Pigments and Their Biological Functions

Pigments

• Pigments are colored substances, either exogenous (from outside) or endogenous (synthesized within the body).
• Examples of exogenous pigments include carbon (coal dust), which can be inhaled and lead to Anthracosis (accumulation of carbon in the lungs).
• Examples of endogenous pigments include melanin, bilirubin, hemosiderin, and lipofuscin.

Melanin

• Melanin is a brownish-black pigment produced by melanocytes in the epidermis.
• Acts as a screen against harmful UV radiation.
• Increased melanin pigmentation can occur due to sun tanning or diseases like nevus or malignant melanoma.
• Decreased melanin pigmentation is seen in conditions like vitiligo (loss of melanocytes) and albinism (melanocytes present but lack tyrosinase enzyme).

Hemosiderin

• Golden yellow-to-brown, granular pigment derived from hemoglobin.
• One of the major storage forms of iron.
• Iron is normally transported by transferrins and stored as ferritin micelles.
• Hemosiderin granules represent aggregates of ferritin micelles.
• Identified by its blue staining reaction with Prussian blue dye.
• Accumulates in tissues due to local or systemic iron excess, leading to hemosiderosis (within macrophages) or hemochromatosis (within parenchymal cells, causing tissue damage).

Lipofuscin

• Known as "wear and tear" pigment, it's an insoluble brownish-yellow granular intracellular material.
• Consists of lipids and phospholipids complexed with protein.
• Derived from free radical-catalyzed peroxidation of polyunsaturated lipids in subcellular membranes.
• Not harmful to the cell but acts as a marker of past free radical injury.
• Seen in tissues like the heart, liver, and brain as a function of age or atrophy.
• Can impart a brown appearance to tissue, known as brown atrophy.

Pathological Calcification

• Abnormal deposition of calcium salts, along with other minerals, in tissues.
• Two forms: Dystrophic and Metastatic calcification.

Dystrophic Calcification

• Deposition of calcium salts in dead or degenerated tissues.
• Occurs with normal calcium metabolism and serum calcium levels.
• Seen in atherosclerosis, damaged heart valves, and necrosis.

Metastatic Calcification

• Deposition of calcium salts in vital tissues, reflecting abnormal calcium metabolism and high serum calcium (hypercalcemia).
• Often a consequence of parathyroid hormone excess.
• May occur in normal tissues, including blood vessels, stomach, kidneys, and lungs.

Metastatic Calcification: Principle Causes

• Hyperparathyroidism due to parathyroid tumors.
• Bone tissue destruction from bone marrow tumors like multiple myeloma.
• Vitamin D-realted disorders like Vitamin D intoxication and sarcoidosis.
• Renal failure causing secondary hyperparathyroidism.

General Morphology of Pathologic Calcification

• Grossly: Appear as fine white granules or clumps, gritty deposits.
• Microscopically: Appear as intracellular and/or extracellular deeply basophilic, irregular, granular clumps.
• Special stains for calcium: von Kossa (black) and Alizarin red S (red).