Tendon Structure

Tendon is a hierarchical composite. Triple-helical collagen molecules assemble into banded fibrils, fibrils into fibres, fibres into fascicles bounded by endotenon, and fascicles into the tendon proper, wrapped by epitenon and then by a paratenon or, where the tendon turns a corner, a synovial sheath. Type I collagen dominates the dry mass, with decorin and other small proteoglycans regulating fibril diameter. A crimped waveform in the resting fibres produces the toe region of the load-elongation curve. Tenocytes lie in rows between fascicles, sparse and metabolically quiet, and the interfascicular matrix lets fascicles slide, which distinguishes energy-storing tendons such as the Achilles from positional ones.

Two measured properties explain most of tendon's clinical behaviour. It is poorly vascularised, with the Achilles supplied mainly through the paratenon and a hypovascular zone a few centimetres above its insertion. And its core turns over extraordinarily slowly: carbon-14 dating using the atmospheric bomb pulse indicated that collagen in the human Achilles core is essentially not replaced after adolescence, so the tissue an adult loads was laid down in childhood.

That combination sets expectations. A tissue with minimal perfusion and near-zero core turnover cannot remodel quickly, which is why tendon rehabilitation is measured in months and why systemic agents face a delivery problem before any pharmacology begins.

The error follows directly. Collagen synthesis measured by microdialysis around a tendon largely reflects peritendinous tissue, not core matrix, and dermal fibroblast data are routinely borrowed for tendon claims despite skin turning over on a completely different schedule. Ultrasound showing a thicker tendon after treatment is read as healing when thickening is equally a feature of tendinopathy.