Shapes of Red Blood Cells: Comparison of 3D Confocal Images with the Bilayer-Couple Model

Abstract

Abstract-Cells and organelles are shaped by the chemical and physical forces that bend cell membranes. The human red blood cell (RBC) is a model system for studying how such forces determine cell morphology. It is thought that RBCs, which are typically biconcave discoids, take the shape that minimizes their membrane-bending energies, subject to the constraints of fixed area and volume. However, recently it has been hypothesized that shear elasticity arising from the membrane-associated cytoskeleton (MS) is necessary to account for shapes of real RBCs, especially ones with highly curved features such as echinocytes. In this work we tested this hypothesis by following RBC shape changes using spherical harmonic series expansions of theoretical cell surfaces and those estimated from 3D confocal microscopy images of live cells. We found (i) quantitative agreement between shapes obtained from the theoretical model including the MS and real cells, (ii) that weakening the MS, by using urea (which denatures spectrin), leads to the theoretically predicted gradual decrease in spicule number of echinocytes, (iii) that the theory predicts that the MS is essential for stabilizing the discocyte morphology against changes in lipid composition, and that without it, the shape would default to the elliptocyte (a biconcave ellipsoid), (iv) that we were able to induce RBCs to adopt the predicted elliptocyte morphology by treating healthy discocytes with urea. The latter observation is consistent with the known connection between the blood disease hereditary elliptocytosis and spectrin mutations that weaken the cell cortex. We conclude that while the discocyte, in absence of shear, is indeed a minimum energy shape, its stabilization in healthy RBCs requires the MS, and that elliptocytosis can be explained based on purely mechanical considerations.

Measurement of the membrane curvature preference of phospholipids reveals only weak coupling between lipid shape and leaflet curvature

“…According to Eq. 5, both the PE and PS lipids should be equally distributed in the bilayer leaflets as the membrane curvature of erythrocyte is very small [<0.001 nm −1 (43)]. Therefore, factors other than membrane curvature-such as lipid flippases-must be driving the lipid asymmetry (27).…”

White-light diffraction tomography of unlabelled live cells

“…This morphological abnormality is well documented and can be an indication of transitory stress (for example, osmotic stress) or a sign of a serious disease 21,43 . We used this interesting three-dimensional morphology as a test sample and used a scanning electron microscopy (SEM) image and a confocal fluorescence microscopy image 44 as control imaging methods (Fig. 2a) .…”

Interplay of Platelet Contractility and Elasticity of Fibrin/Erythrocytes in Blood Clot Retraction

“…The incorporation of RBCs into the blood clot volume disrupts the uniformity of the fibrin network (23), affects the mechanical properties of the clot (24), and influences the process of clot contraction (24). RBCs are easily deformed (25), and when subjected to compressive forces such as those generated by contracting platelets, they become packed in the core of the blood clot and take on a polyhedral shape (14).…”