picroSirius

Picrosirius commonly refers to Picrosirius Red staining, a histological method for visualization of collagen fibers in tissue sections. The technique uses Sirius Red F3B dye dissolved in saturated picric acid and binds selectively to collagen, particularly to its triple-helix structure. When observed under polarized light, the stained fibers exhibit birefringence, providing high-contrast images with minimal background.

Under polarized light, thick, well-organized type I collagen fibers typically appear yellow to orange or red, while thinner type III fibers tend toward green. In practice, color distribution can reflect fiber thickness and packing rather than definitive collagen type, and results depend on sample thickness, staining duration, and microscopy settings. The method is often used for semi-quantitative assessment of collagen content and fiber organization.

Procedure: Tissue sections are deparaffinized and rehydrated, then stained with a saturated Sirius Red solution in picric acid. After staining, sections are washed in acidified water to remove excess dye, dehydrated, cleared, and mounted for observation, often with polarized light to reveal birefringence.

Applications include evaluation of fibrosis in liver, heart, skin, and tumors; assessment of extracellular matrix remodeling; and research into tissue development and repair. The technique complements immunohistochemistry and biochemical assays by providing structural information about collagen organization.

Limitations include potential misinterpretation of collagen type based solely on color, dependency on staining and imaging conditions, and the need for standardized protocols for quantitative comparisons. It is commonly used alongside quantitative assays such as hydroxyproline measurement or collagen type-specific antibodies.