Tank Tread Motion of Red Cell Membranes in Viscometric Flow: Behavior of Intracellular and Extracellular Markers (with Film)

Abstract

With a cone and plate ‘rheoscope’, red cell suspensions can be observed while subjected to shear. Interference contrast optics and high speed cinematography allow resolution of details of deformation in native, unstained red blood cells (RBC) with an optical resolution of 0.3 μm and a time resolution of 2 ms. Movement of membrane and cytoplasm of RBCs is monitored by various markers. At shear rates above 500/s, red cells in whole blood (Hct > 40%) show elongation parallel to flow direction, whereas single red cells suspended in plasma (Hct < 5%) take irregular polyhedral forms and tumble in a shear flow at the same shear rate. Single cells suspended in viscous solutions (e.g. Dextran) under shear are deformed to form flat ellipsoids. Their elongation increases asymptotically towards a maximum with increasing shear stress. Ellipsoidally deformed RBCs assume a stationary orientation in the shear field, the long axis lying in a plane through the center of the cell containing the flow direction and perpendicular to the plate of the rheoscope. The angle between the long axis and the flow direction is zero only for very small and very large elongations. The membrane shows tank tread motion, the frequency of which increases linearly with the shear rate. Shear flow is transmitted from the continuous phase into the cytoplasm of the cell and can be observed directly by cytoplasmic markers. The shear rate within the cytoplasm is approximately constant. Assuming Newtonian behavior of both the continuous phase and the intracellular hemoglobin solution, the calculated shear stresses within the cell are lower by a factor of 4 than those in the continuous phase. Moderate stomatocytes and echinocytes (I and II) are also deformed to form stationary ellipsoids, irrespective of their preparation by different agents. The shear stresses required for their elongation are of the same order of magnitude as those required for discocyte elongation. Fragmentation of RBCs in shear flow and an altered appearance of the membrane after its plastic deformation in excessive shear could be observed.