ASSOCIATION BETWEEN THE EXPRESSION OF MURINE 72 KDA TYPE-IV COLLAGENASE BY ODONTOBLASTS AND BASEMENT-MEMBRANE DEGRADATION DURING MOUSE TOOTH DEVELOPMENT

In situ hybridization was used to study the expression of the 72 kDa type IV collagenase gene and its association with morphogenesis and cell differentiation during advancing mouse tooth development. The epithelia were completely negative during all developmental stages. The dental mesenchyme was uniformly positive during the early stages of tooth morphogenesis, and no association of type IV collagenase with morphogenetic events was observed. However, at the bell stage the expression increased in differentiating preodontoblasts. Expression was intense in the odontoblasts during secretion of the first predentine matrix. The expression was, however, transient; it decreased around the time when mineralization of dentine started until it completely ceased. Transcripts for 72 kDa type IV collagenase also gradually disappeared from the dental pulp. The expression of 72 kDa type IV collagenase was also strong in the osteoblastic cell lineage. The preosteoblasts at the beginning of the formation of mandibular bone as well as the osteoblasts of the alveolar bone expressed more 72 kDa type IV collagenase than did other mesenchymal cells. The increased gene expression in the odontoblasts correlates with the disappearance of the dental basement membrane as shown by immunolabelling with antibodies against type IV collagen. The onset of increased expression in the odontoblasts preceded the disappearance of the basement membrane and at the time when type IV collagenase transcripts were lost from all odontoblasts the basement membrane was completely removed. It can be speculated that during early stages of tooth development the 72 kDa type IV collagenase acts as a gelatinase whereas during later stages, when odontoblasts and ameloblasts differentiate and the deposition of predentine and enamel matrix is initiated, the enzyme may act as a type IV collagenase and contribute to the degradation of the dental basement membrane.