Biochimica et Biophysica Acta, 1071 (1991) 273-290 © 1991 Elsevier Science Publishers B.V. All rights reserved 0304-4157/91/$03.50 273 BBAREV 85386 Review Shapes and shape changes in vitro in normal red blood cells Arnljot Elgsaeter and Arne Mikkelsen Unicersity of Trondheim, Department of Physics and Mathematics, Biophysics Group, Trondheim (Norway) (Received 29 January 1991) Contents !. Introduction ............................................................. 273 II. Membrane organization ..................................................... 274 III. Theoretical analysis of cell shape .............................................. 276 A. Equilibrium shapes ..................................................... 276 B. Shape fluctuations ...................................................... 276 IV. Lipid bilayer shape dependent free energ~ ....................................... 278 A. Helfrich free energy ..................................................... 278 B. Non-local bending ...................................................... 278 V. Protein skeleton shape dependent free energy ..................................... 279 A. Incompressible solid ..................................................... 279 B. Compressible ionic polymer gel ............................................. 279 VI. Glycocalyx shape dependent free energy ......................................... 280 VII. Erythrocyte membrane shape dependent free energy ................................ 280 VIII. Molecular basis for membrane mechanochemicai properties ........................... 281 A. Polymer welt lipid bilayer model ............................................ 281 B. Ionic gel membrane skeleton model .......................................... 284 IX. Conclusions, perspectives and prospects ......................................... 287 Acknowledgements ............................................................ 288 References .................................................................. 288 1. Introduction In humans and other mammals the erythrocytes are highly differentiated and have lost all cell organelles including the nucleus and the transcellular cyto- Correspondence: A. Elgsaeter, University of Trondheim, Depart- ment of Physics and Mathematics, Biophysics Group, Sere Saelands vei 9, NTH N-7034 Trondheim, Norway. skeleton. Normal human red cells have a characteristic axial symmetric biconcave disc shape with an average disk thickness of about 2.5 t~m and an average diame- ter of about 7.8 /zm. Human erythrocytes nave for many years been a popular cell model system. This is partly because these cells can easily be isolated in large quantities and partly because of their intriguing struc- tural and geometrical simplicity. There hardly exists any biophysical technique that has not been applied to