What is red cell deformability?

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Abstract
Microscopic flow visualization of the process of red cell adaptation to flow shows that red cell deformation in flow is the consequence of a continuous viscous rather than an elastic deformation. This fluid drop-like adaptation primarily depends on: (a) the fluidity of the cytoplasm and (b) the favourable surface-area-to-volume ratio, with an excess of surface area allowing strong deformations without an increase in surface area (a real strain). (c) In contrast to previous notions, the modulus of shear elasticity of the membrane is probably less significant. After many attempts to differentiate the contribution of bending and shear stiffness to the elastic recovery of the normal biconcave cell shape have not produced equivocal results, we have changed the elastic shear modulus experimentally by cross-linking the spectrin using the membrane-permeant, bifunctional SH-reagent diamide, which allows to increase the elastic shear modulus in a dose-dependent manner. Despite a 25-fold decrease in compliance the DIAMIDE-treated cells have normal shape and show remarkably small changes in the rheological behaviour when tested in vitro and in vivo.