Abstract A possible physical explanation of the echino- cyte-spheroechinocyte red blood cell (RBC) shape trans- formation induced by the intercalation of amphiphilic molecules into the outer layer of the RBC plasma mem- brane bilayer is given. The stable RBC shape is determined by the minimization of the membrane elastic energy, con- sisting of the bilayer bending energy, the bilayer relative stretching energy and the skeleton shear elastic energy. It is shown that for a given relative cell volume the calcu- lated number of echinocyte spicula increases while their size decreases as the number of the intercalated amphi- philic molecules in the outer layer of the cell membrane bilayer is increased, which is in agreement with experi- mental observations. Further, it is shown that the equilib- rium difference between the outer and the inner membrane leaflet areas of the stable RBC shapes increases if the amount of the intercalated amphiphiles is increased, thereby verifying theoretically the original bilayer couple hypothesis of Sheetz and Singer (1974) and Evans (1974). Key words Red blood cell · Spheroechinocyte · Membrane skeleton · Cell shape · Elastic energy 1 Introduction The red blood cell (RBC) shape may be altered by varying different chemical and physical conditions which affect the properties of the membrane and the volume of the cell (Deuticke 1968; Gimsa and Ried 1995; Seifert and Lipow- sky 1994; Kralj-Iglic ˇ et al. 1996). In particular, in terms of the bilayer couple model (Evans 1974; Sheetz and Singer 1974), by keeping the volume of the cell (V) constant, the RBC shape changes due to the change in the conditions which cause the change of the difference between the outer and the inner monolayer areas (∆A) of the bilayer. Experi- ments (Sheetz and Singer 1974; Isomaa et al. 1987; Gedde et al. 1997) strongly indicate that lowering the area differ- ence ∆A causes the normal (discocytic) RBC shape to change towards the cup (stomatocytic) shape, while an in- crease of ∆A causes the transformation of the discocytic shape into the spiculated (echinocytic) shape. The exoge- neously induced discocyte-echinocyte transformation is generally reversible by washing (Brecher and Bessis 1972) indicating that this RBC shape transformation is usually not connected to some irreversible change in the confor- mation of the membrane skeleton or the membrane bilayer. An increase of ∆A, and the consequent discocyte-echin- ocyte transformation, can be induced by the intercalation of the amphiphilic molecules in the outer layer of the RBC mem- brane bilayer (Sheetz and Singer 1974; Isomaa et al. 1987). Spheroechinocytes, having less prominent spicula than echinocytes (Brecher and Bessis 1972; Bessis 1973), may be developed at higher concentration of the echinocytogenic amphiphilic molecules (Hägerstrand et al. 1992). The sphe- roechinocyte spicula become progressively narrower with increasing amphiphile concentration (Brecher and Bessis 1972; Bessis 1973; Sheetz and Singer 1974; Isomaa et al. 1987). When the concentration of echinocytogenic amphi- philic molecules is further increased, true spheres (sphero- cytes) appear and finally hemolysis occurs (Brecher and Bes- sis 1972; Isomaa et al. 1987). It was also observed that after reaching spherocytic shape RBCs release exomicrovesicles from the membrane (Hägerstrand and Isomaa 1989). Eur Biophys J (1998) 27: 335– 339 © Springer-Verlag 1998 Received: 22 August 1997 / Revised version: 25 November 1997 / Accepted: 11 February 1998 Ales ˇ Iglic ˇ · Veronika Kralj-Iglic ˇ · Henry Hägerstrand Amphiphile induced echinocyte-spheroechinocyte transformation of red blood cell shape ARTICLE A. Iglic ˇ () Laboratory of Applied Physics, Faculty of Electrical Engineering University of Ljubljana, SI-1000 Ljubljana, Slovenia e-mail: ales.iglic@fe.uni-lj.si V. Kralj-Iglic ˇ Institute of Biophysics, Medical Faculty, University of Ljubljana, SI-1000 Ljubljana, Slovenia H. Hägerstrand Department of Biology, Åbo Akademi University, FIN-20520 Åbo/Turku, Finland