Tethering forces as a function of pipette holding pressure.

(A) The square of the equilibrium tethering force as a function of the membrane tension for selected cells. Open symbols represent measurements made on mature RBCs, dark symbols correspond to marrow reticulocytes, and gray symbols correspond to normoblasts. Linear regressions to each group of cells and 95% confidence intervals for the fits are shown as solid lines amid the data. (B) Square of the tethering force as a function of the membrane tension for a broader representation of cells from marrow. A continuous range of behavior was observed, from the force being independent of the membrane tension induced by the aspiration pressure to a linear dependence of the force squared on membrane tension. Intermediate behaviors shown here probably reflect changes in cytoskeletal rigidity and a transition from a highly wrinkled surface to the smooth contour of a nearly mature reticulocyte. Each group of connected symbols represents a different cell, all of which are marrow reticulocytes except the lowermost 4 curves (solid symbols: square, up-triangle, down-triangle and circle), which are normoblasts.