Collagen fibrils are the principal tensile element of the extracellular matrix in a wide range of animal connective tissues. They have a 67 nm axial periodicity in most tissues, 65 nm in vertebrate skin, and are near-circular in transverse section. Fibril diameter depends both on tissue type and stage of development, covering a range of 20-500 nm in vertebrates. Fibril length is less well characterised but fibrils with lengths in the range 1-100 micrometres have been isolated.
Fibril formation is spontaneous (Fallas et al. 2010, Birk & Brückner 2011), but influenced by developmental state and the cellular environment. Several models have been proposed including the simple surface nucleation and propagation (SNAP) model (Trotter et al. 2000) but the mechanism of fibril assembly and regulation of fibril diameter and length are not completely understood (Holmes et al. 2001, Banos et al. 2008). Fibrils frequently contain more than one type of collagen, and the outer surface of fibrils frequently interacts with proteoglycans, fine-tuning its structural and signaling properties (Wess 2005, Kalamajski & Oldberg 2010, Ricard-Blum et al. 2011).
Individual fibril-forming collagen molecules are around 300nm in length. Complete fibrils exhibit a 67 nm periodicity, seen with many different imaging methods. This is due to a staggered overlap of molecules which leads to regions where fewer molecules overlap with a periodicity of 67 nm (Hodge & Petruska 1963, Wess 2005). Laterally, molecules are believed to be packed into a quasi-hexagonal structure (Trus & Piez 1980) resulting in locally ordered crystalline regions interspersed with disordered regions across the lateral plane of the fibril (Hulmes 2002). Interactions between molecules stabilize the fibril, including the formation of divalent and subsequently trivalent crosslinks, unique to collagen, that involve lysine or hydroxylysine residues.