Identification of free coated pinocytic vesicles in Swiss 3T3 cells.

Whether or not free coated vesicles are involved during internalization of ligands bound to the receptors of coated pits is controversial. Free coated vesicles cannot be identified with certainty in random individual thin sections - reconstructions based on consecutive thin sections are required. The thickness of the sections determines the reliability of such reconstructions. In the present study, serial section electron microscopy was applied to Swiss 3T3 cells and the topographical resolution yielded by 80 nm and 20 nm sections was compared. Swiss 3T3 cells in monolayer at 37 degrees C were exposed for 5 min to cationized ferritin (CF) which is a marker of pinocytic vesicles. Subsequently the cells were fixed, pelleted and further processed for electron microscopy. The results showed that reconstructions of coated CF-labeled structures based on consecutive sections of an average thickness of approximately 80 nm could not be performed with certainty. A substantial fraction (25%) of the examined profiles appeared to be free vesicles, but narrow surface connections could easily have been missed in these thick sections. The series of the much thinner 20 nm sections provided a better resolution allowing the narrowest surface connections to be identified. Accordingly, the number of truly free, coated vesicles was much lower than the number of apparently free vesicles in the thick sections. However, free coated vesicles labeled with CF were identified in the consecutive 20 nm sections (4% of the examined profiles).