fibril

A fibril is a slender, elongated filament composed of polymers such as proteins, nucleic acids, or polysaccharides. In biology, protein fibrils are common and arise when individual molecules assemble into long, thread-like structures. Diameter is typically nanometers; length can range from hundreds of nanometers to several micrometers.

Protein fibrils can be functional or pathological. Collagen fibrils in connective tissues provide tensile strength; bacterial curli and other extracellular filaments support adhesion and biofilm formation. Amyloid fibrils are highly ordered, beta-sheet-rich assemblies that can form from various proteins and peptides; they are associated with several diseases when formed abnormally, but amyloid-like fibrils also occur in healthy systems.

Molecular structure and assembly: Many amyloid fibrils exhibit a cross-beta sheet architecture, with beta-strands running perpendicular to the fibril axis. Fibril formation often proceeds via nucleation and elongation, influenced by concentration, temperature, pH, and mutations. Post-translational modifications can modulate assembly.

Functions and relevance: Fibrils contribute to tissue mechanics (collagen), enable microbial adhesion, and can be engineered for biomaterials. Pathological fibrils can disrupt cellular function, sequester proteins, and trigger disease processes in organs such as the brain and pancreas.

Detection and study: Fibrils are observed by electron microscopy and atomic force microscopy; their beta-sheet nature is probed by X-ray diffraction and infrared spectroscopy. Dyes such as thioflavin T and Congo red bind preferentially to amyloid fibrils, aiding detection.