Transient Expression of Type III Collagen by Odontoblasts: Developmental Changes in the Distribution of Pro-α1 (III) and Pro-α1 (I) Collagen mRNAs in Dental Tissues

The expression of pro-_α1 (III) and pro-_α1 (I) collagen mRNAs in mouse and human dental tissues during tooth development and after its completion was analyzed by in situ hybridization, with use of [³⁵S]-labeled RNA probes. The expression of pro-_α1 (III) mRNA was also compared to that of the protein product, as localized by immunostaining with polyclonal antibodies to type III collagen and the N-terminal propeptide of type III procollagen. Contrary to many previous reports, our results suggest that odontoblasts express type III collagen. While proal (III) transcripts were less intensely expressed in odontoblasts than pro-_α1 (I) transcripts, the amounts of both mRNAs increased in odontoblasts with progressing dentin formation, and decreased toward its completion. In contrast to pro-_α1 (III) mRNA, pro-_α1 (I) mRNA was still detectable in odontoblasts of fully developed teeth. Type III collagen immunoreactivity was observed in the early predentin, and again in predentin toward the completion of dentinogenesis, when mRNA was no longer detected. Also in the pulp, the protein product, unlike pro-_α1 (III) mRNA, was relatively strongly expressed. Hence, these immunostaining patterns were inversely related to the expression of pro-_α1 (III) mRNA, suggesting accumulation of the protein. The mesenchymal cells, when condensed in the region of the future mandibular bone, expressed proal (III) mRNA intensely, whereas osteoblasts expressed pro-_α1 (I) but not pro_α1 (III) transcripts strongly. Cell type- and developmental stage-related differences in the expression of the two mRNAs suggest that type I1type III collagen ratio influences the structure of dental tissues.