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Ebook Hurwitz clinical pediatric dermatology - A textbook of skin disorders of childhood and adolescence (5th edition): Part 2

(BQ) Part 2 book "Hurwitz clinical pediatric dermatology - A textbook of skin disorders of childhood and adolescence" presentation of content: Disorders of pigmentation, vascular vascular disorders of infancy and childhood, bullous disorders of childhood, viral diseases of the skin, skin disorders caused by fungi,.and other contents. | 11  Disorders of Pigmentation Although chiefly of cosmetic significance, disorders of pigmentation are among the most conspicuous and thus can have profound psychosocial implications for pediatric patients. The most important pigments in skin are melanin, reduced and oxygenated hemoglobin, and carotene. Melanin is a pigment produced by melanocytes, specialized dendritic cells derived from the neural crest that migrate to the basal layer of the epidermis during embryogenesis. Melanocytes synthesize and package melanin within discrete membrane-bound organelles called melanosomes, which are then transferred via melanocytic dendrites to surrounding keratinocytes of epidermis and hair follicles; on average, there is one melanocyte to every 36 surrounding keratinocytes.1–4 Variations in skin color among different individuals reflect the number and size of mature melanosomes, not the number of melanocytes. Four stages of melanosome maturation have been described and can be distinguished by ultrastructural examination: 1. Membrane vesicles that contain no visible pigment (stage I or premelanosomes) 2. More elongated vesicles with an ordered internal membrane but no pigment (stage II melanosome) 3. The presence of melanin on ordered internal fibers (stage III melanosome) 4. Structures so full of melanin that the luminal structures cannot be seen (mature or stage IV melanosomes). Darkly pigmented individuals have more numerous, larger, singly dispersed melanosomes, whereas individuals with light pigmentation have fewer, smaller melanosomes that are aggregated into complexes and are more rapidly degraded.5 The presence of melanin in the epidermis helps protect against ultraviolet (UV) radiation and associated cutaneous damage, including pigmented nevi,6 actinic damage, and cutaneous neoplasia. Red hair color, usually associated with an inability to tan, increases the risk of developing melanoma fourfold and has been associated with polymorphisms in the melanocortin receptor