Effect of compressive loading on chondrocyte differentiation in agarose cultures of chick limb-bud cells

It is well established that mechanical loading is important to homeostasis of cartilage tissue, and growing evidence suggests that it influences carti lage differentiation as well. Whereas the effect of mechanical forces on ch ondrocyte biosynthesis and gene expression has been vigorously investigated , the effect of the mechanical environment on chondrocyte differentiation h as received little attention. The long-term objective of this research is t o investigate the regulatory role of mechanical loading in cell differentia tion. The goal of this study was to determine if mechanical compression cou ld modulate chondrocyte differentiation in vitro. Stage 23/24, chick limb-b ud cells, embedded in agarose gel, were subjected to either static (constan t 4.5-kPa stress) or cyclic (9.0-kPa peak stress at 0.33 Hz) loading in unc onfined compression during the initial phase of commitment to a phenotypic lineage. Compared with nonloaded controls, cyclic compressive loading rough ly doubled the number of cartilage nodules and the amount of sulfate incorp oration on day 8, whereas static compression had little effect on these two measures. Neither compression protocol significantly affected overall cell viability or the proliferation of cells within nodules. Since limb-bud mes enchymal cells were seeded directly into agarose, an assessment of cartilag e nodules in the agarose reflects the proportion of the original cells that had given rise to chondrocytes. Thus, the results indicate that about twic e as many mesenchymal cells were induced to enter the chondrogenic pathway by cyclic mechanical compression. The coincidence of the increase in sulfat e incorporation and nodule density indicates that the primary effect of mec hanical compression on mesenchymal cells was on cellular differentiation an d not on their subsequent metabolism. Further studies are needed to identif y the primary chondrogenic signal associated with cyclic compressive loadin g and to determine the mechanism by which it influences commitment to or pr ogression through the chondrogenic lineage, or both.