Tendon

Tendons are tough, dense bands of fibrous connective tissue. They serve a vital role by connecting muscles to bones. When a muscle contracts, it creates mechanical force. The tendon transmits this force to the skeletal system to create movement.

While they are often thought of as simple connectors, they also help manage tension. Some tendons even function like springs to make movement more efficient. It is important to distinguish them from ligaments. Ligaments connect bone to bone, while tendons connect muscle to bone.

The structure of a tendon is highly organized and hierarchical. At the microscopic level, special cells called tenocytes, or tendon cells, are the main components. These tenocytes synthesize the extracellular matrix, which is filled with densely packed collagen fibers. These fibers run parallel to one another. They group together into bundles called fascicles. Each fascicle is wrapped in a delicate layer of loose connective tissue called the endotendineum.

A larger sheath called the epitenon binds a set of fascicles together. Finally, the entire tendon is enclosed by a layer called fascia. The space between the fascia and the tendon is filled with paratenon, a fatty tissue.

Building these fibers is a complex biological process. It begins when tenocytes secrete collagen molecules. These molecules are cleaved by specific enzymes called procollagen N- and C-proteases. After this, the tropocollagen molecules spontaneously assemble into insoluble fibrils. A single collagen molecule is roughly 300 nm long and 1 to 2 nm wide. These fibrils then grow to diameters between 50 and 500 nm. Eventually, these fibrils group into fascicles, which are about 10 mm long. These fascicles then form the final tendon fiber.

The chemical makeup of a tendon is precisely balanced. The dry mass of a normal tendon accounts for 30% to 45% of its total mass. Collagen makes up 60% to 85% of this dry mass. Most of this is type I collagen, though type III and type IV are also present. The remaining 15% to 40% consists of non-collagenous components. These include proteoglycans, which are proteins bonded to glycosaminoglycan groups. Specifically, dermatan sulfate and chondroitin sulfate play major roles. Dermatan sulfate helps form associations between fibrils. Chondroitin sulfate helps occupy volume to keep fibrils separated and withstand deformation.

Proteoglycans like decorin are essential for structural integrity. Decorin molecules bind to collagen fibrils at specific locations. The dermatan sulfate chains on decorin can extend to connect with other decorin molecules. This creates interfibrillar bridges that ensure the fibrils align in a parallel fashion. In regions where the tendon is compressed, a proteoglycan called aggrecan is found. This organized chemistry allows the tendon to handle immense mechanical loading. The tenocytes themselves stay connected through a three-dimensional network of cell processes. They communicate using gap junctions to respond to physical stress.

There are approximately 4,000 tendons in the adult human body. Their lengths vary significantly between individuals. This length is determined by genetic predisposition. Unlike muscles, tendon length does not change in response to the environment. This length can influence physical potential. For example, a person with shorter tendons and longer muscles might have more potential for muscle mass. Conversely, athletes who jump or run often benefit from a longer Achilles tendon. This allows the tendon to act as a more effective spring.

Tendons are found in nearly every part of the body. In the shoulders, the rotator cuff tendons include the supraspinatus and infraspinatus. The biceps tendons help bend the elbow. In the legs, the Achilles tendon crosses the ankle to move the foot. The quadriceps tendons help straighten the knee. Even the eyes rely on ocular tendons for movement. While the internal bulk of a tendon lacks nerve fibers, the outer layers do not. The epitenon and paratenon contain nerve endings. Additionally, Golgi tendon organs are located where the tendon meets the muscle.