Figure 1.

Codistribution of TF and IgA tracers applied from basolateral and apical surfaces of polarized cells. Polarized MDCK expressing TR and pIgR were incubated basolaterally with TF-HRP (60 min at 37°C) and apically with IgA-gold (30 min at 37°C). Endosomal elements labeled with both tracers are distributed throughout the cytoplasm and include 0.3-μm-diam vacuoles (V; with a tubular extension, arrowhead), narrower branching tubules (small arrows), larger 100– 150-nm-diam tubules (large arrows) and 60-nm-diam basolateral vesicles (bv). Basolaterally applied TF-HRP is not present on the apical membrane, and the coated pits and vesicles derived from this membrane (ap) are identifiable because they have only been reached by apical tracer. bcp indicates a coated vesicle derived from basolateral surface containing only TF-HRP and has, typically, a peripheral distribution of DAB reaction product. Double arrowheads indicate coated buds on 60-nm tubules. L, lysosome. Bar, 0.1 μm.