Tendons from non-diabetic humans and rats harbor a population of insulin-producing, pancreatic beta cell-like cells.

Diabetes mellitus is a risk factor for various types of tendon disorders. The mechanisms underlying diabetes associated tendinopathies remain unclear, but typically, systemic factors related to high blood glucose levels are thought to be causally involved. We hypothesize that tendon immanent cells might be directly involved in diabetic tendinopathy. We therefore analyzed human and rat tendons by immunohistochemistry, laser capture microdissection, and single cell PCR for pancreatic ß-cell associated markers. Moreover, we examined the short term effects of a single injection of streptozotocin, a toxin for GLUT2 expressing cells, in rats on insulin expression of tendon cells, and on the biomechanical properties of Achilles tendons. Tendon cells, both in the perivascular area and in the dense collagenous tissue express insulin and Glut2 on both protein and mRNA levels. In addition, glucagon and PDX-1 are present in tendon cells. Intraperitoneal injection of streptozotocin caused a loss of insulin and insulin mRNA in rat Achilles tendons after only 5 days, accompanied by a 40% reduction of mechanical strength. In summary, a so far unrecognized, extrapancreatic, insulin-producing cell type, possibly playing a major role in the pathophysiology of diabetic tendinopathy is described. In view of these data, novel strategies in tendon repair may be considered. The potential of the described cells as a tool for treating diabetes needs to be addressed by further studies.