The present study concludes that cells with ruffled borders and clear zones observed by transmission electron microscopy can be identified as odontoclasts or osteoclasts irrespective of the number of nuclei.

belt-like zones, corresponding to the parazone or the diazone of the Hunter-Schreger bands.Reversed images of scanning electron-micrographs of the exposed surfaces of the developing enamel revealed round and bulb-like profiles of Tomes' processes at early amelogenesis and its changes into a characteristic structure combined with flat secretory and enclosing nonsecretory faces that dictated the orientation of corresponding enamel prisms.The results suggest that the groups of enamel prisms oriented in sideward directions first appear as small island-like specks near the enamel-dentin junction, which later merge and form alternating horizontal belt-like zones as a consequence of morphological changes of the Tomes' processes. However, the mechanisms whereby the functional grouping of secretory ameloblasts with similarly oriented Tomes' processes is induced are yet to be determined.

The three-dimensional architecture of enamel prisms and their relationship to Hunter-Schreger bands were examined in the developing enamel of several dog teeth by light and electron microscopy, and computer-assisted reconstruction. Sections were prepared from a single demineralized tooth germ. Longitudinal semithin sections parallel to the meridian of the tooth showed parazones and diazones of the Hunter-Schreger bands in alternate rows at equal intervals. On sections vertical to the tooth crown through the middle region of parazone or diazone, a row of parallel prisms were angulated with the largest tilting angle being 55 degrees to the enamel-dentin junction, running in opposite directions in the respective zones. Tangential sections parallel to the enamel-dentin junction showed numerous belt-like zones arranged perpendicular to the meridian of the tooth. Each belt-like zone consisted of a group of enamel prisms oriented in the same direction, those in the neighboring zones being oriented in an opposite direction. The densely stained boundaries between the adjacent belt-like zones corresponded to the interface between parazone and diazone. Computer-reconstructed enamel prisms in the adjacent two zones were oriented in the opposite sidewards direction with occasional confluence and divergence. Scanning electron-microscopic observation of the developing enamel surface exposed by dissolution of the enamel organ revealed band-like arrangements of groups of pits encasing the Tomes' processes of secretory ameloblasts. The secretory faces of the pits inclined uniformly in the same sidewards direction, with those in the neighboring groups, in the opposite direction.

Immunocompetent cells in the enamel organ of rat incisors were examined immunohistochemically using OX6, ED1, and ED2 monoclonal antibodies known to recognize the Class II MHC molecules, a monocyte-macrophage lineage, and residential macrophages, respectively. The OX6 immunopositive cells (MHC cells) were located exclusively in the enamel maturation zone. MHC cells increased in number in the incisal direction and occasionally extended cytoplasmic processes deep into the ameloblast layer. Migration of MHC cells in the ameloblast layer were also encountered. MHC cells lacked phagolysosomes and could be distinguished from typical macrophages. ED2 immunopositive cells were not seen in the enamel organ. ED1 positive cells displayed identical localization to MHC cells except that some appeared in the transitional zone. MHC cells could not be seen in the enamel organ of rat molar tooth germs. Our data confirmed the presence of a large population of "dendritic" immunocompetent cells in the enamel organ of rat incisors and characterized the ultrastructural features of these cells. Biological significance of the immunocompetent cells in the enamel organ during amelogenesis needs to be clarified.

The relationship between the course of enamel prisms and the direction of secretory face in Tomes' process of ameloblast was examined three-dimensionally in the dog tooth by light and electron microscopy, as well as by reconstruction. Scanning electron microscopic observation on the developing enamel surface after the dissolution of the enamel organ showed numerous groups of pits with bulbous faces inclined in the same sidewards direction, while those faces in neighboring groups were inclined in an opposite direction. Tangential semithin sections from demineralized tooth germs were serially cut from the enamel surface to the enamel-dentin junction. These sections showed numerous belt-like zones arranged perpendicular to the meridian of the tooth. A straight row of enamel prisms perpendicular to the boundaries of the belt-like zones was selected at 142 microns from the enamel-dentin junction. The row of prisms and their successive Tomes' processes were reconstructed from micrographs with a personal computer. The cut ends of the enamel prisms initially perpendicular to the boundaries appeared as a sine curve, viewed from the enamel-dentin junction. In a single belt-like zone, the horizontal tilt angles of the enamel prisms towards the enamel-dentin junction tended to be largest at the center of the zone, and smallest near the boundaries. The long axis of each enamel prism was at right angles to the secretory face of the Tomes' process. The secretory faces in a single belt-like zone were inclined in the same sidewards direction and the secretory faces in neighboring zones were inclined in opposite directions. The results suggest that the direction of the sidewards displacement in ameloblasts is related to that of the secretory face of Tomes' processes. Further, one group of ameloblasts with their secretory faces inclined in the same sidewards direction form one belt-like zone.