Skin changes in psoriasis
NORMAL SKIN
The skin is your largest organ, weighing about four kilograms and covering about two square metres. It is your interface with the environment, protecting you against chemicals, bacteria and radiation, helping you to maintain a stable body temperature, and stopping you from losing fluid and vital body chemicals. Your skin contains nerve endings that allow you to feel objects. Nails, which are also part of your skin layer, enable you to prise things open.
The skin is waterproof, strong and resilient, yet also flexible. It consists of two layers of cells. The outer layer is called the epidermis and is made up of epithelial cells. The epidermis is supported from below by the dermis, which is a network of elastic fibres, blood vessels, hair follicles and roots, nerve endings, and sweat and lymph glands. Underneath the dermis, there is another layer of cells known as the hypodermis, which contains loose connective tissue and fat.
The epidermis contains many layers of closely packed cells. The cells nearest the skin’s surface are flat and filled with a hard substance called keratin. The epidermis contains no blood vessels – these are all in the dermis and deeper layers. The epidermis is thick in some parts (one millimetre thick on the palms and soles) and thin in others (just 0.1 millimetre thick over the eyelids). Dead cells are shed from the surface of the epidermis as very fine scale, and are replaced by other cells that pass from the deepest (basal) layers to the surface layers over a period of four weeks.
SKIN CHANGES
What is going on in the skin of people with psoriasis to produce the red scaly plaques? The epidermis in psoriatic skin is turning over much more rapidly compared with normal skin – possibly seven times as fast. This results in the epidermal cells not having time to mature fully. Instead of normal virtually invisible scale being formed, in people with psoriasis the resulting scale is visible, white and loose, and can be removed easily by gently scraping over the plaques.
Plaques look silvery and scaly because they are full of immature skin cells. If some skin is removed from a plaque of psoriasis (a technique called a biopsy), there are signs of increased cell turnover and inflammation. The increased cell turnover is shown by an increase in the dividing cells that form keratin (keratinocytes). There are also signs that the epidermal cells do not mature fully. Their immaturity is characterised by the presence of their nuclei, which contain the cells’ genetic material. As skin cells travel from the basal layers to the epidermis, their nuclei usually disappear. The outer layers of the plaques still have nuclei present, known as parakeratosis, and this shows that the cells have made their way up to the skin’s surface too quickly; this gives rise to the silvery scale already described.
Plaques may also look red because, in psoriasis, the blood vessels in the dermis are dilated (enlarged). This causes an increased blood flow, which gives the plaques their red colour.
INFLAMMATORY CHANGES
There is considerable inflammation in psoriasis. This causes the red, irritated plaques. No one is quite certain whether the abnormal skin in psoriasis is caused by a dysfunction of the immune system (an over-sensitivity reaction, for example) or whether the immune system is activated because the skin is abnormal.
Either way, many cells that usually fight infections and repair injuries to the skin are found in the epidermis of people with psoriasis.
White blood cells called T lymphocytes (T cells) are often found in the dermis of affected people. The drug ciclosporin A, which can be very beneficial in severe psoriasis, counteracts these T cells. This suggests that the cells may be playing a role in the development of psoriasis.
As mentioned, increased cell turnover is the main cause of the red and scaly plaques of psoriasis.
However, clinical studies and skin biopsies suggest that immunological changes may also be involved. Therefore, many of the treatments of psoriasis are directed against the increased cell turnover, the immunological factors or a mixture of the two. It is now felt that psoriasis is an autoimmune disease in which antigen presentation to cutaneous T-helper cells triggers secretion of chemicals called cytokines, which cause inflammation and proliferation of the keratinocytes. The antigen is so far unknown.
