Lipid microdomains are formed whenlipids undergo lateralphase separations yielding stable coexistinglamellardomains. Thesephase separations can be induced by changes intemperature,pressure,ionic strength or by the addition ofdivalentcations orproteins. The question of whether such lipid microdomains observed in model lipid systems also exist inbiomembranes had motivated considerable research efforts. Lipid domains are not readily isolated and examined as unique species, in contrast to the examples of lateralheterogeneity.One can disrupt the membrane and demonstrate a heterogeneous range ofcomposition in thepopulation of the resultingvesicles orfragments.Electron microscopy can also be used to demonstrate lateral inhomogeneities in biomembranes.
Often, lateral heterogeneity has been inferred from biophysical techniques where the observed signal indicates multiple populations rather than the expected homogeneous population. An example of this is the measurement of thediffusion coefficient of afluorescent lipid analog in soybeanprotoplasts. Membrane microheterogeneity is sometimes inferred from the behavior ofenzymes, where the enzymatic activity does not appear to be correlated with theaverage lipidphysical state exhibited by the bulk of the membrane. Often, the methods suggest regions with different lipidfluidity, as would be expected of coexistinggel andliquid crystalline phases within the biomembrane. This is also the conclusion of a series of studies where differential effects of perturbation caused bycis andtransfatty acids are interpreted in terms of preferential partitioning of the two liquid crystalline and gel-like domains.
