Epifluorescence microscopy of surface domain microheterogeneity in myelin monolayers at the air-water interface

Myelin lipids form liquid-expanded monolayers at the air-water interface, with no evidence of surface pressure-induced two-dimensional phase transition. However, the film doped with 2 mole % of the fluorescent probe N-(7-nitro-2-1,3-benzoxadiazol-4-yl) Diacyl Phosphatidylethanolamine (NBD-PE) shows an irregular pattern of coexisting laterally segregated surface domains with diffuse boundaries that change from smooth patterns to fractal-like structures depending on surface pressure. Successive expansion-recompression cycles lead to more defined domains, with a general reorganization occurring at surface pressures of about 20 mN/m. At least two coexisting phases occur over almost all the compression isotherms. The presence of proteins in whole myelin monolayers induces defined domain textures with relatively sharp boundaries. The patterns during compression and expansion are quite similar and, after the first cycle, little changes occur under recompression. The patterns observed provide topographical evidence for the existence of dynamic domain microheterogeneity in the surface of myelin interfaces.