Behavior of elliptocytes under shear stress in the rheoscope and ektacytometer

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Abstract
When ellipsoidal red cells from patients with hereditary elliptocytosis are subjected to shear stress in a rheoscope the cells rotate around their short axis and align themselves perpendicular to the flow. At high shear stress they deform into an ellipsoid of revolution with the major axis of the ellipse normal to the flow. This change in the direction of the long axis is not the result of simple reorientation of the cells. Rather, the short axis of an elliptocyte exposed to increasing shear stress expands while its long axis contracts. It is this process that eventually brings about the apparent reorientation.Elliptocytes in a well mixed suspension are randomly oriented. As a consequence, the initial diffraction pattern displayed by the ektacytometer is a hazy circle. As the shear stress is increased this becomes rhomboid due to expansion of the central region of each cell (a change confirmed by the rheoscope). Central expansion coexisting with polar remnants of the resting elliptocytic cell shape gives rise to the rhomboid outline. As the shear stress in the ektacytometer approaches 120 dynes/cm2 the characteristic, elliptical diffraction pattern seen with normal cells makes its appearance. As the shear stress is decreased, the rhomboid pattern reappears followed by a new elliptical pattern oriented at right angles to the one seen under high shear. This pattern arises from the resting shape of the elliptocytes which have all been oriented at right angles to the shear stress as it decreased to zero. As a consequence of this uniform cellular orientation, all exposures to shear stress after the initial one start with a deformability index of less than zero if it is calculated in the usual fashion from the axial ratio of the shear deformed cells. Certainly under these circumstances and possibly in other conditions as well the use of the less‐specific term “ektacytometric index” would be less misleading.