It has been reported that, although type III collagen is present in human dentin where there is dentinogenesis imperfecta and in reparative dentin, it is absent in normal dentin. In a preliminary study, however, we observed evidence showing that small amounts of fibers showing positive labeling for type III collagen are present in the molars of normal mice. In the present study, in order to localize type III in normal dentin, immunofluorescent and immunoelectron microscopic examinations of the molars of normal mice were carried out using affinity-purified antibodies to mouse type III and type I collagen. The fibers positive for type III collagen were much more frequently observed in the root than in the crown. These fibers ran in peritubular dentin or near that in parallel to them. The incidence of the existence of dentinal tubules associated with type III collagen-positive fibrils either in or near peritubular dentin was low. These fibrils positive for type III collagen showed a clear cross-banding. In dentinal tubules, unusual collagen aggregations, segment long-spacing-like and fibrous long-spacing-like structures which were intensively stained for type I collagen but weakly so for type III collagen were seldom observed. Type III collagen-positive fibers often extended towards the pulp beyond the odontoblast layer, suggesting that these fibers were produced, at least partly, by the pulp cells.
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