5.4: Skin Pigmentation

Learning Outcomes

  1. Explain how melanin is produced and its role in the integument

  2. Describe the other pigments that contribute to skin color

  3. Explain how skin coloration may indicate pathology

Melanin

Melanin Synthesis

  • Composed of two molecules of the amino acid tyrosine joined by a series of reactions catalyzed by the enzyme tyrosinase

  • This reaction occurs in a vesicle in the melanocyte called a melanosome

  • The melanocyte has “arms“ that extend out to and sometimes inside keratinocytes’ plasma membrane

  • Once inside, melanosomes are absorbed into the cytoplasm of the keratinocytes via phagocytosis

  • Then then melanin is transported superficial to the nucleus, to shield the DNA from UV rays

  • The pigment degrades, so the melanocytes must produce more melanosomes

Melanin’s Role

  • Melanin production increase as a result of sunlight and UV radiation

  • UV rays oxidize melanin immediately, causing it to darken

  • damages melanocyte DNA, causing more melanin to form

  • Major function is to protect keratinocyte DNA from damage

  • Decrease synthesis of vitamin D in the presence of UV radiation

  • This occurs so because an excess amount of vitamin D can lead to calcium ion imbalance and kidney failure

Skin Coloration and Pathology

  • Lighter skin allows for less sunlight to have a more profound affect on vitamin D synthesis

  • A concentration of melanocytes causes freckles and moles (also called nevus)

  • Number of melanocytes is identical across all people, the difference in skin color is the amount of tyrosinase activity and the type of melanin produced

Other Pigments

  • Minor contributors carotene and hemoglobin

  • carotene is obtained from orange and yellow foods in their diet, is lipid-soluble and accumulates in stratum corneum

  • Hemoglobin does not directly affect skin color, however, since much of the blood is found in the dermis, light-skinned individuals will appear with a pink-ish hue

Skin Color as a diagnostic tool

  • Light skin tone individuals, when exercising, injured, or an area is infected, it will appear red, as more blood is directed to that area (Darker skinned individuals may be lighter or darker in that area)

  • In extreme cases, where a person is very light in a cold environment, the collagen becomes visible, giving the skin a white color (called pallor)

  • The color of hemoglobin is also dependent on the amount of oxygen in the cells

  • For example, cyanide poisoning prevents oxygen from leaving the hemoglobin (giving it a brighter red hue), often presenting itself with a victim of bright red cheeks

  • Low amounts of oxygen will turn hemoglobin to a darker red-purple color, making light people bluish. This phenomenon is called cyanosis

  • Cyanosis is more clearly seen in darker people in their lips, gums and nail beds

  • Bilirubin, a breakdown product of red blood cells, causing jaundice