 Collagen fibrils/aggregates are arranged in different combinations and concentrations in various tissues to provide varying tissue properties.In bone, entire collagen triple helices lie in a parallel, staggered array.40 nm gaps between the ends of the tropocollagen subunits (approximately equal to the gap region) probably serve as nucleation sites for the deposition of long, hard, fine crystals of the mineral component, which is hydroxylapatite (approximately) Ca10(OH)2(PO4)6.