Three-dimensional structure of the Golgi apparatus in mammalian cells

The structure of the Golgi apparatus (GA) of 15 mammalian cell types has been examined by three-dimensional electron microscopy. In these cells, the GA appears as a single organelle consisting of a branching and anastomosing ribbon forming a juxta-or perinuclear network. Along the Golgi ribbon, compact regions made up of stacked saccules alternate with non-compact, highly fenestrated or tubular regions that bridge the saccules either of the same or of several adjacent compact zones. On itscis-trans axis, the compact region of the Golgi ribbon may be subdivided into three main compartments. Thecis-compartment, made up exclusively of anastomosed membranous tubules, forms a network referred to as thecis-tubular network (CTN). Themid-compartment comprises poorly fenestrated, more or less dilated saccules depending on the presence of secretory material within their lumen. Thetrans-compartment is composed of three or more sacculo-tubular elements which are not continuous along the Golgi ribbon but display instead a ‘peeling off’ configuration. In cells not engaged in the production of secretion granules, the tubular portions of thetrans-elements are well developed and form extensivetrans-tubular polygonal networks referred to as TTN. In contrast, in glandular cells, where the formation of secretion granules mobilizes most of the membrane of thetrans-Golgi elements, these tubular portions or TTN are small or even absent. The structural changes of the GA under some physiological stimulatory and inhibitory conditions in glandular cells are also described.