What is happening when we get a tan?

Question

Almost everyone nowadays wants that nice summer tan, but what exactly is going on beneath the skin? I've heard a few different theories about tanning - such as a tan is nothing but the pigmentation in your cells burning, but is this true?

My questions:

• What exactly is happening to our skin as we tan?
• Are pigments being burned (like I was told in school), or is pigment being produced by the cells. ?

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Bonus related question: Why do some people burn and others tan?

Bonus related question: https://biology.stackexchange.com/questions/35719/how-does-our-tan-disappear

Answer 1

When our skin is exposed to the sun, this can cause some damage in the upper layers of skin. This activates DNA damage repair and also induces signalling towards the melanocytes (which produce the pigment). Signalling means the excretion of signalling molecules (mostly alpha-Melanocyte stimulating hormone, aMSH) which binds to the MC1R receptor (melanocortin 1 receptor) on the surface of the melanocyte (the cell which produces the pigment) and induces (or increases) the production of melanin. The melanin is then transferred in the melanosomes to the adjacent keratinocytes, which are then better protected from the sun. This figure (from this article) illustrates this process nicely:

Short term tans (the fast reaction) are based on the changed distribution of melanosomes (granules containing pigment) from the melanocytes to keratinocytes. Longer term suntans are caused by increased pigmentation in the skin as a reaction to the UV exposure by the accumulation of melanin and by the burning of cells. So it is no wonder that it looks like increased pigmentation, it really is. People tan differently because of their genetic background. This influences their ability to react to sun exposure.

There is relatively little known about the degradation of the melanosomes (the granules containing the melanin). What is known is that they are degraded in the lysosome, and that the melanin is most likely degraded through oxidation. There happens to be a permanent turnover of the melanosomes, and when the body is producing fewer new melanosomes than are degraded, your skin will get lighter again.

References:

1. Shining light on skin pigmentation: the darker and the brighter side of effects of UV radiation.
2. Melanocyte biology and skin pigmentation.
3. Melanosome Degradation: Fact or Fiction

Answer 2

Some additional details on the same lines as Chris' answer:

Coelho et al. have studied the short and long term effects of UV on human skin and they report that there are different stages of tanning:

• Immediate pigment darkening: happens in a few minutes and persists for some hours. It is most probably due to melanosome redistribution and melanin oxidation by UV.
• Persistent pigment darkening: happens in a few hours and can persist for days. It happens because of new melanin synthesis in the epidermis.
• Delayed pigmentation: happens in a few days and persists for weeks. It results from "prolonged increases in melanin content".
• Long-lasting pigmentation: persists fore more than 9 months after the first UV exposure. The mechanisms underlying this response are not very clear.

IFN$\gamma$ signalling is involved in inhibition of melanogenesis and apoptosis of melanocytes ^[1,2]. This is one possible pathway of detanning. This happens in the order of days.

Answer 3

First, one should mention types of skin: skin type defines how the person will tan -

Second, the solar radiation (UV) absorbed by sun defines the amount of melanin produced by melanocytes of the skin to protect the skin = melanogenesis (which in turn depends on skin type).

When the solar exposure diminish, the natural repair processes causes melanocytes to remove excessive amounts of melanin.

As mention by WYSIWYG, the question too broad, you can continue your education via wiki.