Module 4: Skin Physiology - Skin And Light (Continued)

Basic Principles of Pigmentation:

The phenomenon and the process of post-inflammatory hypo- and hyper pigmentation.

The ovals of dark brown represent the melanin packaged into melanosomes. These packages of melanin are conveyed along the dendritic processes of the melanocyte to keratinocytes in the normal process of pigmentation.

The density of melanocytes at the dermo-epidermal junction is roughly the same per square centimetre of skin irrespective of the colour of the skin.

Stimuli that result in increased pigmentation of the skin are radiation and hormones. Following irradiation, preformed melanosomes are rapidly delivered to the keratinocyte and the process of melanin synthesis augmented. Hormones can have similar effects resulting in increased skin pigmentation as seen, for example, in pregnancy.

Both can stimulate melanin synthesis in selected groups of melanocytes more than others. Melanocyte groups of the facial area that are more sensitive to hormonal stimulation than surrounding ones result in melasma seen with pregnancy or the use of contraceptives. Likewise light can stimulate certain groups of melanocytes more than others and this phenomena is well illustrated by freckles.

The process of melanin synthesis in melanocytes, its packaging into melanosomes and subsequent transfer along the dendritic process of the melanocyte to keratinocytes where it is distributed over the nucleus, is like an industrial manufacturing process. The factory is the melanocyte; the product is melanin, which is packaged into melanosomes then sent down the dendritic conveyor belt to dispatch, where it is transported to the keratinocyte, the end user. It is finally deployed, like an umbrella, over the nucleus of the cell shielding it from irradiation damage. Any interference with this process will result in alterations in pigmentation.

The diagram above illustrates disruption of the epidermis as a consequence of irritation or inflammation. This resultant interference with the process of melanisation and the consequences are healing with altered pigmentation.

If damage occurs without interruption of the synthesis and packaging of the melanin, melanin can find its way into the dermis below the basement membrane. This can be likened to a natural tattoo.

Should damage to melanocytes occur, no melanin will be formed and the skin will appear white (acquired vitiligo). If there is only partial disruption of the process of melanisation, the skin might appear lighter.

Any irritation to the skin, sub-clinical or with clinically obvious inflammation can result in dramatic alteration in skin colour. This is more pronounced in dark skinned individuals and is a cause of great distress to those affected. Unless one halts the process that is causing the pigmentary changes, the problem will persist. Irritation of the skin, even though it is not clinically obvious or causing symptoms, can cause alterations of pigmentation.

• Hyperpigmentation
• Hypopigmentation