 Both receptor and ligand are degraded. This pathway has been identified for epidermal growth factor (EGF) and its receptor. Like many other proteins, EGF binds to its receptor on the cell surface. The complex is internalized and carried to the early endosomes. Here EGF dissociates from its receptor, and both are sorted, packaged in separate MVBs, and transferred to the late endosome. From there, both ligand and receptor are transferred to lysosomes, where they are degraded (Fig. 2.18c).
 Both receptor and ligand are transported through the cell. This pathway is used for secretion of immunoglobulins (secretory IgA) into the saliva and human milk. During this process, commonly referred to as transcytosis, substances can be altered as they are transported across the epithelial cell (Fig. 2.18d). Transport of maternal IgG across the placental barrier into the fetus also follows a similar pathway.
Lysosomes are digestive organelles that were recognized only after histochemical procedures were used to demonstrate lysosomal enzymes.
Lysosomes are organelles rich in hydrolytic enzymes such as proteases, nucleases, glycosidases, lipases, and phospholipases.
FIGURE 2.18 • Fate of receptor and ligand in receptor-mediated endocytosis. This diagram shows four major pathways along which the fate of internalized ligand–receptor complexes is determined. a. The internalized ligand–receptor complex dissociates, the receptor is recycled to the cell surface, and the ligand is directed to late endosomes and eventually degraded within lysosomes. This processing pathway is used by the LDL–receptor complex, insulin–GLUT receptor complex, and a variety of peptide hormone– receptor complexes. MVB, multivesicular bodies. b. Both internalized receptor and ligand are recycled. Ligand–receptor complex dissociation does not occur, and the entire complex is recycled to the surface. An example is the iron–transferrin–transferrin receptor complex that uses this processing pathway. Once iron (Fe) is released in the endosome, the transferrin–transferrin receptor complex returns to the cell surface, where transferrin is released. c. The internalized ligand–receptor complex dissociates in the early endosome. The free ligand and the receptor are directed to the late endosomal compartment for further degradation. This pathway is used by many growth factors (i.e., the EGF–receptor complex). d. The internalized ligand–receptor complex is transported through the cell. Dissociation does not occur, and the entire complex undergoes transcytosis and release at a different site of the cell surface. This pathway is used during secretion of immunoglobulins (secretory IgA) into saliva. The antibody IgA–receptor complex is internalized at the basal surface of the secretory cells in the salivary gland and released at the apical surface.
A lysosome represents a major digestive compartment in the cell that degrades macromolecules derived from endocytotic pathways as well as from the cell itself in a process known as autophagy (removal of cytoplasmic components, particularly membrane-bounded organelles, by digesting them within lysosomes). For more information about autophagy, see page 41.