Indian hedgehog roles in post-natal TMJ development and organization

Area of Science:

• Developmental Biology
• Orthodontics
• Regenerative Medicine

Background:

• Indian hedgehog (Ihh) signaling is vital for embryonic mandibular condylar and temporomandibular joint (TMJ) disc development.
• The role of Ihh in post-natal TMJ growth and organization remains largely unexplored.

Purpose of the Study:

• To investigate the necessity of Indian hedgehog (Ihh) signaling for post-natal temporomandibular joint (TMJ) development and organization.
• To elucidate the effects of Ihh ablation on TMJ growth, cellular proliferation, and extracellular matrix composition.

Main Methods:

• Conditional ablation of Indian hedgehog (Ihh) in the cartilage of neonatal mice.
• Assessment of temporomandibular joint (TMJ) morphology, growth, and cellular organization over time.
• Analysis of gene and protein expression related to chondrogenesis and fibrocartilage formation.

Main Results:

• Ihh deficiency in neonatal mice led to significant condylar disorganization and growth retardation.
• Reduced proliferation in the polymorphic cell layer and altered expression of key chondrogenic markers (Sox9, Runx2, Osterix) and matrix components (collagen II, collagen I, aggrecan).
• TMJ disc formation occurred but with morphological defects, partial fusion, reduced synovial cavity, and unexpectedly increased lubricin expression.

Conclusions:

• Continuous Indian hedgehog (Ihh) signaling is essential for the complete post-natal growth and organization of the temporomandibular joint (TMJ).
• Ihh deficiency disrupts the fibrocartilaginous nature of the condyle and affects TMJ disc development.
• Overexpression of lubricin in Ihh-deficient mutants may indicate a compensatory mechanism to maintain TMJ function.