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Lipid domains on the growth cone. The plasma membrane of Xenopus growth cones was stained with FITC-Cholera Toxin B (CTxB), DiIC₁₈, and filipin to examine the distribution of different lipid constituents. Staining of the membrane with the lipophilic dye, DiIC₁₈, resulted in a relatively uniform fluorescent signal. On the other hand, labeling of ganglioside G_M1by CTxB or cholesterol by filipin showed an apparent heterogeneous distribution, indicating the existence of microdomains. The image of FITC-CTxB staining was processed by applying a digital threshold for better illustration of domains. Scale bar = 10 μm.

Freeze-fracture electron microscopy of autophagosomes from mouse hepatocytes. Panels A and C are from intact hepatocytes; B and C show isolated autophagosome fractions. Note that autophagosome membranes are almost completely free of intramembrane particles, putative integral membrane proteins. In panel C, arrows indicate a fusion event or the autophagosome with a smaller vesicles, putative endosome, which has intramembrane particles (arrowheads). P indicates the cytoplasmic fracture face of the outer limiting membrane; E is the extracytoplasmic fracture face. I indicates the inner limiting membrane. Bar, 200 nm. Reproduced, with permission, from: Punnonen EL, Pihakaski K, Mattila K, Lounatmaa K, and Hirsimäki P: Intramembrane particles and filipin labelling on the membranes of autophagic vacuoles and lysosomes in mouse liver. Cell Tissue Res 1989; 258: 269-276 (Fig. 1).