ultraviolet radiation (UVR) by taking up melanosomes, vesicles containing the endogenous photoprotectant melanin, from epidermal melanocytes. Each melanocyte in the epidermis has several dendrites that stretch out to connect it with many keratinocytes. The melanin is then stored within keratinocytes and melanocytes in the perinuclear area as supranuclear “caps”, where it protects the DNA from UVR-induced damage. Role in wound healing Wounds to the skin will be repaired in part by the migration of keratinocytes to fill in the gap created by the wound. The first set of keratinocytes to participate in that repair come from the bulge region of the hair follicle and will only survive transiently. Within the healed epidermis they will be replaced by keratinocytes originating from the epidermis. At the opposite, epidermal keratinocytes, can contribute to de novo hair follicle formation during the healing of large wounds. Functional keratinocytes are needed for tympanic perforation healing. Sunburn cells A sunburn cell is a keratinocyte with a pyknotic nucleus and eosinophilic cytoplasm that appears after exposure to UVC or UVB radiation or UVA in the presence of psoralens. It shows premature and abnormal keratinization, and has been described as an example of apoptosis. Aging With age, tissue homeostasis declines partly because stem/progenitor cells fail to self-renew or differentiate. DNA damage caused by exposure of stem/progenitor cells to reactive oxygen species (ROS) may play a key role in epidermal stem cell aging. Mitochondrial superoxide dismutase (SOD2) ordinarily protects against ROS. Loss of SOD2 in mouse epidermal cells was observed to cause cellular senescence that irreversibly arrested proliferation in a fraction of keratinocytes. In older mice, SOD2 deficiency delayed wound closure and reduced epidermal thickness. Civatte body A Civatte body (named after the French dermatologist Achille Civatte, 1877–1956) is a damaged basal keratinocyte that has undergone apoptosis, and consist largely of keratin intermediate filaments, and are almost invariably covered with immunoglobulins, mainly IgM. Civatte bodies are characteristically found in skin lesions of various dermatoses, particularly lichen planus and discoid lupus erythematosus. They may also be found in graft-versus-host disease, adverse drug reactions, inflammatory keratosis (such as lichenoid actinic keratosis and lichen planus-like keratosis), erythema multiforme, bullous pemphigoid, eczema, lichen planopilaris, febrile neutrophilic dermatosis, toxic epidermal necrolysis, herpes simplex and varicella zoster lesions, dermatitis herpetiformis, porphyria cutanea tarda, sarcoidosis, subcorneal pustular dermatosis, transient acantholytic dermatosis and epidermolytic hyperkeratosis. See also Epidermis Skin Corneocyte Keratin HaCaT List of human cell types derived