The Reason Behind Skin Colour
Skin colour is a function of the size, number and the distribution of
melanin cells (not their density).?Melanin cells of darkly pigmented skin
have thicker, longer and branched dendrites. The differences in racial
skin pigmentation depend on the quantity of melanin pigments produced
and on the distribution and the deposition of these pigments throughout
the epidermis.
Tyrosinase is a rate-limiting enzyme for melanin synthesis, and defects
in the enzymes activity lead to the albinism in human. Differences in
human colour may primarily be due to differences in the tyrosinase activity
in the melanin cells from varying skin types.
Skin Pigmentation
Pigments are organic
coloring molecules within the body. They may be found in the bile, blood,
urine, eye, or skin and hair. Blood, bile and urinary pigments are derived
from hemoglobin or the products of hemoglobin catabolism. The endogenous
pigments of the choroid, skin and hair are called melanins.
Melanins are synthesized in melanosomes from the amino acid tyrosine
into dopa and dopaquinone. After the tyrosinase steps, the pathways to
produce black/brown and amber/red pigments diverge and involve many other
enzymes. Melanocyte Stimulating Hormone (MSH) treatment accelerates melanin
synthesis and causes the skin to visibly darken.
Differences in skin color are due mainly to differences in the number
of melanin granules in the keratinocytes.
What Is Melanin Pigment?
Melanin is a dark
compound that is called a photoprotective pigment.
There are two forms of the pigment melanin: Eumelanin granules and phaeomelanin
granules. These two forms of melanin are often both present together,
and occur in varying proportions.
The major role of melanin pigment in the skin is to absorb ultraviolet
(UV) light that comes from the sun.
How Does Melanin Form?
Melanin
forms in a special cell called the melanocyte. This cell is found in the
skin, in the hair follicle, and in the iris and retina of the eye. There
are many steps in the process of converting the amino acid tyrosine to
melanin pigment.
As with most metabolic pathways in our body, the first compound in a
pathway is converted to the next compound by the action of an enzyme.
The formation of melanin pigment is more complex and not all of the steps
are known.
Tyrosinase is the major enzyme involved in the formation of melanin pigment.
Tyrosinase is responsible for converting tyrosine to DOPA and on to dopaquinone.
The dopaquinone then forms black-brown eumelanin or red-yellow pheomelanin.
Two additional enzymes called tyrosinase-related protein 1 or DHICA oxidase
and tyrosinase-related protein 2 or dopachrome tautomerase are important
in the formation of eumelanin pigment.
Factors Affecting Skin Pigmentation
Skin pigmentation depends upon the organization and function of epidermal
melanin unit and several other separate but related events:
• Melanoblast migration from the neural crest.
• Melanoblast differentiation into melanin cells.
• The rate of synthesis and melanization of melanosomes.
• The size of melanosomes.
• Synthesis of melanin.
• The efficacy of melanosome transfer into keratin layer.
• The rate of melanosome degradation within the keratin layer.
• The rate of synthesis and inhibition of the tyrosinase enzyme.
• Activity of tyrosinase in melanosomes.
Melanin cells work in close harmony with their neighbouring cells in
the epidermis. They are influenced by a variety of biological factors
and environmental factors. The most important factor is UV exposure (
sun exposure) which increase the melanin cells activity up to 100-fold.
Physiology of Skin Pigmentation
All
skin colour stems from melanin. Melanocytes are spider-shaped cells with
long irregular arms that extend from the cell body. The arms of each melanocyte
link it with surrounding skin cells. They produce pigment granules called
melanosomes. As the skin regenerates, these neighbouring cells migrate
towards the skin surface and carry the pigment with them. In this way,
melanin is spread across the skin. Radiation from the sun stimulates melanocytes
to produce more melanin and results in skin tanning.
Formation of Melanin

The synthesis of melanin requires the amino acid tyrosine and a specific
enzyme called tyrosinase. Tyrosinase catalyzes the conversion of tyrosine
into biochemical intermediates like dopa and dopaquinone before producing
the final melanin pigment.
Skin Pigmentation After UV Radiation

The skin pigmentation response following UV irradiation comprises immediate
pigment darkening (IPD) and delayed tanning (DT).
Picture A: IPD results from the alteration and redistribution of melanin
moieties already present in the skin by forming ‘nuclear caps’ to protect
cell nuclei from UV-induced damage.
Picture B: DT is associated with an increase in the activity and number
of melanocytes. In repeated UV exposure, the number of melanocytes increases,
the number and size of melanosomes increases, and melanocyte dendrites
elongate and branch. Accelerated melanin transfer to keratinocytes then
results in a large increase in melanin granules in the epidermis.
Reference : Images adapted from www.adam.com
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