I've seen plenty of photos of people with albinism/leucism, but what about its opposite? I've been looking and haven't found any photos or reliable references to humans with melanism. The closest I came was finding a photograph of a 'piebald' child, but it was from the very early 20th century and it wasn't known whether it was vitiglio, partial albinism or partial melanism.
Dermal melanin is produced by skin cells called melanocytes. Humans generally possess a similar concentration of melanocytes in their skin. However, melanocytes in some individuals or ethnic groups express the melanin producing gene more or less frequently resulting in a greater or lesser concentration of skin melanin.
I am only aware of melanism or melanosis in humans occurring in specific organs. For example, melanosis coli refers to increased pigmentation of the lining of the colon. This is actually due to the excess use of laxatives and is a bit of a misnomer because it is not due to increased melanin pigment, but rather, lipofuscin in macrophages. There is a condition called Peutz-Jhegers Syndrome which is an inherited condition characterised by colonic polyps and patches of hyperpigmentation on the lips, oral mucosa (lining), face, genitalia and palmar surface of the hands. Freckles are the most common pigmented lesions of childhood. Melasma is the term for a mask-like zone of facial hyperpigmentation commonly seen in pregnancy (but can occur during use of oral contraceptives, other drugs or for unknown reasons). It presents as poorly defined, blotchy patches involving the cheeks, temples and forehead on both sides. Acanthosis nigricans describes thickened, hyperpigmented zones of skin involving most commonly flexural areas (axillae, skinfolds of the neck, groin and anogenital regions). It is an important skin marker of some conditions including insulin resistance and various tumours. Localised melanocyte proliferation can produce skin lesions such as lentigo, nevi (mole) and of course malignant melanoma.
One condition I can think of that causes more generalised increased pigmentation is Addison's Disease. In this condition, the cortex of the adrenal glands (which sit on top of each kidney) is destroyed and therefore the adrenals fail to produce the hormone cortisol. Cortisol is normally produced in response to a stimulating hormone called ACTH (adrenocorticotrophic hormone) produced by the pituitary gland at the base of the brain. When cortisol levels are low the pituitary produces more ACTH in an attempt to stimulate the
adrenal glands to produce cortisol. ACTH is derived from a precursor called POMC (pro-opiomelanocortin) which too will be increased, and it contains melanocyte-stimulating-hormone (MSH). It was originally thought that the increased MSH, as a by-product of increased POMC expression, produced the increased pigmentation of Addison's disease, but it is now thought to reflect increased stimulation of the melanocortin-1 receptor by ACTH itself. The pigmentation of Addison's disease is seen in sun-exposed areas, recent rather than old scars, axillae, nipples, palmar creases, pressure points (elbows, knees, knuckles) and in mucous membranes (oral, vaginal, vulval, anal).
David got in ahead of me and mentioned Addisons and the reasons it causes an increase in melanin (but then we are both endocrinologists). I can't think of any other causes of generalised melanism nor am I aware of any genetic/inherited causes.
FYI the "piebald" mutation that occur in many species (inc humans, rodents and cats) is due to a mutation in a tyrosine kinase (c-kit) receptor and has very little to do with melanism.
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