The organization of endoplasmic reticulum export complexes

where I is the number of intersections on a grid, P is the number of points on the grid, and d is the distance between the points (Weibel, 1979) . Both procedures yielded nearly identical results for RBL cells, The values obtained from the second procedure are reported in Table I . The stereological parameters of nuclei, ER, and Golgi apparatus were determined using the same sections. Randomly taken cell contours were enlarged to a total magnification of 14,000 as described above. Using the point-counting method, we determined the nucleus volume to cell volume ratio and a nucleus surface to nucleus volume ratio as described (Weibel, 1979) . Randomly chosen fields of cells containing ER or Golgi membranes were magnified to 31,000 to establish the ER(Golgi) volume to cell cytoplasm volume ratio by the point-counting method (Weibel, 1979) . Some of these images were photoenlarged to 120,000 to determine the ER(Golgi) surface to volume ratio. The ER(Golgi) surface was found by formula 3. Corrections of bias due to section thickness were done as described (Weibel and Paumgartner, 1978) . For the ER volume, the correction factor was 0.51 and 0.65 for the ER surface. Assuming that two-thirds of the Golgi complex was composed of cisternae and one-third of tubules, the correction factors for Golgi volume and surface were estimated to be 0.6 and 0.64, respectively.