The organization of endoplasmic reticulum export complexes

Buds in the GolgiExclusion Zone. The Golgi exclusion zone is defined as the Golgi-containing region found in the pericentrosomal region of RBL cells and includes directly adjacent bud-bearing cisternae of the ER. The surface area of the Golgi exclusion zone in each individual section within a stack of 11-20 consecutive sections (referred to as a disector [Sterio, 1984; Lucocq et aL 1989] ) was determined by the point-counting method (Weibel, 1979) , and the mean surface area in each disector was calculated for 12 different cells (range from 30-90 ~tm2). To find the volume of the Golgi exclusion zone included in the disector, the mean surface area of the Golgi exclusion zone in each disector was multiplied by the thickness of the section and the number of individual sections (35 to 105 ixm 3 for 12 individual disectors). The total number of ER buds within each disector of a given cell was found by reconstruction from serial sections. The volumetric density of ER buds was calculated by dividing the number of buds found within each disector by the volume of the disector. The total volume and surface of ER membranes in the Golgi exclusion zone was found by the point-counting method (Weibel, 1979) . By dividing the total number of ER buds by the ER surface within the Golgi exclusion zone, we calculated the density of ER buds in this region.