation of the chromatin. The cytoplasmic changes varied from altered organelle morphology to fragmentation and almost complete shedding of the whole cytoplasm. In the ameloblasts of the cystic wall secretory products accumulated intracellularly, in distended rough endoplasmatic reticulum, in vesicles of the Golgi region and extracellularly between ameloblasts as well as between cells in the stratum intermedium, indicating an altered matrix secretion. Electron lucent material, cell and cell fragments were found in the cystic lumina, the two latter apparently originating from the ameloblastic layer. The degenerative changes seemed to follow the normal pattern of cell degeneration.

Ultrastructure of altered rat enamel beneath fluoride-induced cysts.

The effect of a single injection of sodium fluoride (60 mg/kg) on the development of rat molar enamel beneath fluoride-induced subameloblastic cysts was studied by transmission electron microscopy using undecalcified sections. Three bands of altered enamel were identified and defined as the cyst surface band, the hypoplastic band, and the hypercalcified band. The irregular cyst surface band, not previously described, was found to have two components: electron-dense enamel globules and organic spherules. The electron-dense globules consisted of small, randomly arranged crystals (confirmed by selected area electron diffraction) occurring within a stippled organic matrix. The organic spherules have staining properties similar to stippled material and lack a crystalline component. They may be a form of organic material being extruded from the underlying developing enamel. The critical role of normal matrix production and ameloblast Tomes' process structure on the development of the crystal orientation and rod pattern is discussed.

Mode of enamel matrix secretion.

Evidence for direct fusion of ameloblast secretory granules to the plasma membrane at the enamel forming front is minimal. Typically, secretion granules cluster in the center of the Tomes' process and frequently show continuity with a tubular reticulum containing a similar material. Secretion granules seem to exit primarily by way of plasma membrane invaginations connected to the secretory granule via tubular pseudopodia. Enamel formed initially at the dentino-enamel junction, as well as interrod enamel, is also characterized by enamel matrix release into deep plasma membrane invaginations. This modified type of merocrine secretory process may play a role in enamel crystal orientation.

Endotoxin penetration into root cementum of periodontally healthy and diseased human teeth.

This study was undertaken to determine the extent of in vitro penetration of E. coli endotoxin into the root cementum of periodontally healthy and diseased teeth. Freshly extracted teeth were washed in distilled water, scaled and divided into two groups of 16 teeth each. Nine diseased and five healthy teeth in the first group were immersed in various concentrations of E. coli endotoxin for 2 to 12 weeks. They were then prepared for indirect immunofluorescence examination after treatment with anti-endotoxin antibody and rhodamine conjugated secondary antibody. Teeth in the second group were prepared for autoradiographic examination by immersing nine diseased and five healthy teeth in tritium labelled E. coli endotoxin for 2 to 12 weeks. The latter technique also allowed for semi-quantitative study of the depth of endotoxin penetration by creating facets on the root at various depths after endotoxin exposure. This technique was also used to investigate the strength of endotoxin binding to the tooth surface by brushing for 1 minute and re-examining the tooth for the presence of endotoxin. Controls included periodontally diseased and healthy teeth. Results of the study showed that (1) endotoxin adheres to the tooth surface without penetration into the root cementum of either periodontally healthy or diseased teeth, and (2) the binding of the endotoxin to the root surface appears to be weak.

Disturbances and defects in enamel development.

Ameloblastic disturbances are correlated with defects in enamel. Experimental disturbances created by tetracyclines, fluorides, and antimitotic drugs are emphasized. Cellular alterations are shown to interfere with both matrix production and secretion. Defects in enamel formation are characterized by abnormal crystal size and distribution. Further research in normal and particularly abnormal enamel development is needed to establish: 1. exact mode of matrix secretion and its relation to crystal nucleation and orientation 2. function of maturative ameloblast and its relation to the stratum intermedium and papillary layer during that phase 3. cause for lack of mineralization of matrix, particularly in ectopic areas.