SOOOO. How often do you actually use sunscreen? . I think a lot of black people, including myself are very guilty of avoiding the use of sunscreen. So in this post, i explain how humans tan in general and the effects of UV-radiation on the skin. A follow up on my post about melanin.
If you haven't already read that... click here then come back :)
Tanning is an increase in the skin pigmentation greater than normal levels which is stimulated by UV exposure. The main risk of UV exposure is the production of Reactive Oxidative Species (ROS) which can go on and cause damage to DNA. DNA damage is disastrous beacuse DNA is basically the blueprint for everything that happens in our bodies. The ROS cause havock to the DNA and therefore messes up the information that is stored in our DNA.
The skin responds to UV radiation in two ways
1. thickening of the stratum corneum - this is the outer layer of the epidermis – this layer is thickest on the palms of your hands and the soles of your feet and are the hardest regions for UV radiation to penetrate.
2. an increase in the melanin filter - Melanin is a natural sunscreen thst neutralises the ROS generated due to UV exposure. Tanning involves an increase in the melanin synthesis and melanocyte dendricity which increase melanin transfer to keratinocytes. Simply explained in the diagram below (B) the melanocyte dendrites mover further into the layer of keratinocytes.
UV absorption is triggered by keratinocytes . UV radiation causes an increases in division of the keratinocytes which in turn causes an increase in the number of melanocytes. The rate of mitosis in keratinocytes increases a day after UV expose and then reaches its max after two days where it remains at this level for about a week .
There are two types of tanning
1. immediate tanning – occurs after exposure to UV-A for between 1-2 hours and fades after 5-24 hours . Some changes occur to melanocytes but no drastic change. The Skin detects the UV-A component of the sunlight through specific receptors that are sensitive to light which are found on the melanocytes . This stimulates early synthesis of melanin by activating the pre-existing melanin synthesising machinery.
2. delayed tanning- this is what happens when you go on a long holiday or are under constant exposure to the sun .This happens due to repeated exposure to UV-B and darkening usually occurs after 48-72 hours and reaches it maximum after about 3weeks. The skin does not return to normal levels for at least 8-10 months.The skin responds to UV-B via keratinocytes Several things occur within the keratinocytes that enhance the production of melanin after several days
UV-B stimulation does have other benefits such as the stimulation of vitamin D in the epidermis. You do not need to bask under the sun for the whole day or weeks for your body to produce enough vitamin D , if only 5% of the body is exposed to UV-B it is sufficient enough for vitamin D synthesis. Vitamin D promotes the absorption of calcium ions and ensures the mineralisation of bone.
To summarise,just because you produce more melanin does not mean you are completely exempt from the risks of UV radiation. You are still at risk of DNA damage. Use sunscreen and don't go sitting under the sun the whole day.
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Natarajan, V. T. et al., 2014. Multifaceted pathways protect human skin from UV radiation. nature:chemical biology, 10(1552-4469), pp. 542-551.
Slominsky, A., Tobin, D. J., Shibahara, S. & Wortsman, J., 2004. Melanin Pigmentation in Mammalian Skin and Its Hormonal Regulation. American Physiological Society , 84(4), pp. 1155-1228.
Tadokoro, T. et al., 2003. UV-induced DNA damage and melanin content in human skin differing in racial/ethnic origin. FASEB journal, 17(9).