Skin Disorders Guide

Melanocytes account for about 1 % of the cells of the epidermis. These dendritic, pigment-producing cells are derived from the neural crest and are first identified in the epidermis in 8-week-old embryos. In normal skin, they are found exclusively in the basal layer where they are interspersed among the basal keratinocytes. Each melanocyte, via its dendritic processes, is in contact with 30 to 40 nearby keratinocytes. Melanin pigment, which occurs through polymerization of tyrosine-derived indole quinones, develops in membrane-bound organelles known as melanosomes in the cytoplasm of these cells. These pigment granules then move from the central cytoplasm into the finger-like dendritic processes. From there they are transferred into the cytoplasm of the multiple keratinocytes which surround each melanocyte. Variation in nonnal skin color, including that due to racial differences, is determined not by the number of melanocytes but rather by the number and size of the melanosomes that are produced and transferred to keratinocytes.Ultraviolet light, primarily that of 280- to 320-nm wavelengths, darkens the skin through two pathways, immediate photooxidation of preformed melanin and, more importantly, delayed new melanin production in the process known as tanning. An increased amount of melanin pigmentation in the epidermis, whether resulting from genetic determination or from tanning, decreases the amount of ultraviolet light that can pass through the epidermis. This protective mechanism helps keep mutational DNA damage from occurring to the underlying epidermal and dermal cells. Ultraviolet light, probably through its damaging effect on antigen-presenting Langerhans’ cells, also adversely modifies cutaneous immune response. Presumably, melanin formation is protective in this situation also. The beneficial role of melanin is dramatically demonstrated by the ease with which those without (albinos) and those with minimal (Celtic people) melanin develop skin cancers following prolonged exposure to ultraviolet light.

The skin darkens in response to stimuli other than ultraviolet light irradiation. Endocrine changes associated with increased elaboration of Melanocyte-stimulating hormone (MSH) and Adrenal Corticotropic hormone (ACTH) cause darkening, as does the presence of inflammation through the poorly understood process known as postinflammatory hyperpigmentation.

Nevus cells are pigment-producing cells derived from melanocytes. These rounded-up cells lack dendritic processes and occur in nests or clusters. Pigmented lesions containing such clusters are termed nevi. Lesions containing nests of nevus cells only at the dermal-epidermal junction are known as junctional nevi, lesions containing nests of nevus cells both at the dermal-epidermal junction and within the dermis are known as compound nevi, and lesions containing nests of nevus cells only within the dermis are known as intradermal or, more simply, dermal nevi.