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PH-FD12

Endogenous and exogenous pigments

Where the colours in tissues come from: melanin from melanocytes, haemosiderin and bilirubin from broken-down haemoglobin, lipofuscin from ageing cells, and carbon, ink and metals from outside, with what each looks like and causes.

By the end, you can

  • Describe how melanocytes make melanin and explain the main causes of too much or too little.
  • Trace haemoglobin breakdown to haemosiderin and bilirubin, and relate it to bruises, heart failure cells and jaundice.
  • Distinguish haemosiderosis from hereditary haemochromatosis by where the iron is stored and what it damages.
  • Explain what lipofuscin is and why it is a marker rather than a cause of disease.
  • Describe how inhaled carbon and tattoo ink are handled by macrophages, and tell common pigments apart.
  • Recognise the classic appearances, special stains and tests of tissue pigments, and explain how venesection, chelation, phototherapy and other treatments act on the mechanism.

What it is

A pigment is a coloured substance in tissue. Endogenous pigments are made in the body: melanin; the haemoglobin-derived pigments haemosiderin (iron-containing) and bilirubin (iron-free); and lipofuscin, the residue of oxidised membranes. Exogenous pigments come from outside, most often inhaled carbon and tattoo ink, and less often metals such as silver or dietary carotene. Some are harmless markers of past events; others show, or cause, disease.

4 types to tell apart
Melanin
Brown-black pigment made by melanocytes; increased by sunlight, some hormones and inflammation, and lost in vitiligo and albinism.
Haemoglobin-derived pigments
Haemosiderin (golden-brown, iron, Prussian blue-positive) and bilirubin (yellow, iron-free) from the breakdown of red cells.
Lipofuscin
Yellow-brown "wear-and-tear" pigment of oxidised lipid in the lysosomes of long-lived cells.
Exogenous pigments
Carbon (anthracosis), tattoo ink, metals such as silver (argyria) and dietary carotene, held in macrophages or deposited in skin.

US spelling: coloured → colored, haemoglobin → hemoglobin, haemosiderin → hemosiderin, colour → color, haem → hem.

Normal function

Pigments the body makes on purpose

Melanocytes sit among the cells of the of the epidermis. In membrane-bound melanosomes they turn the amino acid tyrosine into melanin, using the enzyme tyrosinase, and pass the melanosomes along their branches to keratinocytes in the . There the melanin forms a cap over each nucleus that absorbs ultraviolet light and protects the DNA.

People of different skin colour have about the same number of melanocytes; colour depends on how much melanin they make, its type (brown-black eumelanin or red-yellow pheomelanin) and how it is distributed. Ultraviolet light increases production within days, which is tanning.

Haemoglobin is the body’s other main pigment. Old red cells, after about 120 days, are eaten by macrophages, mainly in the spleen. The iron is kept and reused, stored in the protein ferritin; the rest of the haem becomes green biliverdin, then yellow bilirubin, which the liver excretes in bile.

Point to it on the model