Age spots, also called Lentigo senilis, are small, localised, sharply circumscribed sites of hyperpigmentation of the skin. Contrary to freckles, which are due to an UV-induced increased, but reversible production of melanin, lentigines are caused by local hyperplasia (cell proliferation) at the basal layer of the epidermis.
Melasma, another symptom of hyperpigmentation and of aesthetic concern to Asian population, is characterised by diffuse patches of brownish colour, often induced by hormonal imbalance (‘pregnancy masks’) and sometimes reversible.
Physiology of pigmentation
In order to understand the causes of age spots, it is worth first looking at the physiology of skin pigmentation (Fig. 1). Melanocytes are embedded at the base of the epidermis, close to the epidermal/ dermal junction. From a single melanocyte, dendrites (finger like extensions) reach out to the surrounding keratinocytes (about 35-40 of those are in contact with one melanocyte, this entity being called the melanin unit). In the melanocyte, small organelles, called melanosomes, are formed continuously and migrate to the extremities of the dendrites. These melanosomes are the true ‘factories’ of melanin production: they contain a large number of copies of the enzyme tyrosinase, which is a key molecule in the synthetic pathway of melanin. Tyrosinase needs the amino acid tyrosine as the initial substrate for melanin synthesis, which proceeds from tyrosine to dopa, to dopaquinone, to leucodopachrome, to 5,6 indolequinone and via polymerisation steps to melanin. The melanosomes are then transferred to the surrounding keratinocytes in a process that resembles phagocytosis.
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