Freeze-fracture studies on lipids and membranes

Freeze-fracture electron microscopy is especially useful for investigation of lipid structures by the advantageous fracture course within hydrophobic zones. Freezing is, on the other hand, a restriction because the structures of lamellar and non-lamellar phase states with disordered acyl chains (L(alpha), H(II,) cubic) are difficult to preserve. An important aspect of this method is therefore the lipid structure of phase states with ordered acyl chains (crystal, gel), and with a different degree of hydration. Freeze-fracture of pure lipid systems creates a valid representation of the structure of non-lamellar phases and of the general structure of the "lamellar" lipid bilayer, and lamellar phases with characteristic deformations (ripples, curvatures, plane sectors) can be identified. Fracture through the hydrophobic bilayer centre of biological membranes reveals characteristic protein components, the intramembraneous particles (IMPs). The lateral distribution of the IMPs is a helpful marker for fluid and rigid phase states, also without deformation of the lamella. The overall history and the present state of knowledge concerning the different structures revealed by the freeze-fracture and freeze-etch techniques in lipid systems, and to a limited extent in biological membranes, is reviewed, taking into account studies from our own laboratory.