The epidermis is made up of 2 cell types - melanocytes:
The 2nd cell type in the epidermis are the melanocytes. The melanocytes are pigment producing cells which lie in the basal layer of the epidermis. The number of melanocytes in the basal layer determines our genetic skin colour. The role of the melanocytes is to protect the keratinocytes from mutation from UV radiation the skin receives during sun exposure. Understanding this role is the key to understanding why sun damage and abnormal pigmentation occur and how we can prevent and treat these skin conditions.
Melanocytes have long arms that stretch up into the upper layers of the epidermis, allowing them to make contact with many keratinocytes. When the skin is exposed to UV radiation, the melanin granules contained in structures called melanosomes, migrate from the melanocytes into the surrounding keratinocytes. This is called a tan. The melanosomes coat the nuclei of the keratinocytes, protecting them from UV radiation and the risk of mutation. As these keratinocytes are pushed upwards to the surface of the skin, they exfoliate and the skin returns to its natural colour.
The number of melanocytes each skin has, is the result of 1000s of years of evolution. The skin has evolved to have the right number of melanocytes to protect the skin from the UV radiation risk associated with the geographic region the skin inhabits. If a skin type, with a melanocyte count evolved for one region, is then relocated to a region with a much higher UV exposure, the inbuilt melanin keratinocyte protective system can be very easily overwhelmed.
The first sign that this melanin protective system (MPS) has been overwhelmed is the vertical clumping of melanin. We call this a freckle. Over time as the skin receives continued UV overexposure, melanin starts to horizontally clump, producing a chloasma (dermal pigment staining) type hyperpigmentation, which, unlike hormone induced chloasma, is very difficult to treat.
Following years of overexposure to UV, the melanocytes can be completely overwhelmed and die, leaving small round patches of depigmented skin. This is evident on the forearms of most Caucasian Australians, over the age of 40.
Melanocytes can also overproduce melanin in response to hormones, skin injury, inflammation and some drugs. The most common reason for hyperpigmentation in the skin is UV overexposure and the second, scarring, in particular, acne scarring.
Some skins are more prone to hyperpigmenting than others and one theory postulates this may be due to genetic mixing of different skin types eg Caucasian/Asian: African/Hispanic: Asian/Hispanic.