A Tendon injury is known as a tendinopathy (NOT tendinitis). It is a non-inflammatory degenerative condition,most commonly due to too much loading. Histologically (at cellular levels) it is associated with changes in the collagen bundles, the tenocytes & the ground substance (proteoglycans, aggrecan & decorin). The end stage is when the disroganised infiltration of new blood vessels into the holes of disrupted matrix is seen. This increased vascularity is not helping to heal, only to fill the holes.
The Achilles is the only tendon that can have a midportion tendinopathy. All other tendinopathies are at the tendon attachment to the bone.