Introduction to skin and hair pigmentation

39 Introduction to skin and hair pigmentation


Skin and hair colour is genetically determined and is mainly due to the presence of melanin pigment. Melanin is produced by melanocytes, cells present in the epidermis and hair follicles, in a highly organized multiple-step process. Within melanocytes, melanin is synthesized in intracellular cell organelles called melanosomes.


Melanocytes are derived from melanoblasts, precursor cells, which originate in the neural crest and under the control of a number of growth factors, migrate to the skin, eyes, meninges and inner ear during embryonic development. Melanocytes are dendritic cells and in the skin are found mainly in the basal layer of the epidermis and hair follicle. Each melanocyte interacts with a number of keratinocytes by way of dendritic processes. The ‘epidermal melanin unit’ consists of one melanocyte interacting with around 36 keratinocytes. In a ‘follicular melanin unit’, one melanocyte interacts with around five hair follicle keratinocytes.


The melanosome is the principal product of melanocytes and is a highly organized, membrane-bound organelle within which melanin synthesis takes place. The initial steps (Fig. 39.1) of melanin synthesis are under the control of the enzyme tyrosinase, which oxidizes tyrosine to DOPA and then to dopaquinone. This undergoes a series of further complex reactions, involving cyclization and oxidative polymerization, to form the different types of melanins. The process of melanin transfer to surrounding keratinocytes involves transport of melanosomes to the tips of the dendrites and then subsequent transfer across to keratinocytes (Fig. 39.2). The transfer of melanosomes to neighbouring cells occurs by a combination of phagocytosis and diffusion through cell membranes, and is known as cytocrinia. As keratinocytes ascend through the epidermis and undergo differentiation, transferred melanins are released from melanosomes by lysosomal acid hydrolases. The visible skin colour depends on the stage of melanosome transfer, the size of the melanosomes and on how widely they are dispersed.


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Sep 3, 2016 | Posted by in SMALL ANIMAL | Comments Off on Introduction to skin and hair pigmentation

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