Cochleate lipid cylinders: formation by fusion of unilamellar lipid vesicles

Freeze-fracture electron microscopy was used to study the morphological changes occurring following the addition of Ca²⁺ to sonicated preparations of phosphatidylserine in aqueous NaCl buffer. Before the addition of Ca²⁺, preparations contained only small (200-500 Å diameter) spheroidal vesicles. After the addition of Ca²⁺ (10 mM) and incubation for 1 h at 37°C preparations contained only large (2000-10 000 Å) apparently multilamellar structures many of which were cylindrical in shape. The lamellae in these cylinders appear to be folded in a spiral configuration. Addition of EDTA to these preparations produced large, closed, spherical, unilamellar vesicles. We suggest the name cochleate lipid cylinders for the spiral structures and propose that they are formed by fusion of unilamellar vesicles into large sheets which fold spirally to form cylinders.