Figure 1.

Illustration showing the molecular, microfibrillar, fibrillar and fascicular structure of tendon. Collagen molecules are composed of flexible Gly-X-Y (light regions on left of collagen molecule) sequences that alternate with rigid sequences of Gly-Pro-Hyp (dark regions of collagen molecule) that make a triple helix that is a hybrid of flexible and rigid mechanical elements. The microfibril is composed of 5 quarter-staggered collagen molecules with all the flexible and rigid regions in register. The 67 nm repeat period (D period in tendon) occurs at the fibrillar level when observed in the electron microscope and has all the flexible and rigid regions laterally aligned. Fibroblasts form longitudinal columns of cells on the surface of collagen fibrils. Collagen fibers are made up of laterally fused collagen fibrils and are wrapped with an ECM termed endotendon. Collagen fibers form fascicles and are bound into tendon units by additional ECM termed epitendon. The flexibility of collagen molecules is preserved in the D period of collagen fibrils and fibers and provides the energy storage to the ECM.