Science of Skin Color

 

Science of Human skin color

PKGhatak, MD

The skin is one of the body's large organs. The color of the skin prevents or minimizes the harmful effects of Ultra Violet (UV) light of the sun.

The darker the shade of color the lesser amount of UV light penetrates the skin and decreases Vitamin D synthesis, the reverse is true for the lighter shades of color. It is said that women have lighter skin color because more vitamin D is needed for childbearing.

Not every part of the skin of an individual has the same shade of color and variability depends on the degree of exposure to sunlight. The sole of the feet and palms of the hands have different thicknesses of skin and a much lighter color, and that lighter shade is most noticeable in very dark colored people.

Theory of color variation in humans.

Scientific communities believe that the human race originated in equatorial Africa. Having dark skin gave humans an advantage over the other predators to hunt for a longer time each day. Because the black skin color developed as the coarse hairs were replaced by fine body hair. This combination allowed the body from overheating. That advantage led to a population boom and subsequent migration of people to other lands. When people settled in the subtropical area, their dark skin turned to a disadvantage; the lack of calcium produced weaker siblings, and most did not survive beyond their early childhood. A genetic mutation made the color of the skin lighter shade which allowed more UV light to penetrate the skin and generate more vitamin D. and the population bounced back.

The so called white colored skin is the color of the pale white color of the subcutaneous tissue shining through the light-colored skin.

What are the color pigments of human skin:

Melanin is the primary color and minor color contributions come from hemoglobin carotenoids and oxyhemoglobin.

Perception of color:

The combination of colors of melanin, oxyhemoglobin (proportional to blood flow) and carcinoids produce the final color perceived by the observer's retina.

Melanin:

Melanin is a group of chemicals very closely resemble each other in molecular structure. The skin Melanin in humans is Eumelanin and Pheomelanin. Neuromelanin is present in the brain.

Eumelanin and Pheomelanin are the same chemical but differ only in the number of polymer bonds in them.

Two colors of melanin:

Eumelanin is a dark pigment, has a range from brown to black, and Pheomelanin has a shade between yellow to pink color. The pink color of the lips is due to pheomelanin.

Eumelanin synthesis.

Amino acid Tyrosine is converted to Dihydrophlylalamine (DOPA) by the enzyme Tyrosinehydrolase and in the presence of cofactor Tetrahydrobiopterin. DOPA is converted to Dopaquinone by Tyrosinase and then undergoes several steps of polymerization to form eumelanin or pheomelanin.

Site of melanin production:

Melanin is produced by the Melanocytes present in the basal layer of the skin. The pigment granules are nicely packed in an envelope and are called melanosomes. Melanosomes are carried along the long arm of the melanocyte and the pigment packets are deposited within the Granular cells of the keratinocyte layer. One melanocyte and 36 granular cells and one Langerhans cell make one keratinocyte unit. It is estimated one melanocyte can supply 500 granular cells with melanosomes in its lifetime.

Stimuli for melanin formation:

UV radiation stimulates the synthesis of melanin. A hormone, produced by the Pars Intermedia of the Pituitary gland, Pro-Opiomelanocortin, is turned into Alpha Menaocytotropic hormone (alpha MSH). UV light also increases ACTH (adrenocorticotropic hormone) secretion. MSH and ACTH increase melanin production. Androgen also influences melanin deposit of pigment in the form of male patterns.

Genes controlling Melanin.

In humans chromosome 11 is the primary chromosome and three additional chromosomes, 17, 6 and 13 have also influence melanin. The location of the genes on these chromosomes are 11q24.2, 17q23.2, 6q25.1 and 13q33.2.

Defective genes and abnormal skin color:

1. Albinism.  It is a Hereditary disease due to an inheritance of defective genes. Skin, hair, retina of the eyes are devoid of melanin and is known as Oculocutaneous albinism (OCA). It is inherited as an autosomal recessive trait. At present 7 varieties of OCA have been identified.

2. Hermansky-Pudlak Syndrome.  Hermansky-Pudlak patients lack skin pigment and in addition, have a tendency to bruise easily and bleed spontaneously. Patients have additional symptoms which may involve GI, Pulmonary and CNS systems. So far 11 gene mutations have been identified. This abnormality is also inherited as an autosomal recessive pattern.

3. Chediak-Higashi syndrome.  It is an autosomal recessive hereditary disorder consists of partial OCA, platelet disorder, immune deficiency due to abnormal Lysosomes and in addition to hematophagy lymphohistiocytosis syndrome

4. There are many other syndromes due to inherited defective genes. Symptoms involve various organ systems and skin color. Just to name a few – Acanthosis nigricans, Xeroderma pigmentosa, Treitz syndrome, Histiocytosis lymphadenopathy plus syndrome, Lengius syndrome, Cruzion syndrome, Incontigentia pigmenti, etc.

5. Loss of melanocytes.  Skin abrasions may peel off the basal layer of skin containing the Melanocytes, when skin lesion heals the area is devoid of skin color.

5. Autoimmune diseases. Some acquired autoimmune diseases may attack melanocytes producing pigment voided spaces.

6. Vitiligo. It is suspected to be an auto-immune disease. The color free areas are patchy and can be any spot on the skin. No genetic link has so far been documented.

7. Localized dark pigmentation on skin:

Freckles, moles, Mongolian spots on buttocks, Caffe au lait spots, and Birthmarks are examples of uneven distribution of melanin in keratinocytes. Some of these spots are transient, and others disappear and reappear later in life. Caffe au lait spots, if multiple, are associated with neurofibromatosis.

All these are benign conditions unless one starts to grow in size and becomes darker or bleeds.

 8. Melasma. Symmetrical dark patches on the face due to estrogen – progesterone concentration variation, as seen in pregnancy and menopause. Melasma has no health consequences and is important from the cosmetic point of view.

Carotenoids:

Carotenoids are Terpenoids having formula C40H56. Carotenoids are plant pigments and humans must depend on plant sources for carotenoid supply. Yellow, orange, or red vegetables and fruits contain carotenoids. They are fat soluble chemicals; animal fat is also a source like Salmon. Cooking shredded vegetables in oil increases the bioavailability of carotenoids. In humans these molecules are antioxidants, boost immunity and provitamin A. Vitamin A is an essential vitamin for vision.

An essential organ:

The skin is an essential organ. The skin protects the body from the physical mechanical and thermal injury, prevents harmful agents from getting inside, maintains body temperature, synthesizes vitamin D, performs immune functions and the sensory receptors of pain, touch, temperature and pressure are located in the skin. Melanin in the skin protects tissues from the radiation damage of the sun.

All 8 billion people:

The color of the skin comes from one pigment - Melanin. All 8 billion people on earth have the same melanin. Color variation of the skin is due to the variation of melanin content in the skin cells.

People seeking advantage and try to divide human families into many color groups, like India did through the caste system. Businesses interested in selling their products claim about 110 tones of skin colors and have marketed matching colored products for sale.

Science has steadfastly maintained the scientific data which anyone can verify and as it stands today, all humans carry the same skin pigment melanin.

The difference in skin color is only in the view of the observer.

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