Melanin is the brown black, non-haemoglobin-derived pigment normally present in the hair, skin choroid of the eye, meninges and adrenal medulla. It is synthesized in the melanocytes and dendritic cells, both of which are present in the basal cells of the epidermis. Melanin is stored in the form of cytoplasmic granules in the phagocytic cells called the melanophore, present in the under lying dermis. Melanocytes possess the enzyme tyrosinase necessary for the synthesis of melanin from the amino acid called tyrosine
Tyrosine to Melanin
Various disorders of melanin pigmentation cause generalized and localized hyperpigmentation and hypopigmentation.
a) In Addison's disease, there is generalized hyperpigmentation of the skin, especially in areas exposed to light and buccal mucosa.
b) Hyper pigmentation on skin of face, nipples and genitalia during pregnancy under the influence of oestrogen.
c)In chronic arsenical poisoning, there is characteristic rain-drop pigmentation of the skin.
Albinism is an extreme degree of generalized hypo pigmentation in which tyrosinase activity of the melanocytes is genetically defective and no melanin is formed. Albinoes have blonde hair, poor vision and severe photophobia. They are highly sensitive to sunlight. Chronic sun exposure may lead to pre-cancerous lesions, squamous and basal cell cancers of the skin.
a) Leucoderma is a form of partial albinism and is an inherited disorder.
b) Vitiligo is local hypopigmentation in the skin.
c) Acquired local hypopigmentation can result from various causes such as leprosy, healing of wounds, radiation dermatitis, etc.
Invisible light forms a part of the electromagnetic spectrum immediately beyond the violet end of the visible light spectrum (i.e. between visible light and X-rays). Long wavelength ultraviolet light (i.e. that nearest visible light) is often termed as UVA; intermediate wavelength ultraviolet light is designated UVB; and short wavelength ultraviolet light (i.e. that nearest X-rays) is called UVC.
The main source of UV radiation is the sunlight; others are UV lamps and welder' arcs. UV light penetrates the skin for a few millimeters only so that its effect is limited to epidermis. In human excessive exposure to UV rays can cause various forms of skin cancer such as squamous cell carcinoma, basal cell carcinoma and malignant melanoma. The efficiency of UV light as carcinogen depends upon the extent of light-absorbing protective melanin pigmetation of the skin. Fair skinned people and whites are more prone to skin complications due to UV exposure. People living close to the equator, outdoor workers and farmers in Tropics are exposed to light radiations.