liver and excreted in bile. When liver function is compromised, there is a reduction in the rate at which bilirubin is processed and it “backs-up” in the blood causing hyperbilirubinemia. Cases of massive RBC lysis may also show hyperbilirubinemia, as the liver cannot process haemoglobin as quickly as blood cells are being destroyed. In such cases of massive haemolysis, hyperbilirubinemia is also accompanied by the presence of the pigment in the blood and tissues. Hematin is produced in the liver from the metabolism of bilirubin and is also excreted in bile.
The non-haematogenous endogenous pigments are lipofuscin and melanin .
Lipofuscin is an insoluble yellow-brown pigment. Lipofuscin is made of the insoluble leftovers of the breakdown of cell components. Oxidative damage seems to be the instigating factor in the production of lipofuscin, leading us to associate the accumulation of lipofuscin in a tissue with . Lipofuscin is mostly seen in the cells of the liver and heart of geriatric individuals but may also be deposited in the skin. Other causes of lipofuscin accumulation are starvation, atrophy and an insufficient intake of dietary antioxidants. Melanin is a brown-black pigment made by melanocytes. Melanin is derived from the amino acid , and its production is catalyzed by the enzyme tyrosinase. Melanocytes are specialized epithelial cells that are capable of producing tyrosinase, and therefore melanin. Melanocytes are found in skin, hair follicles, the retina, the inner ear, the leptomeninges (the arachnoid matter and the pia matter covering the brain and spinal cord), the ovaries, the adrenal medulla and the substancia nigra (the area of the brain associated with Parkinson’s disease). is a hereditary condition in which there is hypopigmentation of the skin. This is due to a hereditary defect in the production of tyrosinase that prevents the synthesis of melanin. True albinism does not occur in horses, but a condition called leucism does. Leucism is a reduction in the presence of pigment producing cells, either throughout the body or in localized regions of the body. The localized form of this condition is responsible for coloration seen in paint horses and some appaloosas.
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