MECHANOTRANSDUCTION IN BONE - OSTEOBLASTS ARE MORE RESPONSIVE TO FLUID FORCES THAN MECHANICAL STRAIN

Mechanical force applied to bone produces two localized mechanical sig nals on the cell: deformation of the extracellular matrix (substrate s train) and extracellular fluid flow. To study the effects of these sti muli on osteoblasts, MC3T3-E1 cells were grown on type I collagen-coat ed plastic plates and subjected to four-point bending. This technique produces uniform levels of physiological strain and fluid forces on th e cells. Each of these parameters can be varied independently. Osteopo ntin (OPN) mRNA expression was used to assess the anabolic response of MC3T3-E1 cells. When fluid forces were low, neither strain magnitude nor strain rate was correlated with OPN expression. However, higher-ma gnitude fluid forces significantly increased OPN message levels indepe ndently of the strain magnitude or rate. These data indicate that flui d forces, and not mechanical stretch, influence OPN expression in oste oblasts and suggest that fluid forces induced by extracellular fluid f low within the bone matrix may play an important role in bone formatio n in response to mechanical loading.