Sinew

Tendon
The Achilles tendon, one of the tendons in the human body.
Micrograph of a piece of tendon. H&E stain.
Details
Identifiers
Latin	tendo
Code	TH H3.03.00.0.00020
FMA	9721
Anatomical terminology []

A tendon or sinew is a tough band of fibrous connective tissue that usually connects muscle to bone and is capable of withstanding tension.

Tendons are similar to ligaments; both are made of collagen. Ligaments join one bone to another bone, while tendons connect muscle to bone.

Histologically, tendons consist of dense regular connective tissue fascicles encased in dense irregular connective tissue sheaths. Normal healthy tendons are composed mostly of parallel arrays of collagen fibers closely packed together. The dry mass of normal tendons, which makes up about 30% of their total mass, is composed of about 86% collagen, 2% elastin, 1–5% proteoglycans, and 0.2% inorganic components such as copper, manganese, and calcium. The collagen portion is made up of 97–98% type I collagen, with small amounts of other types of collagen. These include type II collagen in the cartilaginous zones, type III collagen in the reticulin fibres of the vascular walls, type IX collagen, type IV collagen in the basement membranes of the capillaries, type V collagen in the vascular walls, and type X collagen in the mineralized fibrocartilage near the interface with the bone. Collagen fibres coalesce into macroaggregates. After secretion from the cell, the terminal peptides are cleaved by procollagen N- and C-proteinases, and the tropocollagen molecules spontaneously assemble into insoluble fibrils. A collagen molecule is about 300 nm long and 1–2 nm wide, and the diameter of the fibrils that are formed can range from 50–500 nm. In tendons, the fibrils then assemble further to form fascicles, which are about 10 mm in length with a diameter of 50–300 μm, and finally into a tendon fibre with a diameter of 100–500 μm. Fascicles are bound by the endotendineum, which is a delicate loose connective tissue containing thin collagen fibrils. and elastic fibres. Groups of fascicles are bounded by the epitenon. Filling the interstitia within the fascia where the tendon is located is the paratenon a fatty areolar tissue.