Why do I get freckles in the sun?

Chris put Jane Sterling on the spot, with this question about freckles...

Chris - Now talking of the weather we've got this question for you Jane, which Adam has sent in…

Adam - Why does the sun bring out freckles?

Chris - So what is a freckle and why does it make Adam go freckly, or anyone in fact?

Jane - So I’m detecting a little bit of an Irish accent with Adam there, and it may be that if he's from Ireland, has seen a lot of people with freckles because they are much more common in people with red hair or reddish hair, and particularly from the Celtic races that have moved to the British Isles many many years ago.

So a freckle is a little patch of skin where the melanocytes, that's the cells that make the pigment in the skin the melanin, they're a little bit more efficient at changing the effects of the ultraviolet light to produce more melanin. So between the freckles obviously, is much paler skin that'll tend to burn more easily. So in those areas of skin the melanocytes just don’t make as much pigment and as efficiently. If you got very dark skin then your melanocytes sites are very very efficient at making the pigment and obviously the skin looks darker with or without sun.

Chris - Does that mean that if you then retreat from the Sun or when winter comes, same thing pretty much isn't it, certainly in high latitudes like where we are, that your skin then regresses to a non-freckled state. freckles are not permanent?

Jane - You can usually see the freckles even if you never go out in the sun or at the end of winter but they become much more marked once spring and summer comes because those freckly bits, the darker bits, start making more pigment and the bits in-between don’t.