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.
Your skin
protects you against chemicals, bacteria and radiation, helps
you maintain a stable body temperature, and stops you from losing
fluid and vital body chemicals. |
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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 cells are 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.
Dead cells
are shed from the surface of the epidermis as a fine scale and
are replaced by other cells that pass from the deepest layers
to the surface layers over a period of about four weeks. |
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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 oversensitivity 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 cyclosporin 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.
The epidermis
in psoriatic skin is turning over much more rapidly than normal
skin. Immature skin cells reach the surface forming plaques of
loose visible scale. |
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KEY POINTS
- In psoriasis, the skin renews itself in four days
instead of 28 days
- The silvery, scaly patches of skin are called
plaques
- Plaques consist of immature skin cells
- Inflammation may play a role in the formation
of plaques
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