Humans have an amazing ability to change skin tone as a part of adaptation to environmental challenges as well as changes in metabolism or health issues. Several pigments define our skin tone, but the most important of them is melanin, produced by skin pigment cells called melanocytes.
Melanin is a dark natural pigment produced by the pigment cells. Melanin pigment is vital for our skin health due to its sun protection factor from ultraviolet radiation and free radicals, constantly adjusting to changing environmental conditions.
The shielding effect of melanin, especially dark eumelanin, is achieved by its ability to serve as a physical barrier that scatters ultraviolet radiation. Also, it acts as an absorbent filter that reduces the penetration of ultraviolet through a superficial skin layer.
The efficacy of melanin as a sunscreen was assumed to be about 1.5-2.0 sun protective factors (SPF). Possibly, it can be as high as 4 SFP, implying that melanin absorbs 50% to 75% of harmful radiation. An SPF of 2 means the natural melanin doubles our skin's protection against sunburn.
You can guess that is not enough to protect the skin from damage, but our skin melanin creates a natural barrier that helps protect our skin at some basic point. In the case of a skin condition called albinism, when the natural pigment melanin is absent, there is an increased risk of sun damage. The skin is more vulnerable and needs to be protected carefully, even when the intensity of ultraviolet rays is very low.
Melanin produced by specific skin cells is called melanocytes. They are located between the superficial skin layer called the epidermis and the deep skin layer called the dermis.
Melanocytes have a very similar structure to nervous cells and are sometimes called "skin neurons" as they are able to interact with the nervous system using the same biochemical language. Melanocytes form about 1% of all epidermal cells, but each pigment cell is associated with about 36 of the main epidermal cells called keratinocytes and one immune Langerhans cell, forming a small molecular structure known as the epidermal melanin unit.
Each melanocyte plays the role of a sun umbrella for 30-40 keratinocytes (main epidermis cells), protecting them from damage and mutations. MEach pigment cell has a special structure called melanosome which produces melanin. Melanin doesn't stay inside pigment cells for long, as pigment cells are able to transfer protective pigment to other cells using dendrites. Dendrites look like octopus tentacles growing from melanocytes, adjusting to other cells in the skin and transferring melanin to them, helping to protect their DNA from within.
While skin is exposed to ultraviolet radiation, it triggers special receptors of pigment cells, which activate a reaction of melanin synthesis. The main source of melanin is the amino acid tyrosine, which circulates in our body. Tyrosine is oxidized and transformed into a new substance called DOPA, which can be transformed into dark brown eumelanin or yellow/red pheomelanin if reacting with sulfur-based amino acid cysteine.
Ultraviolet radiation triggers mostly brown melanin production, changing skin color to darker. A key substance transforming tyrosine to melanin is called tyrosinase. It is an enzyme produced by pigment cells while they are exposed to hormonal influence, inflammation, trauma, or stress, as well as ultraviolet.
Simply speaking, more tyrosinase means darker skin color in the area exposed to sun rays. Skin damage or sensitization by different agents provoques higher sensitivity of pigment cells' receptors and higher activity of tyrosinase, leading to excessive melanin production and formation of dark spots or discoloration.
Skin light sensitivity (photosensitivity) means you have an unusual reaction to sun exposure. This can cause symptoms like pain, redness, skin inflammation, itching, or a rash when the skin is exposed to light, followed by pigmentation issues like the formation of dark spots and melasma. You can become photosensitive from:
If you have a family history of skin photosensitivity or it was increased by some sensitizing agents, it is important to be very careful and use broad spectrum sunscreen avoiding sun exposure during mid-day when the intensity of ultraviolet rays is higher.
Individuals with a darker skin tone are better protected against sun-induced damage, as their skin contains more of protective eumelanin which is superior to pheomelanin in its photoprotective properties. Interestingly, the melanin in darker skin tones is twice as effective compared to lighter skin in inhibiting UVB radiation from penetrating into the skin.
Scientists found that the darker skin allows only 7.4% of UVB and 17.5% of UVA to penetrate through epidermis, but 24% of UVB and 55% of UVA pass through fair skin structure. Melanosomes in dark skin are more stable and resistant to degradation, while in lightly pigmented skin, melanosomes are easily degraded, distributing a "melanin dust" in the superficial skin’s layers. That explains the higher risk of skin cancers in people with fair skin types and the importance of intense and persistent sun protection.
Differences in skin pigmentation do not result from differences in the number of pigment cells in the skin, as one might assume, but from differences in their activity, the type of melanin produced, and the size of melanosomes. The amount of pigment in one pigment cell can vary, with the melanin content of melanosomes ranging from 18% to 72%, defining a darkness of skin color.
Melanocyte density in the skin of the palms and soles is only about 10-20% of that in the skin on the trunk or limbs, so their color is always lighter. Red hair can contain high levels of pheomelanin, but generally, all types of skin contain more of dark eumelanin than red/yellow pheomelanin.
While total melanin content in the epidermis differs only by 2-fold in Asian and Caucasian skin, Black skin contains ∼3 to 6-fold higher levels of melanin, more eumelanin and has more and larger melanosomes than fair skin.
However, it doesn't mean that people with darker skin types don't need to protect their skin from ultraviolet light! The World Health Organisation recommends using sunscreens and seeking shade when an ultraviolet index is 3 or higher. But even for very sensitive, fair-skinned people, the risk of short-term and long-term UV damage below a UVI of 2 is limited, and under normal circumstances, no special protective measures are needed.
The latest research found a correlation between characteristics of the skin microbiome and the pigmentation status of the skin. Specific strains of pro-inflammatory bacteria, including C. acnes, have been found to be prevalent in pigmented skin.
In non-pigmented skin prevailed other bacteria strains known for their antioxidant activities. "Antioxidant" bacteria help moderate skin immune reactions, reduce inflammatory reactions, and improve natural protection from ultraviolet. Apparently, the skin microbiome plays an important role in skin protective function but also in skin aging, pigmentation, and the development of skin conditions.
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