We report on budding and fission of protein-free vesicles swollen from
a natural lipid mixture of bovine brain sphingomyelins. Budding was i
nduced by increasing the area-to-volume ratio through heating. Morphol
ogical changes were monitored by phase contrast microscopy and correla
ted with the thermal behavior of the bilayer by differential scanning
calorimetry. Freeze fracture electron microscopy revealed that budding
and fission are not restricted to giant vesicles but also occur on le
ngth scales relevant for cellular processes. We also observed osmotica
lly induced budding and fission in mixtures of dimyristoyl phosphatidy
lcholine with cholesterol. We find that these shape transitions are dr
iven by liquid/gel domain formation and/or coupling of the spontaneous
curvature of the membrane to the local lipid composition. Our results
provide evidence that coat proteins are not necessary for budding and
fission of vesicles. The physics of the lipid bilayer is rich enough
to explain the observed behavior.