Ultrastructural features of adult human tendon.

A variety of human tendons have been studied at the electron microscope level. The fibers of these tendons are composed of collagen fibrils that average 1,750 A and 600 A in diameter. A third population that measures 100 A in diameter may represent immature collagen or filaments that are incorporated into tendon elastic fibers. The larger collagen fibrils vary in ratio with respect to one another, and are connected by interfibrillar bridges which in some cases appear to extend through the substance of the fibril. The collagen fibrils of the paratenon are less-well organized than those of the tendon proper and average 600 A in diameter. Tendons that exhibit the property of lateral stretch (plantaris and palmaris) were compared at the ultrastructural level with tendons that do not have this property. No differences between the two tendon types could be determined in normal or spread preparations, indicating that the differences in physical characteristics are a result of fiber rather than fibril organization.