Tendons are made primarily of water and collagen, a tough structural protein that gives them their remarkable strength. By dry weight, collagen accounts for 50 to 80 percent of a tendon’s mass, making it by far the dominant building material. The rest is a mix of specialized cells, smaller proteins that act as molecular spacers, and a gel-like ground substance that holds everything together.
Collagen: The Core Building Block
The collagen in tendons isn’t just one type. About 95 percent of it is type I collagen, the same variety found in bone and skin. This form arranges itself into long, tightly packed fibers that resist being pulled apart, which is exactly what tendons need to do every time a muscle contracts. The remaining roughly 5 percent is type III collagen, a thinner, more flexible form that plays a supporting role.
These collagen molecules don’t just float around randomly. They organize themselves into a precise hierarchy. Individual collagen molecules twist together into fibrils, fibrils bundle into fibers, fibers group into fascicles, and fascicles combine to form the complete tendon. Think of it like a rope: each level of bundling adds strength far beyond what any single strand could provide.
The Cells That Maintain It All
Living cells make up a small fraction of a tendon’s volume, but they’re essential. The primary residents are tenocytes, specialized cells that produce collagen and maintain the surrounding matrix. Tenocytes and their precursors (tendon stem and progenitor cells) account for 90 to 95 percent of all cells in a tendon. The remainder includes blood vessel cells, cartilage-like cells near the bone attachment point, and smooth muscle cells.
Tenocytes are relatively quiet cells in a healthy tendon. They slowly turn over old collagen and replace it with new fibers, keeping the tissue strong over time. When a tendon is injured, though, the repair process gets more complicated. Both the tendon’s own tenocytes and outside fibroblasts that migrate into the wound begin laying down type III collagen as a quick patch. Over weeks to months, this type III collagen is gradually replaced with the stronger type I collagen during a remodeling phase. If that process goes wrong, the result is disorganized scar tissue that never quite matches the original tendon’s strength.
The Gel Between the Fibers
Collagen fibers don’t work alone. They’re embedded in a gel-like ground substance made of water, proteoglycans, and glycoproteins. Two proteoglycans in particular, called decorin and biglycan, act as key regulators of how collagen fibrils assemble and how much space sits between them. Without these molecules, collagen fibrils shift toward abnormally large diameters and lose their uniform structure, which compromises the tendon’s mechanical properties.
Proteoglycans also attract and hold water within the tendon matrix. This water content is critical: it cushions the collagen fibers during loading, helps nutrients diffuse through the tissue, and allows the tendon to absorb shock. When proteoglycan levels drop (as they do with aging or overuse), the tendon becomes stiffer and more brittle.
Protective Wrapping Layers
A tendon isn’t just a bare cord of collagen. It’s wrapped in several layers of connective tissue, each with a distinct job:
- Endotenon: the innermost layer, surrounding individual fiber bundles and allowing them to glide against each other inside the tendon. It also carries blood vessels, lymphatic channels, and nerves deeper into the tissue.
- Epitenon: a thin sheath enclosing the entire tendon, holding all the fascicles together as a unit.
- Paratenon: a loose outer layer that lets the tendon slide smoothly against surrounding tissues during movement.
Some tendons, particularly in the hands and feet, have an additional synovial sheath that produces a lubricating fluid. This extra lubrication is necessary wherever tendons pass through tight spaces or wrap around bony pulleys, reducing friction that would otherwise wear the tissue down.
Why Tendons Have Poor Blood Supply
One of the most important things about tendon composition is what’s largely absent: blood vessels. Tendons are hypovascular, meaning they receive far less blood flow than muscle or skin. The small vessels that do exist enter through the surrounding connective tissue layers and through structures called vincula, small folds of tissue that deliver branches from nearby arteries.
Certain zones within a tendon are almost completely avascular. In the finger’s deep flexor tendon, for example, there is a known avascular zone along the palm-side surface at the level of the first finger bone. The Achilles tendon has a similarly vulnerable zone a few centimeters above its heel attachment. These low-blood-flow areas are precisely where tendon injuries and degenerative tears most commonly occur, because the tissue can’t deliver enough oxygen and nutrients to keep up with repair demands.
Where Tendon Meets Bone
The point where a tendon attaches to bone, called the enthesis, has its own specialized composition. Rather than a sudden transition from soft tissue to hard bone, the enthesis passes through four distinct zones: dense fibrous connective tissue (essentially the end of the tendon), uncalcified fibrocartilage, calcified fibrocartilage, and finally bone. Each zone is progressively stiffer than the last.
This gradient exists for a good mechanical reason. If a flexible tendon attached directly to rigid bone, stress would concentrate at that single junction and the tendon would tear away under load. The gradual shift in stiffness across the four zones distributes force over a wider area, protecting the attachment. Injuries at the enthesis (common in conditions like tennis elbow and Achilles tendinopathy) often involve a breakdown in one of these transitional zones rather than a clean tear.
How Composition Changes With Age and Injury
Tendon composition is not static. With aging, the water content of the matrix decreases, collagen cross-links become stiffer, and the ratio of type III to type I collagen can shift. Proteoglycan levels decline, further reducing the tissue’s ability to manage mechanical stress. The result is a tendon that’s less elastic and more prone to micro-damage with repetitive loading.
In tendinopathy, the chronic overuse condition that affects millions of people, the composition shifts even more dramatically. The normally organized collagen fibers become disorganized, proteoglycan content changes, and new blood vessels and nerve fibers grow into areas that were previously avascular. This ingrowth of nerves is one reason tendinopathy can be so painful, even though the tendon may not be visibly torn. The fundamental problem is a breakdown in the precise architecture that gives a healthy tendon its strength: the right collagen types, in the right arrangement, held together by the right molecular spacers, with just enough water to keep it all working smoothly.