Expression of Indian Hedgehog during postnatal development of mandibular condyle

Objectives: The temporo-mandibular joint (TMJ) is essential for jaw function, but the mechanisms regulating its growth remain poorly understood. Since Indian hedgehog (Ihh) regulates trunk and limb skeletogenesis, we studied its expression pattern during postnatal development of the mandibular condyle. Methods: This study was approved by the Animal Research Committee of Tokyo Dental College. Five postnatal male ICR mice aged 1, 7, 21, 35 and 56 days were used. TMJ samples were analyzed by anatomical, histological and in situ hybridization procedures. Gene expression in the mandibular condylar cartilage was monitored by real-time PCR and laser microdissection. Results: At each stage examined, the condyles displayed distinct growth plate-like zones including a superficial layer, a polymorphic precursor cell layer, a flatten chondrocyte zone and a hypertrophic chondrocyte zone. In neonatal condyles, Ihh transcripts were abundant and clearly detectable in flatten chondrocyte- and upper hypertrophic zones, but undetectable in the polymorphic zone. Ihh receptor (Ptch1) and effector genes (Gli1 and PTHrP) transcripts were clearly detectable in the polymorphic zone. These patterns were maintained in 56-day condyles, although Ihh expression levels decreased. Ihh gene expression level peaked at 7 days and decreased from 21 days onward. PTHrP gene expression level peaked at 1 and 7 days and tended to decrease from 21 days onward. Expression levels of the osteopontin gene increased gradually over 7-56 days. Conclusion: The data suggest that the polymorphic zone of the mandibular condyle is a major target of Hh signaling activity and action and that the expression level of Ihh is high in the relatively early stages of development prior to weaning.