Expression of type I and XII collagen during development of the periodontal ligament in the mouse

The purpose (of this study) was to determine the temporal and spatial pattern of type XII collagen expression during murine tooth/root development. Using in situ hybridization techniques, expression of type XII collagen was compared with that of type I collagen, the most abundant collagen in periodontal tissues. Mouse first mandibular molars were examined at the following developmental periods: pre-root formation, early root formation, initial alignment of the periodontal ligament (PDL) fibres, and PDL maturation as the tooth erupted and attained occlusal function. Transcripts for type I collagen were identified in bone cells and odontoblasts at all times but not in the dental follicle before root formation. As root formation progressed, type I collagen expression became apparent within cells of the dental follicle and forming PDL. During early stages of tooth development, signal for type XII collagen was not observed in any cells/tissues. Type XII collagen expression was first detected in the dental follicle/PDL region during tooth eruption and increased in the PDL as the molar tooth erupted into the mouth and achieved occlusal contact. These findings suggest that type XII expression is timed with the alignment and organization of PDL fibres and is limited in tooth development to cells within the periodontal ligament.