Apoptosis and the skin

In multicellular organisms, homeostasis is maintained by a balance between cell proliferation and cell death. Two common forms of cell death, called a poptosis and necrosis, have been described. Apoptosis, which is often equat ed with programmed cell death, is a physiological form of cell death that i s responsible for the deletion of cells. Apoptosis is morphologically and b iochemically characterized by cell shrinkage, dense chromatin condensation, cellular budding, fragmentation, rapid phagocytosis by nearby cells, and D NA fragmentation into units of approximately 200 base pairs. Apoptosis can be triggered by a wide variety of stimuli such as cytokines, hormones, drug s, and viruses, and their signal transduction tightly regulated by genes su ch as Bcl-2. Effector caspases are finally activated, resulting in apoptoti c cell death. In the skin, there is considerable evidence that apoptosis pl ays an important role in the pathogenesis of a wide variety of skin disease s. In lichenoid tissue re:actions, the Civatte body or colloid body is a fo rm of apoptotic keratinocytes which is mediated by T lymphocytes via Fas-Fa sL interaction or through the perforin-granzyme B pathway. In several skin tumors,:Bcl-2 or Fast expression is involved in the proliferation or regres sion of the tumors, or in the escape from immune attack by T cells. Moreove r, apoptosis is also responsible for the homeostasis of skin, such as the k eratinocyte differentiation and hair cycle. In this review, we describe the basic concept of apoptosis and its relevance to skin diseases.