Tendon

A tendon, also known as a sinew, is a strong, dense band of fibrous connective tissue linking muscle to bone. Its job is to transmit the mechanical force of muscle contraction to the skeleton, all while resisting tension. Like ligaments, tendons are made of collagen, but ligaments connect bone to bone, whereas tendons connect muscle to bone. The adult human body contains roughly 4,000 tendons.

Structurally, a tendon is composed of dense regular connective tissue. Its main cells are specialized fibroblasts called tenocytes, which produce the tendon’s extracellular matrix—a material packed with parallel collagen fibers. These fibers are grouped into bundles called fascicles. Each fascicle is wrapped in the endotendineum, a delicate loose connective tissue with thin collagen fibrils and elastic fibers. A set of fascicles is encased by the epitenon, a sheath of dense irregular connective tissue. The entire tendon is enclosed by a fascia, and the space between the fascia and the tendon is filled with the paratenon, a fatty loose connective tissue. Healthy tendons are anchored to bone by Sharpey’s fibers.

The dry mass of a normal tendon makes up 30–45% of its total weight. Of this dry mass, 60–85% is collagen: 60–80% is type I collagen, 0–10% is type III, and 2% is type IV, with small amounts of types V, VI, and others. The remaining 15–40% consists of non-collagenous components, including 3% cartilage oligomeric matrix protein, 1–2% elastin, 1–5% proteoglycans, and 0.2% inorganic elements like copper, manganese, and calcium. Though type I collagen dominates, minor collagens are crucial for tendon development and function. For instance, type II collagen appears in cartilaginous zones, type III in the reticulin fibers of vascular walls, type IX in certain regions, type IV in capillary basement membranes, type V in vascular walls, and type X in the mineralized fibrocartilage near the bone interface.

At the ultrastructural level, collagen fibers merge into macroaggregates. After secretion from the cell, procollagen is cleaved by N- and C-proteases, allowing tropocollagen molecules to spontaneously assemble into insoluble fibrils. A collagen molecule is about 300 nm long and 1–2 nm wide, and fibril diameters range from 50–500 nm. In tendons, these fibrils further assemble into fascicles roughly 10 mm long and 50–300 μm in diameter, and finally into tendon fibers 100–500 μm in diameter. Collagen in tendons is held together by proteoglycans, such as decorin and, in compressed regions, aggrecan, which bind to collagen fibrils at specific spots. The proteoglycans’ glycosaminoglycan (GAG) side chains interact with fibril surfaces, making them structurally important for interconnection. The main GAGs are dermatan sulfate and chondroitin sulfate. Dermatan sulfate is thought to form associations between fibrils, while chondroitin sulfate occupies volume between fibrils, keeping them separated and helping resist deformation. Decorin’s dermatan sulfate chains can aggregate in solution, aiding collagen fibril assembly. When decorin binds to a fibril, its dermatan sulfate chains may extend and link with chains on decorin bound to other fibrils, creating interfibrillar bridges that promote parallel alignment.

Tenocytes produce collagen molecules, which aggregate end-to-end and side-to-side to form fibrils. Fibril bundles organize into fibers, with elongated tenocytes packed tightly between them. A three-dimensional network of cell processes is associated with the collagen. These cells communicate via gap junctions, allowing them to detect and respond to mechanical loading. This communication relies on two proteins: connexin 43, found where cell processes meet and in cell bodies, and connexin 32, present only where processes meet. Blood vessels run within the endotendon parallel to collagen fibers, with occasional branching transverse anastomoses. The internal tendon bulk likely contains no nerve fibers, but the epitenon and paratenon have nerve endings, and Golgi tendon organs are located at the myotendinous junction between tendon and muscle.

Tendon length varies across major muscle groups and from person to person. In practice, tendon length is the deciding factor for actual and potential muscle size. For example, all else being equal, a person with shorter tendons and a longer biceps muscle has greater potential for muscle mass than someone with a longer tendon and shorter muscle. Successful bodybuilders generally have shorter tendons. Conversely, in sports requiring running or jumping, it is beneficial to have a longer-than-average Achilles tendon and a shorter calf muscle. Tendon length is determined by genetics and has not been shown to increase or decrease in response to environment, unlike muscles, which can be shortened by trauma, use imbalances, or lack of recovery and stretching.