The occurrence of phase separation structures in biological membranes as derived from freeze-etch observations

Some structural specializations revealed in biological membranes by freeze-fracturing are probably attributed to phase separation in the lipid matrix. The visibility of rigid domains within the fluid bilayer has been demonstrated in model membranes. Characteristics of rigid domains are the ripple- or band-pattern or a change of the surface curvature. Additionally in biological membranes an exclusion of particles or a regular incorporation of particles are the most important properties of rigid domains. By the combination of different features 8 basic domain types can be postulated. Examples for 6 of these types have been found as normally occurring structures or by induction. These are in the group of smooth particle-free domains the flat circular type (SFC), the bulged circular type (SBC) and the bulged elongated type (SBE). In the group with regularly arranged particles are known the flat circular type (RFC), the flat elongated type (RFE) and the bulged elongated type (RBE).