The major organelles of the endomembrane system are: the endoplasmic reticulum (ER), with its rough (RER) and smooth (SER) components; the Golgi apparatus (GA); endosomes (endo); lysosomes (lys); secretory vesicles (sec ves) and granules (SG) and the plasma membrane (PM). The outer membrane of the nuclear envelope (NE) is studded with ribosomes and is continuous with the rough ER. Transitional elements (TE) are specialized ER cisternae from which membrane vesicles (MV) bud off that deliver materials from the ER to the cis region of the Golgi apparatus, possibly by first fusing with each other to form structures that together with the first (cis) Golgi cisterna, constitute the cis Golgi network (CGN). Transport across the Golgi apparatus, from the cis to the trans face is also mediated by membrane vesicles (MV). The trans Golgi network (TGN) is a region in the trans most side of the Golgi apparatus where sorting of proteins destined to distal portions of the endomembrane system takes place. In the TGN, some secretory proteins and plasma membrane proteins are incorporated into secretory vesicles (sec ves). Other secretory proteins are packaged into immature secretory granules or condensing vacuoles (CV), that mature into secretory granules (SG). Secretory vesicles and granules release their contents into the extracellular space by exocytosis, a process that involves the fusion of their membranes with the plasma membrane. Secretory vesicles undergo constitutive exocytosis whereas secretory granules undergo regulated exocytosis in response to signals received at the plasma membrane. Proteins of the membranes of secretory vesicles or granules are incorporated into the plasma membrane during these exocytic events. In polarized epithelial cells in which the plasma membrane has distinct apical and basolateral domains, two populations of secretory vesicles destined to the two aspects of the cell surface emerge from the TGN.

Materials taken into the cell by endocytosis are incorporated into membrane vesicles derived from the plasma membrane and transported to early endosomes (e. endo). The endosomal compartment is polymorphic and includes several classes of endosomes that represent stages in their development and their conversion into lysosomes. These include the CURL (Compartment for Uncoupling of Receptors from Ligands), or sorting endosome, from which membrane vesicles bud to return interiorized receptors to the cell surface, late endosomes (l. endo) and multivesicular bodies (MVB), which receive lysosomal hydrolases brought by vesicles derived from the TGN and are converted to lysosomes (lys). To maintain the characteristic structure and composition of all the organelles of the endomembrane system that communicate with each other by a forward vesicular flow, a retrograde vesicular flow must also take place.