What is Pigment?
Pigments are substances that impart color to tissues and cells. In histology, pigments can be endogenous (originating within the organism) or exogenous (originating outside the organism). These pigments play a significant role in diagnostic pathology as their presence, absence, or accumulation can indicate various physiological and pathological states.
Types of Endogenous Pigments
Endogenous pigments are produced within the body and include:-
Melanin: Provides pigmentation to the skin, hair, and eyes and protects against UV radiation.
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Hemosiderin: An iron-storage complex found within macrophages, usually indicative of hemorrhage or iron overload.
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Bilirubin: Accumulates in jaundice, usually due to liver dysfunction or hemolysis.
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Lipofuscin: Often called the "aging pigment," it accumulates over time in long-lived cells such as neurons and cardiac myocytes.
Types of Exogenous Pigments
Exogenous pigments are introduced from outside the body and include:-
Carbon: Also known as anthracotic pigment, it accumulates in lung tissue due to inhalation of polluted air or smoking.
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Copper: Seen in conditions like Wilson's disease, where copper accumulates in various tissues.
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Tattoo ink: Deposited in the dermis and remains largely unchanged for the individual's lifetime.
Diagnostic Importance of Pigments
The presence and distribution of pigments in tissues can provide crucial diagnostic information. For instance:-
Hemochromatosis can be diagnosed by the presence of hemosiderin in liver biopsy samples.
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Jaundice is indicated by elevated bilirubin levels, which can be observed in liver biopsies.
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Wilson's disease can be identified by copper deposits in liver and brain tissues.
Histological Techniques for Pigment Identification
Various staining techniques are employed to identify and differentiate pigments in histological samples:Common Artifacts
Certain pigments can be confused with artifacts, which are extraneous substances introduced during tissue processing. These include:-
Formalin pigment: A dark brown artifact resulting from the reaction of formalin with hemoglobin.
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Mercury pigment: Associated with the use of mercuric chloride fixatives, appearing as dark brown granules.
Conclusion
Understanding the nature and significance of pigments in histological samples is crucial for accurate diagnosis and research. Identification and differentiation of endogenous and exogenous pigments, as well as recognizing common artifacts, are essential skills in histology.
