The organization of endoplasmic reticulum export complexes

To determine the number of elements in VTCs, the value reported assumes that all elements found in consecutive sections are discontinuous with one another. This value is likely to be an overestimate, as some of the tubular structures within VTCs may extend across several sections. The number of elements in VTCs was determined by direct counting of individual profiles on each serial section. These were summed throughout all sections through a given VTC to obtain a total value. Probability Measurements. To estimate the probability of a bud having proximity to a second bud in the cell, images of serial sections were photoenlarged to 40,000 to reconstruct a section of the cell in three dimensions. 300 randomly chosen ER buds were taken as the center of reference (referred to as reference buds), and the distance between each individual reference bud and other ER buds in the same and consecutive serial sections encompassing up to 1.3 p.m distance above and below the reference bud was measured directly. A series of concentric shells with a volume equal to the volume of the most internal sphere having a 0.2-1xm diam (corresponding to a volume of 0.0042 i~m 3) was constructed around the reference bud. Subsequently, each measured bud was assigned to a shell with its distance from the reference bud being that of the corresponding outer diameter of the shell. The probability of finding a bud within a particular shell was determined by dividing the number of positive shells by the number of reference buds, and the value was reported as a percentage.