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Our skin responds to sunlight by tanning. It is trying to protect its genetic material from UV radiation – with remarkable precision. Why a tan alone is still not enough.
After a few days in the sun, many people develop a tan. The colour comes from a pigment called melanin. Melanin absorbs some UV radiation, helping to protect the skin from damage. But tanned skin still needs sun protection.Protecting the cell nucleus is particularly important. It contains the genetic material, the DNA, with all the information a cell needs to live and divide. UV radiation can damage this material, and pigment helps shield it.
The protective pigment is produced by melanocytes, the skin's pigment cells. They sit in the deepest layer of the epidermis and produce melanin, which is packaged into tiny cellular structures called melanosomes. Through finely branched extensions, the melanocytes pass the pigment to neighbouring cells. In this way, a single melanocyte supplies several dozen epidermal cells with protective pigment.The amount of pigment in the skin varies from person to person, which is reflected in different skin colours. Interestingly, lighter and darker skin contain roughly the same number of melanocytes in comparable areas of the body. What differs is the amount of melanin they produce, as well as the size and distribution of the melanosomes.
Once transferred, the pigment forms a cap over the nucleus of the epidermal cell. This places it between the nucleus and incoming UV radiation. You can picture it as a tiny parasol: it intercepts some of the radiation before it reaches the sensitive genetic material below.These pigment caps can be seen under a light microscope in a suitably prepared skin sample. And their protective effect can be measured: after exposure to UV light, cells with a pigment cap showed less damage to their genetic material than cells without one. It is not enough for pigment simply to be present. It must also be in the right place. And even then, the cap does not provide complete protection.
The skin has this pigment protection even without sunbathing. But when it is exposed to the sun, it can produce additional melanin and become darker. For that to happen, the melanocytes first need a signal to make more pigment.That signal arises when UV radiation damages the genetic material of the epidermal cells. The affected cells respond with the help of a protein called p53, a kind of internal alarm button. It sets off a chain of signals that leads to more melanin being produced. This additional pigment, in turn, helps protect the cells from further UV radiation.The sequence matters. First comes the damage. Only then does the skin strengthen its protection. In other words, the skin tans because it has been damaged.Producing this additional pigment takes days, and a stronger adaptation can take weeks. The tan that appears later cannot undo the damage already done.
A darker appearance does not always mean that the skin has produced more protective pigment. Sunlight contains both UVA and UVB radiation. It is mainly UVB that stimulates the production of new melanin. UVA, by contrast, can quickly darken pigment that is already present. The skin then looks browner without a corresponding increase in melanin. It has more colour, but no more protection.Skin colour therefore does not tell us how well the skin can withstand UV exposure. Pre-tanning on a sunbed is not a sensible way to prepare for a holiday in the sun.
The protection provided by additional pigment does not last indefinitely either. The epidermis renews itself continuously. Pigment-containing cells migrate towards the surface and are eventually shed. When less pigment is supplied, the tan fades and the additional pigment protection declines with it.The skin can also adapt to the sun in a second way: UVB radiation thickens its outermost layer, the stratum corneum. This adaptive thickening also provides a degree of protection. Without further UV exposure, however, it gradually recedes.After a winter with little sun, the skin may therefore be more sensitive than it is in late summer, even when the sun is at a similar height in the sky. On the first sunny days of spring, it needs extra protection.
I find it remarkable how our skin protects its genetic material with these small parasols. But even with them, it remains vulnerable to UV radiation.That is why it is worth checking the UV index before spending time outdoors. It indicates the level of UV radiation the skin is exposed to, and many weather apps display it. At a UV index of 3 or above, protective measures are recommended: shade, clothing and a broad-spectrum sunscreen with UVA and UVB protection on exposed skin. This applies even when we are already tanned.Our skin raises its tiny parasols. The rest of the protection is up to us.
