Golgi structure in three dimensions: Functional insights from the normal rat kidney cell

Three-dimensional reconstructions of portions of the Golgi complex from cry ofixed, freeze-substituted normal rat kidney cells have been made by dual-a xis, high-voltage EM tomography at similar to 7-nm resolution. The reconstr uction shown here (similar to 1 x 1 x 4 mu m(3)) contains two stacks of sev en cisternae separated by a noncompact region across which bridges connect some cisternae at equivalent levels, but none at nonequivalent levels. The rest of the noncompact region is filled with both vesicles and polymorphic membranous elements. All cisternae are fenestrated and display coated buds. They all have about the same surface area, but they differ in volume by as much as 50%. The trans-most cisterna produces exclusively clathrin-coated buds, whereas the others display only nonclathrin coated buds. This finding challenges traditional views of where sorting occurs within the Golgi comp lex. Tubules with budding profiles extend from the margins of both cis and trans cisternae. They pass beyond neighboring cisternae, suggesting that th ese tubules contribute to traffic to and/or from the Golgi. Vesicle-filled "wells" open to both the cis and lateral sides of the stacks. The stacks of cisternae are positioned between two types of ER, cis and trans. The cis E R lies adjacent to the ER-Golgi intermediate compartment, which consists of discrete polymorphic membranous elements layered in front of the cis-most Golgi cisterna. The extensive trans ER forms close contacts with the two tr ans-most cisternae; this apposition may permit direct transfer of lipids be tween ER and Golgi membranes. Within 0.2 mu m of the cisternae studied, the re are 394 vesicles (8 clathrin coated, 190 nonclathrin coated, and 196 non coated), indicating considerable vesicular traffic in this Golgi region. Ou r data place structural constraints on models of trafficking to, through, a nd from the Golgi complex.