Defining Cartilage Stem/Progenitor Cells During TMJ Development and Homeostasis

Objectives: The temporomandibular joint (TMJ) is a site for mandibular growth and is crucial for dental occlusion, speaking, and chewing. It is well known that stem cells have an important role in TMJ health and diseases. Previously, we identified Lgr5-Cre as a marker for progenitors which differentiate into disc, perichondrium, periosteum, and bone in the TMJ. However, cartilage stem/progenitor cells and their markers are still unknown. Our objective is to define cartilage stem/progenitor cells critical for TMJ development and homeostasis.
Methods: To identify TMJ cell populations in the mandibular condyles during development, we performed scRNA-seq analyses using E16.5 C57BL6 mice. To explore the localization for potential stem/progenitor cells, we performed in situ hybridization for each developmental stage. To confirm their contribution to TMJ development and homeostasis, we performed linage tracing experiments with Cre/loxP system.
Results: Histological analysis showed that the polymorphic zone harbors cartilage progenitor cells in the TMJ. scRNA-seq analysis using TMJ condyles from E16.5 showed Lgr5-expressing perichondrium/disc cell populations (5) and Acan-expressing cartilage cell populations (6). Trajectory analysis suggested that Thy1⁺ and Gli1⁺ populations are immature compared to other cartilage cell populations in E16.5 TMJ. In situ hybridization analysis showed that Gli1⁺ and Thy1⁺cells were localized in the polymorphic zone. We confirmed their contribution to TMJ development and homeostasis by using Gli1-CreERT; tdTomato mice and Thy1-Cre; tdTomato mice.
Conclusions: We found that Gli1⁺ and/or Thy1⁺ cells are cartilage progenitor cells critical for TMJ development and tissue homeostasis. Our on-going studies aim to determine key signal pathways which determine TMJ stem cell fate.