Cell and nucleus deformation in compressed chondrocyte-alginate constructs: temporal changes and calculation of cell modulus.

Mechanical loading is essential for the homeostasis of articular cartilage and may be necessary for achieving functional tissue engineered cartilage repair using isolated cells seeded in scaffolds such as alginate. Chondrocyte mechanotransduction is poorly understood, but may involve cell deformation and associated distortion of intracellular organelles. The present study used confocal microscopy to examine cell and nucleus morphology in isolated chondrocytes compressed in alginate constructs. Compression of 2% alginate resulted in cell deformation from a spherical to an oblate ellipsoid morphology with conservation of cell volume. Cell deformation was associated with deformation, to a lesser degree, of the nucleus. Despite constant cell deformation over a 25 min period of static compression, the nucleus deformation reduced significantly, particularly in the axis perpendicular to the applied compression. Constructs made of a lower alginate concentration exhibited a reduced compressive modulus with an altered cellular response to compression. In 1.2% alginate, compression resulted in cell deformation which was initially of a similar magnitude to that in 2% alginate but subsequently reduced over a 60 min period reflecting the viscoelastic behaviour of the gel. This phenomenon enabled the calculation of a stress-strain relationship for the cell with an estimated Young's modulus value of approx. 3 kPa.