The organization of endoplasmic reticulum export complexes

Stereological Parameters. An estimation of the mean volume of the cells was performed by two independent methods (Baddeley et al., 1985; Griffiths et al., 1989) . In the first case, RBL cells were grown on glass coverslips. Cells were fixed with 2.5% GA in PBS, pH 7.4, overnight. The cells were then viewed and photographed under phase contrast with an Axiophot (Zeiss, Oberkochen, Germany) with a ×20 objective lens. The images were enlarged photographically 10 times, and the mean surface of the cell projection was determined by the point-counting method (Weibel, 1979) using a 5-ram square lattice grid. Approximately 10,000 RBL cells were used for the estimation of the mean surface of the cell projection onto a planar surface. The cells on coverslips were stained and embedded in Epon as described above. After polymerization, the coverslips were detached from the resin by plunging into liquid nitrogen. Thin-strips of embedded cells in resin were then combined face to face and reembedded on top of an Epon block to produce "vertical" sections (Griffiths et al., 1984) . Images of cells were taken with a transmission electron microscope (1200EX; JEOL USA, Peabody, MA) and photoenlarged (×3,500 total magnification), and the mean height of ~250 cells in a monolayer was measured by the point-counting method (Weibel, 1979) . The mean cell volume (Vc.r.) was then determined according to formula: