Primary Chondrocyte Cilium Strains Increase with Depth Within the Growth Plate Reserve Zone and Reflect Loading Direction Across the Growth Plate

Abstract

Background. It is known that reserve zone (RZ) chondrocytes regulate the organization of the physis and that chondrocyte primary cilia (PC) act as mechanotransducers. Objective. To explore if physeal loading-induced strains in RZ chondrocyte PC can enable cells to sense whether the physis is subjected to increasing or decreasing levels of compression. Methods. Finite element models of RZ chondrocytes within the physis were created to examine strains developed in the PC when the physis is subjected to either 10% compression or tension. The PC were oriented either toward the epiphysis or metaphysis. Findings. PC basal body axial and transverse strains were negative when the physis was compressed and positive when subjected to tension. Axial strains in the cilium body transition zone also followed the applied loading and were amplified up to 4X, whereas membrane strains transverse to the cilium were tensile under physeal compression, possibly stretching integrin receptors. Tension applied to the physis stretched the cilium membrane in the axial direction. Conclusion. PC perceive strains in ways that may explain the regulatory function of RZ chondrocytes and their role in modulating bone growth.