The organization of endoplasmic reticulum export complexes

The basic stereological parameters of the RBL and NRK cell line used in these studies are shown in Table I . To morphometrically evaluate the distribution of export sites, ER-budding structures were identified as an elevation on the surface of the ER with a width of 65-85 nm, extruded from the membrane by at least 50% of their diameter, and covered with a distinctive electron-dense coat ( Fig. 1-3, arrowheads). Budding sites emanate from three locations in the cell including (a) those associated with the nuclear envelope ( Fig. 1) , (b) those associated with the Golgi apparatus that are analogous in structure to classical ER transitional elements (Palade, 1975; Sesso et al., 1994) (Fig. 2) , and (c) those found in more peripheral regions that lack detectable Golgi (Fig. 3) . Buds found at the tip of tubular projections from the surface of the ER had an average diameter of 78 ± 6 nm. Although tubular projections were generally shorter than 150 nm (Fig. 1 , section 5; arrowheads), they could be as long as 350 nm based on reconstruction from serial-thin sections. Buds were covered with an ~8-10-nm-thick electron-dense coat. On grazing sections and at high magnification, the coat of ER buds possessed a lattice-like appearance due to a semi-regular array of 4-5-nm elongated particles ( Fig. 1, inset) . These coats are similar to those observed in pancreatic acinar cells (Merisko et al., 1986) . Based on analysis of random sections through ~300 cells (see Materials and Methods), the average number of buds in a cell was found to be 250 ± 10.45% of budding profiles were found on ER tubules located in the vicinity of Golgi complexes, whereas 55% were located in regions without noticeable juxtaposition to Golgi. 11% of total buds were observed emerging from the nuclear envelope. Thus, it is apparent that a substantial level of membrane exiting the ER appears to do so from sites distant from Golgi elements.