The organization of endoplasmic reticulum export complexes

To estimate the total number of elements within a VTC, we assumed that each profile appearing in an individual section was, in fact, a separate vesicle or tubule. From this assumption, the average number of elements was determined to be 32 +__ 4 (Table II) . Clusters generally contained between 10 and 60 vesicular-tubular elements, although some of the clusters contained as many as 150 elements (Fig. 5 C) . Although these values are undoubtedly an overestimate of the total number of luminally distinct elements, given that tubules within VTCs are likely to extend across several consecutive thin-sections, they serve as a useful numerical approximation for the relative size and composition of VTCs. When we compared the number of elements found in peripheral vs Golgi-adjacent VTCs, we observed very similar values, suggesting they are of comparable size (Fig. 5 C) . Whereas peripheral VTCs were generally spherical in shape, those adjacent to Golgi possessed a semispherical shape with the CGN occupying one hemisphere, and the other hemispheres facing ER membranes. Using the disector method to estimate the total number of VTCs per cell (Sterio, 1984 ) (see Materials and Methods), we found an average value of 74 _ 11 clusters per cell. This level is similar to that observed using more indirect TEM methods (Buccione et al., 1996) or indirect immunofluorescence in other cell lines (Plutner et al., 1992; Aridor et al., 1995) .