A new stretching apparatus for applying anisotropic mechanical strain to bone cells in-vitro

Bone is adapting to in-vivo loading by modeling and remodeling processes. The sensors of the external forces acting on the bone matrix seem to be the bone cells. Osteocytes, osteoblasts, and bone lining cells have been shown to respond to mechanical forces in-vitro. In this work, we describe a new in-vitro system which applies anisotropic stress conditions to MC3T3-E1, osteoblast-like mouse calvaria derived cells. The system allows stretching of cell cultures under well-defined stretching conditions. Cells are grown on an elastic polyurethane culture support (PUCS) that is subjected to uniaxial tensile stress using a direct current (dc) motor-driven linear positioning stage, situated within the incubator. The physical stretching parameters, the maximum elongation of the PUCS (the maximum strain applied to the cells), the strain rate, and the number of cycles, can be varied. First, the actual strains occurring at different locations of the PUCS were determined using optical methods. The surface strain appeared to be uniform over the PUCS and biaxial with a Poisson contraction nearly 80% in magnitude to the axial extension. Second, we tested the behavior of the MC3T3-E1 cells on PUCS compared to the cells grown in petridishes (PD). After 11 days of culture, cell number per dish on PUCS was significantly reduced to PD cultures (20% of control). At that time, cultures on PUCS reached confluency as compared to day 4 for the PD cultures. However, histochemical staining of alkaline phosphatase (ALP) and multilayer formation of the PUCS cultures appeared to be not significantly different from PD cultures. We also looked at the cytoskeleton by phalloidin staining, at vinculin, a protein of the cell-matrix and cell-cell interaction, and at fibronectin, a protein of the extracellular matrix using immuno staining methods. All these features tested so far seemed not to be different in cells cultured on PUCS compared to cultures in PD. Third, the responsiveness to the external force was tested using confluent cells on PUCS. A strain of 6.8 millistrain (6800 microstrain) was applied to the cells, using a strain rate of 4.9 millistrain/s and 350 cycles/h for a period of 48 h. These loading conditions led to significantly decreased cell proliferation, as measured by [H-3] deoxythymidine ([H-3] dT) incorporation, and significantly increased ALP activity. These data show that the stretching device introduced in this paper offers new possibilities to study the response of osteoblast-like cells to anisotropic forces. (C) 2000 American Institute of Physics. [S0034- 6748(00)02109-2].