Tangent counting for objective assessment of erythrocyte shape changes.

The resting shape of the human erythrocyte is maintained mainly by the elastic properties of its membrane. There is evidence that negative spontaneous membrane curvature is the cause of the biconcave shape; also negative membrane curvature is thought to be enhanced under "stomatocytogenic" influences and reduced under "echinocytogenic" influences. Experimental tests of these hypotheses are difficult in the absence of objective methods to quantify cell shape and/or membrane curvature. A new method is introduced to estimate the changes in convex and concave curvatures of the cell outlines in larger populations of RBC. It is based on the so-called "tangent count" procedure, carried out on microphotographs of freely suspended, randomly oriented RBCs in isotonic solutions. The method was validated by subjective shape assessment according to BESSIS' classification. It proved to be most sensitive in the range of moderate shape changes (classified subjectively as echinocyte II and stomatocyte II).