Lysosomes and the skin

Lysosomes were first described in 1955 by deDuve at al. They are cytoplasmic organelles which contain hydrolytic enzymes that are capable of digesting many tissue constituents. Most digestion occurs intracellularly, but hydrolytic enzymes can be secreted by exocytosis into the extracellular compartment where they may also act on extracellular macromolecules. Lysosomal enzymes are produced in the rough endoplasmic reticulum and are then transported to a specialized region of the smooth endoplasmic reticulum located at the inner surface of the Golgi stack (GERL). Here these proteins are concentrated and packaged into membrane bound primary lysosomes. Primary lysosomes can fuse with organelles containing substrates for digestion; the single membrane bound vacuole which containes hydrolases and substrate is shown as a secondary lysosome or digestive vacuole. The substrate containing organelles can originate by several distinct mechanisms. Heterophagy is a process by which the cell can engulf foreign material into heterophagosomes by either phagocytosis or pinocytosis. The two processes are known collectively as endocytosis. The fusion of a heterophagosome with a primary lysosome produces a digestive vacuole. Autophagy is a method by which the cell can sequester part of its own cytoplasm in autophagosomes for digestion. In this way, the cell is able to rid itself of damaged constituents. Fusion of the autophagic vacuole with a primary lysosome results in the formation of a digestive vacuole. The membrane that delimits the digestive vacuole is ideally suited for cellular economy. Large macromolecules readily enter the lysosome by heterophagy or autophagy but they are unable to diffuse from the vacuole because of their size. After extensive enzymic digestion, the breakdown products of proteins, carbohydrates, nucleic acids, mucopolysaccharides, and glycoproteins are small enough to pass through the lysosomal membrane where they may be used in biosynthetic processes in other parts of the cell. A residual body or telolysosome if formed when substances in the digestive vacuole are incompletely digested so that they are too large to pass through the lysosomal membrane. When these organelles have no further demonstrable hydrolytic enzyme activity, they are called postlysosomes. The residual body can be extruded from the cell by the process of exocytosis or it may remain within the cytoplasm of the cell. In this review the morphologic evidence for the presence of lysosomes in the epidermis and its appendages is discussed. The connective tissue cells of the dermis are not dealt with.