Fig. 3. PrP^C immunolabelling of sensory neurons, seen by transmission electron microscopy of sectioned material (A–K) or of tear-off samples of the cell surface (L–S), using anti-PrP Fab directly coupled to 5 nm gold with biotinylated Tf coupled to 20 nm avidin–gold (A–K) or AP2 labelled with 10 nm gold (L–S). (A–E) Examples of PrP^C entering coated pits or vesicles after 2 min at 37°C, with occasional co-localization with TfR (A and E). (F–K) Co-localized TfR and PrP on the cell surface. (L) Overlapping PrP and AP2 at electron-dense cytoplasmic tubular structures (M and N) Regions arrowed in (L) at higher power; arrows in (M) and (N) point to 10 nm AP2 label. (P) A cluster of PrP^C partially overlapping an AP2-labelled electron-dense region (arrowed). (Q) Clusters of PrP^C overlying AP2 (small arrows), with another PrP^C cluster (large arrow) not associated with AP2. In this sample, the cytoplasmic surface is less distinct (presumably masked by cytoplasmic protein), somewhat obscuring the AP2-positive electron-dense structures. (R and S) Examples of clusters of PrP not associated with AP2 label (the nearest AP2 label is arrowed). Scale bars are 50 nm.