10. Histochemical characteristics of the different pigments. Exogenous pigments

First of all, what is a pigment? A pigment is a colored substance that can be both exogenous or endogenous. Exogenous pigments come from the outside of the body, like carbon dust or a tattoo. Endogenous pigments are synthetized within the body and can be lipofuscin, melanin and derivatives of hemoglobin. Accumulation of these pigments is associated with dysfunction of the cell.

Endogenous pigments

Histochemical characteristics

Let’s see how we can stain the different pigments to be able to see them better in the microscope. Some histochemical methods may also bleach pigments, as in case of melanin.

Haemosiderin

This hemoglobin-derived granular pigment accumulates in tissues when there is a local or systemic excess of iron. Its color varies from yellow to brown. More about hemosiderin can be studied in topic 13.

Hemosiderin stains blue with Prussian Blue and is useful when you want to know the accurate amounts of hemosiderin in a tissue.

Melanin

If we wish to remove melanin from e.g. tissues rich in it, it’s possible to bleach them away. Strong oxidizing agents like hydrogen peroxide or potassium permanganate can bleach melanin. On the other hand, if we wish to highlight melanin, no staining is necessary. That’s because formaldehyde imparts a strong yellow autofluorescence to the tissue where melanin is present.

Lipofuscin

Lipofuscin can be stained by sudan black B and PAS. More can be studied about each in the Histopathology slide section.

Exogenous pigments

These group of pigments are usually inhaled, digested or from a traumatic entry, like an injury or a tattoo.

Mineral dust

These dust particles can originate from coal, sawdust, silica and asbestos, and can result in anthracosis or pneumoconiosis. Anthracosis is an asymptomatic pigment accumulation without any cellular response, while pneumoconiosis is symptomatic with macrophage accumulation and fibrosis. Any person who has lived many years close to a city has some kind of (harmless) anthracosis.

Particles smaller than 0,5 μm can reach the alveoli and move in and out of them, while particles greater than 5 to 10 μm won’t even reach the distal airways. However, particles between 1 to 5 μm are dangerous because they get stuck in the distal airways.

Coal dust is relatively harmless, and large accumulations must be present to cause disease. However, silica and asbestos are much more reactive than coal and result in fibrotic reactions. Coal miners have no increased risk of lung cancer whereas workers exposed to silica dust and asbestos do. Usually, the particles don’t cause any harm itself, but when a pulmonary alveolar macrophage encounter them, an immune response will be initiated which results in lung injury and fibrosis which is the disease known as pneumoconiosis.

Coal Workers Pneumoconiosis is a special type of pneumoconiosis found in coal workers. Pigments from the coal accumulate in the lungs and cause immune response. Morphologically, its characterized with coal macules and larger coal nodules scattered in the lungs where the macules consist of dust-laden macrophages. The nodules contain collagen fibers as well. In Complicated Coal Workers Pneumoconiosis, black scars may be seen. It’s less severe than asbestos or silica-induced pneumoconiosis.

Tattoos

The pigment in a tattoo is the ink, which will be taken up by macrophages, but will not be recognized as antigens, so there’s no immune response. The ink granules will eventually be engulfed by fibroblasts and remain in the epidermis, while a scar tissue is formed above it.