Temporo-mandibular Joint Development Requires Indian hedgehog Signaling

Objectives: The temporo-mandibular joint (TMJ) is essential for jaw function, but the mechanisms regulating its development remain poorly understood. Since Indian hedgehog (Ihh) regulates trunk and limb skeletogenesis, we studied its possible roles in TMJ development. Methods: TMJ samples from E15.5 to E18.5 Ihh^-/- and double Ihh^-/-;Gli3^-/- null mouse embryos and their wild type littermates were analyzed by anatomical, histological and in situ hybridization procedures. Results: In wild type mouse embryos, Ihh expression was already strong in condylar cartilage by E15.5, and expression of Ihh receptors (Ptch1 and Smoothened) and effector genes (Gli1, Gli2, Gli3 and PTHrP) indicated that Ihh range of action normally reached the apical condylar tissue layers, including the polymorphic chondroprogenitor layer and articular disc primordia. In Ihh^-/- embryos, TMJ development was severely compromised. Condylar cartilage growth, polymorphic cell proliferation and PTHrP expression were all inhibited, and growth plate organization and chondrocyte gene expression patterns including Collagen type II, and type X, osteopontin and PTHrP Receptor were altered. These severe defects were partially corrected in double Ihh^-/-/Gli3^-/- mutants, signifying that Ihh action in condylar cartilage is normally modulated and delimited by Gli3 action and Gli3 repressor in particular. Both single and double mutants, however, failed to form an articular disc primordium, normally appreciable as an independent condensation located between condylar apex and neighboring developing temporal bone . This failure persisted at later stages, leading to complete absence of a normal functional disc and lubricin-expressing joint cavities. Conclusion: The data demonstrate that Ihh is pivotal for TMJ development where it appears to regulate growth and elongation events, condylar cartilage phenotype and chondroprogenitor cell function. Absence of articular disc and joint cavities in single and double mutants points to irreplaceable Ihh roles in formation of those critical TMJ components. Supported by NIH R01AR47543 and AG25868.