How UV radiation affects your skin

YOUR SKIN

The skin is the largest organ of the body, weighing about four kilo­grams and covering about two square metres. It helps maintain our body temperature, prevents dehyd­ration and protects us relatively effectively from harmful environ­mental agents, particularly infective organisms (bacteria or viruses – ‘germs’), dirt, dust and sunlight. It is also, of course, extremely important in terms of our appearance, and in enabling us to receive sensory messages from our environment. Finally, it helps to dispose of any organisms that do penetrate and probably also destroys early skin cancers.
Skin is made up of several layers, each having a specific function:

• The stratum corneum: the outer­most protective layer, which is a horny strip of inert dead cells created from the underlying epidermis; these are continually rubbed off to be replaced from below.

• The epidermis: a ‘brick wall’ of cells known as keratinocytes, which gradually move towards the surface, mature and die to form the stratum corneum.

• The dermis, which lies beneath the epidermis: a supportive supply layer, carrying sensory nerves and blood vessels to provide the skin with the nutrients and oxygen that it needs to grow.

• The hypodermis, lying below the dermis: essentially acts as a protective mechanical buffer joining the skin to the body itself; it contains connective tissue and fat.

In more detail, the superficial stratum corneum is tough and inert, helps prevent dehydration and protects the underlying living epidermal layers from mild-to­moderate injury, including UV radiation damage and penetration by ‘germs’.
The living cells beneath are of several types, most particularly keratinocytes, which are the building blocks of this layer and evolve as they move upwards to form the stratum corneum. At their base are evenly scattered melanocytes which produce melanin, a UV radiation-absorbing pigment; these give us a tan. A third type of cell is the Langerhans’ cell, an important part of the skin’s defence or immune system against infection and incipient tumours. Like all cells, these keratinocytes, melanocytes and Langerhans’ cells have a central nucleus – a headquarters that contains their genetic programming material, or DNA. Damage to this, particularly in certain basal cells that continuously divide to form the keratinocytes, is thought to be important in the development of skin ageing and cancer. Such damage is regularly caused in the skin by UV radiation, and is probably augmented by exposure to various harmful chemicals, particularly cigarette smoke.
The dermis, which underlies the epidermis, consists of a network of supportive fibres, blood and lymphatic vessels, hair follicles, nerve endings and sweat glands. The fibres are made of collagen and elastin and give strength, elasticity, shape and firmness to this layer and the skin overall.
Beneath the dermis is the hypodermis, containing loose connective tissue and fat. This layer binds the skin to the rest of the body, while also providing pro­tection as a mechanical buffer against damage from above.

ULTRAVIOLET RADIATION AND THE SKIN

Approximately five per cent of UV radiation striking the skin is immediately reflected; the remainder passes into the tissue, is bounced around by the contained particles and then passes back out again, or else is absorbed by molecules in the stratum corneum, epidermis or dermis. Absorption of the short-wavelength UVB (290 to 320 nm), if it occurs, is largely in the dead stratum corneum and the living epidermis, particularly in the latter by DNA and melanin, whereas the longer UVA (320 to 400 nm) is mostly transmitted to the dermis and absorbed there, predominantly by the haemoglobin in blood.
UV radiation has many secondary effects as a result of this absorption by skin molecules, called chromophores if they absorb; the most important are on DNA. Thus, this essential structure may undergo any of a variety of chemical changes, the best recognised being the formation of what are known as pyrimidine dimers. If these are not rapidly repaired, they are highly disruptive to cell function and division. It has in fact been shown that even minute amounts of sunlight, less than required to cause sunburn, can cause such damage throughout the entire thickness of the epidermis. Fortunately, however, most is repaired within hours to days, although a minute legacy of permanent damage generally remains to contribute eventually to skin photoageing and sometimes cancer.

HOW DOES SUNLIGHT CAUSE VISIBLE SKIN DAMAGE?

The absorption of UV radiation by skin chromophores (predominantly of UVB by DNA) and the conse­quent damage to these are the main cause of the skin changes that we can see, particularly sunburn, photoageing and cancer. Although such damage may occur to any part of the skin, most of the immediate effects, namely sunburn if severe enough to see, are epidermal because UVB is largely absorbed in the epidermis; however, UVB, and also UVA, do cause deeper damage as well.
During these changes after skin DNA is damaged by UVB, one of three things happens. The most likely is that the cell accurately repairs the damage through a sophisticated and complex process undertaken by specialised enzymes. At the same time, chemicals important in repair of the damage act on other skin molecular structures to cause the underlying blood vessels to dilate. This induces what we perceive as sunburn, an example of the body’s overall repair process of inflammation. The chemicals may also perhaps simultaneously damage collagen and elastin fibres in the dermis, thereby accentuating skin ageing, although direct radiation damage to these fibres and associated DNA probably plays a greater part.
The second possibility, which is in fact safe, is that the damage is so extensive that the cell dies. This happens particularly in severe sunburn, when the skin blisters and peels. However, less severely damaged cells may not die and thus leave important DNA damage, which is inaccurately repaired. This incorrect piece of DNA is then passed on when the cell divides, and leads to incorrect cell function. Such an alteration is called a mutation, and it is almost certain that the step-wise accumulation of these, mostly in the basal epidermal layer, may eventually lead to skin photoageing and cancer.