Veterinary Pathology - Pigmentation & Calcification

Here, we will look at the role of pigmentation and calcification in disease and how they are used in pathology, how to stain for them and discuss why they are important in pathology.


This is when there is coloured material in tissue, such as hair and skin. If there is colour where there isn't usually colour, then that is considered abnormal pigmentation. Pigments come from either inside or outside the animal. If they come from outside the animal they likely have no pathological relevance.

Firstly, let's look at pigments from outside the animal, which are classified as Exogenous pigments.

Exogenous pigments

  1. Carbon

  2. Carotenoids

  3. Iatrogenic pigmentation


If you see animals with dark lungs it could be carbon pigmentation which is also known as anthracosis.

This happens because of carbon in the air. When it is breathed in, especially by city-dwelling animals, it can gather in the lungs over time.

Under the microscope, black granules can be seen.

It isn't to be confused with melanosis and pneumoconiosis.

Carotenoids or Lipochrome

These are:

  • Yellow-orange pigments

  • Bind to lipids

  • Fat-soluble, they are found anywhere in the body where fat is stored

  • The yellow in eggs & butter

  • In different animals, you'll see different amounts of yellow in the fat. For example, horse fat is much yellower than the fat of cattle etc.

These are not to be confused with jaundice. You can differentiate this pigment from jaundice by examining the sclera of the eye which will be yellow in jaundice and white with this pigmentation.

Iatrogenic pigmentation

This pigment is brought on by humans, such as branding, tattoos or other reasons associated with husbandry. Care must be taken when diagnosing to ensure that other factors such as inflammation are evident before pigments can be used to diagnose a disease.

Endogenous pigments (from inside the animal)


  • This is the pigment that makes up most of the body colouring, inside and out

  • Colours range from light brown to dark black

  • The pituitary hormone controls its production

  • Many animals have melanin patches naturally on their internal organs with no pathological relevance

  • You'll see melanocytes under the microscope

Melanocytes can accumulate naturally from birth, the skin can become irritated or have a scar and so hyperpigmentation can result and in some cases, cells melanocytes can multiply into a tumour (melanoma).

Blood pigments

Blood pigments can be separated into two groups, abnormal or pathological or normal derivatives of blood.


  • Haematin occurs after lots of blood breakdown (haemolysis) and can be seen most commonly in parasite infections such as liver fluke and babesiosis.

  • Methaemoglobin - this is when blood is oxidised. Nitrate and chlorate poisoning can do this and when it happens, the blood goes a chocolate brown colour.

  • Carboxyhaemoglobin - this occurs when the haemoglobin binds with carbon monoxide instead of carbon dioxide and turns the blood and tissue a cherry red colour, in the event of carbon monoxide poisoning.


  • Haemoglobin is released from the red blood cells when they're damaged (haemolysis or erythrolysis). Haemolysed blood is red and sometimes can be excreted in kidneys and urine.

  • Haemosiderin is the result of iron splitting from haemoglobin. It can look like gold granules in unstained tissues and it stains bright blue under the Perl's Prussian Blue stain.

  • Bilirubin is a process of the degradation of haemoglobin and it is orange/yellow. It is conjugated in the liver and excreted into bile there. If there is too much bilirubin it can cause jaundice/icterus, making all tissues yellow.

  • Porphyrins are made when haemoglobin is being made. When there is a metabolic disturbance in making haemoglobin, too many can be produced. Under the microscope, they look brown but under ultra-violet light, they can look red in urine or pink in bone.

  • Lipofuscins are brown and are known as the "wear and tear" pigment, seen in older animals, especially in heart muscle and neurones. Lipofuscin can be defined as a yellowish-brown, autofluorescent, lipid-containing pigment that accumulates in the cytoplasm of cells during ageing.

This video explains some of the mentioned pigments nicely!


Calcification in Pathology is the laying down of calcium salts in tissues that don't usually contain any bone.

There are two types of calcification, dystrophic and metastatic.

Dystrophic calcification

  • Happens in damaged tissue

  • Sometimes happens after cell necrosis

  • Happens as a result of ageing

  • Examples of dystrophic calcification are parasitic lesions such as lungworm in sheep, tubercles and white muscle disease.

  • If you look at the gross calcified tissue they look grey, chalky-white with a gritty appearance.

  • Stain with H & E to see them under the microscope, where they'll appear blue/purple

Metastatic calcification

  • Caused by too much calcium or phosphate in the blood

To see other pathology posts click here.