A Microfluidic Platform and Atomic Force Microscopy for a Mechanobiology Study: Dynamic Compressive Stress Reduces Growth Plate Chondrocyte Stiffness

The growth plate plays a crucial role in the development of long bone in children and adolescents. Chondrocytes are the only cell type in the growth plate, and they respond to mechanical stimulations. Compressive stress is one of the major mechanical stimulations acting on the growth plate chondrocytes, and it changes the function of the growth plate chondrocytes as measured by gene expression, extracellular matrix (ECM) release, and the physical dimensions and alignment of chondrocytes in the growth plate. These adjusted chondrocyte functions appear to be due to the changes of chondrocyte cytoskeleton configurations which determine the stiffness of chondrocytes. Although the stiffness of chondrocytes is closely related to the integrity of the growth plate and chondrocyte function, it is largely unknown how chondrocyte stiffness changes in response to compressive stress. I hypothesized that the dynamic compressive stress reduces the stiffness of growth plate chondrocytes because the actin cytoskeleton of articular chondrocytes is known to be depolymerized due to dynamic compression.^ To study the effects of compressive stress on the stiffness of growth plate chondrocytes, I developed a microfluidic cell compression device to simultaneously apply different magnitudes of dynamic compressive stress on multiple alginate hydrogel constructs containing chondrocytes. After compression of the alginate-chondrocyte constructs, the change in chondrocyte stiffness was examined by the atomic force microscopy (AFM) indentation method. The chondrocytes, which were exposed to dynamic compression for 1 hour (14% pre-strain + 34% dynamic strain at 1 Hz), became softer than the uncompressed chondrocytes. Therefore, the proposed hypothesis was validated with the microfluidic platform and the AFM indentation method. This study has laid a foundation to study further how mechanically regulated chondrocyte stiffness affects the integrity of growth plate architecture.