Fig. 1.

Simplified cartoon model of Achilles tendon anatomical structure. Paratenon sheet surrounds the whole tendon, while endotenon sheets surround individual tendon fascicles. Scarce blood vessels and nerve fibres (not shown) are located within the sheets. Each fascicle is composed of collagen fibres with variable diameter size situated in proteoglycan-rich matrix, while each collagen fibre is composed of multiple collagen type I fibrils. In between fibres, tenocytes (terminally differentiated cells) are found. Tendon stem cells are suggested to be located in the tendon sheets, near the blood vessels or within the intra-fascicular matrix. Based on Docheva et al.⁴⁶