Reticulocyte rigidity and passage through endothelial-like pores

The importance of cell rigidity in regulating the release of reticulocytes from the bone marrow has been investigated in a model system. Reticulocytes were obtained from phlebotomized rabbits and separated from whole blood by discontinuous density gradient centrifugation. The mechanical properties of the cells were tested. Using single-cell micromechanical techniques, the membrane elastic rigidity and the viscoelastic response of reticulocyte and mature cell populations were measured. The reticulocyte membranes were more rigid than the mature membranes, but the reticulocyte properties were heterogeneous, and some cells exhibited behavior indistinguishable from the mature cells. The mean time constant for viscoelastic recovery was the same for reticulocytes as for mature cells, but the variability within the reticulocyte population was greater. The possible influence of this increased rigidity on cell egress from the bone marrow was tested using an in vitro model of the thin endothelial pores found within the marrow. A silicon wafer approximately 0.1 microns in thickness and containing a small (1.2-microns diameter) pore in its center was cemented over the tip of a large (15.0-microns ID) micropipette. The passage of cells through the pore was observed as a function of the pressure across the pore. Consistent with the difference in mechanical properties, the reticulocytes required greater pressures (as great as 4.0 mm Hg compared with less than 1.0 mm Hg) and took longer to traverse the pore. These measurements support the postulate that deformability is important in the regulation of the release of cells from bone marrow.