Confocal microscopy of Tomes' granular layer in dog premolar teeth

Tomes' granular layer is the hypomineralized area of radicular dentin, but knowledge concerning it is limited. The present study was designed to inves tigate the structural characteristics of Tomes' granular layer in the dog's teeth by confocal microscopy. Permanent premolars of four beagles, two at 7 months and the other two at 14 months of age, were used for observation. During premolar root formation, the 7-month-old dogs were injected with cal cien and alizarin red S for vital staining of dentin, and ground sections o f the teeth were prepared. Both ground and decalcified-paraffin sections we re made from the teeth of the 14-month-old dogs and stained with basic fuch sin or with hematoxylin and eosin. All sections were examined by fluorescen ce and confocal microscopy. In the ground sections, granules of Tomes' laye r and dentinal tubules were stained with basic fuchsin and with calcein. Th e granules of Tomes' layer stained with calcein were seen only near the lab eling lines by calcein. The granules of Tomes' layer appeared as bright spo ts in cross sections, and as lines in longitudinal sections. When the secti ons were cut tangentially through the surface of dentin, the granules of To mes' layer showed a reticular structure. Most of the dentinal tubules were seen to pass between the granules and terminated in the dentin-cementum jun ction. Looped tubules were not found in this area. In the paraffin sections stained with hematoxylin and eosin, extracellular matrix of dentin showed fluorescence of various intensities and dentinal tubules appeared dark. At the surface of the radicular dentin, the granules of Tomes' layer appeared as fluorescent fibers running parallel to the surface of dentin in the long itudinal sections. The fibers appeared as bright spots in the cross section s and as a mesh in the tangential sections. In the periodontal ligament, co llagen fibers showed intense fluorescence, whereas most cells were negative . From these results we conclude that Tomes' granular layer of dog's teeth may be the collagen fiber bundles that remained uncalcified or hypocalcifie d within the radicular dentin.