Chondrocyte Deformation Under Extreme Tissue Strains

Abstract

Articular cartilage cells (chondrocytes) are responsible for maintaining the health and integrity of the tissue extracellular matrix (ECM). Chondrocyte activity under mechanical loading has been linked to changes in joints leading to osteoarthritis (OA). The purpose of this study was to investigate the deformation behavior of chondrocytes in their native environment using a novel in situ experimental approach.Patellae were extracted from New Zealand White rabbits immediately after sacrifice. Tissue samples were incubated in ECM-marking dye for 5 hours prior to testing. Static compressive loads were applied to the samples in this order: 10%, 20%, 30%, 40%, 60%, and 80% strain. Confocal image sections were recorded before and at each load after 15 minutes. Local ECM strain and cell deformations were analyzed to quantify the overall response of the cartilage and cells.Local compressive ECM strains increased with increasing applied nominal tissue deformation, from 20.4% to 58.6% at 10% and 80% applied tissue strain, respectively. Chondrocyte volume increased slightly under a 10% nominal tissue strain, then decreased at a 20% strain, followed by a further decrease at 30%, before remaining essentially constant up to 80% applied tissue strain. Cell height compressive strain increased in a similar fashion to the ECM strain. Cell strains in the transverse directions (width and depth) remained approximately uniform for nominal tissue strains ranging from 10% to 60%, however at 80% nominal tissue strain, the transverse cell dimensions increased greatly.These results provide new insight into the deformation behavior of chondrocytes in their native environment under physiological and extreme nominal tissue strains. The relative stability of cell volumes and the transverse dimensions of cells under increasing tissue loads is consistent with the hypothesis that the extra- and peri-cellular matrices protect cells from excessive strains in situations of large cartilage deformation.