Effects of compressive loading on chondrocyte differentiation in a novel chick limb bud cell-agarose model of chondrogenesis.

Abstract

Although the mechanical environment is widely believed to help regulate connective-tissue cell differentiation, details of the regulation are not well understood. The purpose of this research was to quantify the effects of mechanical loading on chondrocyte differentiation in vitro. A computer-controlled, electropneumatic device was used to apply compressive loads to chick limb bud cells embedded in agarose gel, and chondrogenesis was assessed using histological and biochemical techniques. In addition, finite element models were used to improve characterization of the loading in terms of cell stress and deformation. Cyclic compression at 0.33 Hz to a peak stress of 9.0 kPa for two hours during each of the initial three days in culture caused a two-fold increase in chondrogenesis (assessed on Day 8). The effect was diminished by loading at lower frequencies, and chondrogenesis was not enhanced by a 4.5 kPa static compression. Furthermore, conditioned medium from cyclically loaded cultures increased chondrogenesis in non-loaded cultures to the same extent as cyclic loading. Finite element (FE) analysis predicted axial cell strain that was above the level of applied axial gel strain and that increased at a higher loading rate. FE analysis also predicted that cell deformation was coupled with increased hydrostatic pressure, both in the cell and in the agarose matrix. It was concluded that cyclic compressive loading can increase the proportion of chick limb bud cells that become chondrocytes, and that this effect is mediated by secretion of a soluble differentiation factor. Results of the finite element analysis suggest that the biological response to dynamic hydrostatic pressure at the macroscopic level may be related to the associated cellular deformation at the microscopic level.