Freeze-fracture studies of the developing cell surface. II. Particle- free membrane blisters on glutaraldehyde-fixed corneal fibroblasts are artefacts

We describe, in sections and by freeze-fracture, four classes of intramembrane particle (IMP)-free membrane blebs or "blisters" associated with glutaraldehyde-fixed embryonic corneal fibroblasts: (a) Single blisters attached to the cell membrane; (b) free (detached) vesicles; (c) myelin figures; (d) multivesicular protrusions which resemble the "mounds" described by others on nerve growth cones. The IMP-free, membrane-bounded blisters contain no ground cytoplasm or organelles, in contrast to blebs on trypsin-isolated fibroblasts, which we show here do contain cytoplasm and IMP-rich membranes. That the IMP- free membrane blisters in embryonic corneas are artefacts of fixation is demonstrated by (a) their absence in replicas of fibroblasts frozen and fractured without prior aldehyde fixation and (b) their absence in sections of fibroblasts fixed in a combination of glutaraldehyde and osmium tetroxide. We suggest that the addition of osmium prevents postfixation movement of membrane lipids, especially the negatively charged "fluid" lipids which others have shown are capable of considerable mobility after aldehyde fixation alone. Recent literature has implicated membrane blistering in secretory processes and in growth of nerves, but before the functional significance of such IMP-free blisters is assessed, membrane mobility of the type shown here should be taken into consideration.