Articular cartilage and growth plate defects are associated with chondrocyte cytoskeletal abnormalities in Tg737orpk mice lacking the primary cilia protein polaris

Primary cilia are highly conserved organelles found on almost all eukaryotic cells. Tg737orpk (orpk) mice carry a hypomorphic mutation in the Tg737 gene resulting in the loss of polaris, a protein essential for ciliogenesis. Orpk mice have an array of skeletal patterning defects and show stunted growth after birth, suggesting defects in appositional and endochondral development. This study investigated the association between orpk tibial long bone growth and chondrocyte primary cilia expression using histomorphometric and immunohistochemical analysis. Wild-type chondrocytes throughout the developing epiphysis and growth plate expressed primary cilia, which showed a specific orientation away from the articular surface in the first 7–10 cell layers. In orpk mice, primary cilia were identified on very few cells and were significantly shorter. Orpk chondrocytes also showed significant increases in cytoplasmic tubulin, a likely result of failed ciliary assembly. The growth plates of orpk mice were significantly smaller in length and width, with marked changes in cellular organization in the presumptive articular cartilage, proliferative and hypertrophic zones. Cell density at the articular surface and in the hypertrophic zone was significantly altered, suggesting defects in both appositional and endochondral growth. In addition, orpk hypertrophic chondrocytes showed re-organization of the F-actin network into stress fibres and failed to fully undergo hypertrophy, while there was a marked reduction in type X collagen sequestration. These data suggest that failure to form a functional primary cilium affects chondrocyte differentiation and results in delayed chondrocyte hypertrophy within the orpk growth plate.