What cause skin darkening?

It is melanin, produced by melanocytes, that causes the skin to darken which is found on the skin's epidermis. Overproduction or underproduction of melanin results to skin discoloration.

That is the reason why too much sun exposure results to skin darkening - when melanocytes produce more melanin (hyperpigmentation) than they should, they turn into freckles or moles.

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