7.6: Additional Cells of the Epidermis
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Although keratinocytes comprise the vast majority of epidermal cells, there are a number of accessory cells present in the normal epidermis that provide important contributions to the function of the skin.
Melanocytes are professional melanin synthesizing cells located primarily within the basilar (deeper) regions of the epidermis, and are histologically recognized as poorly staining “clear cells”. Variation in epidermal melanin content is responsible for the color variety in most domestic species. The density of melanocytes within the epidermis is relatively uniform between individual animals of a species. Pigmentation is therefore largely dependent on the amount of melanin produced by melanocytes. Melanocytes have complex dendritic processes that lie adjacent to keratinocytes and facilitate the transfer of melanin to keratinocytes. Although melanocytes are the producers of melanin, keratinocytes often contain the greatest amount of melanin, via transfer from melanocytes.
Melanogenesis, the process of melanin generation, occurs within specialized, melanocyte-specific organelles called melanosomes. Production of melanin can vary based on anatomic location (e.g. oral mucosa pigmentation in some dogs) and/or in response to physiologic need (e.g. chronically sun-exposed skin). Tyrosine is an amino acid required for melanogenesis. Within melanosomes, tyrosine is converted into intermediate compounds (e.g. dihydroxyphenylalanine, DOPA) by tyrosinase. Tyrosinase is a copper-containing enzyme. For this reason, copper deficiency in sheep and cattle results in a characteristic loss of hair coat pigmentation.
Melanin serves unique protective roles in the epidermis, including the ability to both scatter and reflect ultraviolet (UV) rays, and as a scavenger of potentially damaging free radicals. Melanization is also a dynamic process, and increased pigmentation (hyperpigmentation) or reduced pigmentation (hypopigmentation) may be associated with various disease cutaneous processes.
Langerhans cells are dendritic cells sparsely located within the stratum basale. Although they are functionally different from melanocytes, they also appear histologically as “clear cells”. As dendritic cells, the primary function of Langerhans cells is to take up antigens (antigen = molecules that induce an immune response) and present them to and activate cells of the immune system, such as T cells. In this role, Langerhans cells serve as “sentinels” at the interface of the external environment and the immune system. For example, in a bacterial skin infection, Langerhans cells may take up fragments of the bacteria (bacterial antigens), migrate to the regional lymph node, and present these bacterial antigens to T cells present in the lymph node. In doing so, the Langerhans cell bridges aspects of the innate (skin) and adaptive (T cells) immune responses.
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