Effects of age and shear rate on the rheological properties of human yellow bone marrow

Bone marrow is the niche for stem cells and is within close proximity to bone lining cells. Forces experienced by these cells guide their differentiation and proliferation. As these forces are dependent on the viscosity of the medium, the knowledge about the viscosity of marrow is essential to modeling the mechanical environment of bone. This study sought to examine the effects of age on the rheological properties of human yellow bone marrow. Samples were harvested from the femurs of male donors ranging from 22 to 82 years of age (N=19) and subjected to stress and frequency sweeps to determine viscosity and dynamic moduli, respectively. The viscosity of bone marrow at physiologically plausible shear rates ranged from 44 to 142 mPa·s. The coefficients of variation ranged up to 0.40 within subjects and 0.14 between subjects. Regressions of viscosity values against age did not generate a strong level of significance; therefore, earlier reported changes in the composition of marrow with age did not translate into variation in viscosity of marrow. Since age does not seem to offer a governing effect, the observed variation within and between donors may stem from other factors (genetic, nutrition, etc.). The wide range of variation in the viscosity of marrow within subject, between subjects and with age implies that the fluid shear experienced by cells resident in marrow may also vary substantially.