Tanning is the result of a complex biological mechanism of skin pigmentation under the influence of UV rays. Whether it is artificial or natural, the process at work remains the same. In this article, we offer you an insight into how tanning occurs.
How does tanning occur?
Tanning is a natural skin colouring process that occurs when it is exposed to UV rays. It is the result of the melanogenesisprocess, that is, the synthesis of melanin, a brown pigment.
Upon penetrating the epidermis, UV rays trigger several biological reactions, including the binding of themelanotropin hormone (MSH) to the melanocortin 1 receptor (MCR1), located on the membranes of melanocytes. This binding activates the cAMP/PKA signalling pathway, which leads to the phosphorylation of the ubiquitous protein CREB, essential for the regulation of the expression of certain genes, including the MITF gene (Melanocyte Inducing Transcription Factor). The increase in MITF expression causes both an increase in the expression of the TYR gene coding for tyrosinase, a key enzyme in melanogenesis, and the induction of the latter.
The process of melanogenesis takes place in the melanocytes, which are globular cells in the basal layer contained within specific organelles known as melanosomes. Melanocytes are equipped with numerous extensions (dendrites), allowing them to be in contact with keratinocytes, the cells present in each stratum of the corneal layer.
The synthesis of melanin occurs via a series of reactions, the last of which is the transformation of tyrosine, an amino acid present in the bloodstream, under the action of the enzyme tyrosinase. It's worth noting that two types of melanin can be produced, theeumelanin, which is dark and has a strong protective power, and the pheomelanin, which is lighter.
The melanosomes are then transferred from the melanocytes to the keratinocytes to allow for the pigmentation of the epidermis by melanin. Following this, the melanosomes are destroyed in a process called lysosomal autophagy by certain lysosomal proteases such as cathepsin.
Note : The various phototypes are due to the differences in the proportion of each type of melanin synthesised and the size of the melanosomes between each individual. For example, there is more eumelanin in people with darker skin and their melanosomes have a larger diameter.
Tanning, a defence mechanism.
Tanning is often appreciated for its aesthetic appeal. However, its role goes beyond that and it primarily has a photoprotective function for the skin.
The UVA and UVB rays emitted by the sun have a detrimental effect on the skin, as they generate an excess of free radicals in the dermis and epidermis. Specifically, when UV rays reach the skin, they are absorbed by molecules present in skin cells, such as lipids and proteins. However, when a molecule absorbs a photon of energy from the UV rays, an electron within this molecule can be excited to a higher, unstable and reactive energy level. In some cases, it dissociates from the molecule, thus creating a free radical, that is, an unstable species containing an unpaired electron.
Once formed, free radicals can initiate chain reactions. Indeed, a free radical can react with a neighbouring molecule, stripping it of an electron and transforming it into a free radical itself. This chain reaction can spread, generating numerous free radicals that damage cells and DNA, leading to various skin disorders (accelerated ageing, hyperpigmentation, inflammation, cancers...).
Melanin has the ability to act by enveloping the nucleus of keratinocytes to form a veil that protects DNA from the mutagenic and carcinogenic effects of UV rays. It can also directly capture free radicals, which are responsible for the damage.
However, tanning is not a sufficient mechanism to be fully protected from UV rays. Despite the protection provided by melanin, approximately 15% of UVB still manage to reach the basal layer and 50% of UVA the dermis. Therefore, when one wishes to tan, it is important to apply a sunscreen before each exposure. For this purpose, we offer a wide range of broad-spectrum sun care products at Typology, effectively protecting against UVA and UVB rays.
HERLYN M. & al. Crosstalk in skin : melanocytes, keratinocytes, stem cells and melanoma. Journal of Cell Communication and Signaling (2016).
ASKARIAN-AMIRI M. E. & al. Signaling pathways in melanogenesis. International Journal of Molecular Sciences (2016).