Figure 13–61 A neutrophil reshaping its plasma membrane during phagocytosis. (a) an electron micrograph of a neutrophil phagocytosing a bacterium, which is in the process of dividing. (b) pseudopod extension and phagosome formation are driven by actin polymerization and reorganization, which respond to the accumulation of specific phosphoinositides in the membrane of the forming phagosome: pI(4,5)p2 stimulates actin polymerization, which promotes pseudopod formation, and then pI(3,4,5)p3 depolymerizes actin filaments at the base. (a, courtesy of dorothy f. bainton, phagocytic Mechanisms in health and disease. new york: Intercontinental Medical book corporation, 1971.) in the acidic environment of the endosome and eventually end up in lysosomes, while most of the receptors are recycled via transport vesicles back to the cell surface for reuse. Many cell-surface signaling receptors become tagged with ubiquitin when activated by binding their extracellular ligands. Ubiquitylation guides activated receptors into clathrin-coated pits, they and their ligands are efficiently internalized and delivered to early endosomes.
Early endosomes, rapidly mature into late endosomes. During maturation, patches of the endosomal membrane containing ubiquitylated receptors invaginate and pinch off to form intralumenal vesicles. This process is mediated by ESCRT complexes and sequesters the receptors away from the cytosol, which terminates their signaling activity. Late endosomes migrate along microtubules toward the interior of the cell where they fuse with one another and with lysosomes to form endolysosomes, where degradation occurs.
In some cases, both receptor and ligand are transferred to a different plasma membrane domain, causing the ligand to be released at a different surface from where it originated, a process called transcytosis. In some cells, endocytosed plasma membrane proteins and lipids can be stored in recycling endosomes, for as long as necessary until they are needed.
TransporT froM The Trans golgI neTwork To The cell exTerIor: exocyTosIs
Having considered the cell’s endocytic and internal digestive systems and the various types of incoming membrane traffic that converge on lysosomes, we now return to the Golgi apparatus and examine the secretory pathways that lead outward to the cell exterior. Transport vesicles destined for the plasma membrane normally leave the TGN in a steady stream as irregularly shaped tubules. The membrane proteins and the lipids in these vesicles provide new components for the cell’s plasma membrane, while the soluble proteins inside the vesicles are secreted to the extracellular space. The fusion of the vesicles with the plasma membrane is called exocytosis. This is the route, for example, by which cells secrete most of the proteoglycans and glycoproteins of the extracellular matrix, as discussed in Chapter 19.
All cells require this constitutive secretory pathway, which operates continuously (Movie 13.6). Specialized secretory cells, however, have a second secretory pathway in which soluble proteins and other substances are initially stored in secretory vesicles for later release by exocytosis. This is the regulated secretory pathway, found mainly in cells specialized for secreting products rapidly on demand—such as hormones, neurotransmitters, or digestive enzymes (Figure 13–62). In this section, we consider the role of the Golgi apparatus in both of these pathways and compare the two mechanisms of secretion.
Many proteins and lipids are carried automatically from the  Trans golgi network (Tgn) to the cell surface