Sympathetic innervation of the suprasesamoidean region of the deep digital flexor tendon in the forelimbs of horses

• The sympathetic innervation of the normal equine DDFT in the foot was investigated.
• Most of the sympathetic innervation was in the walls of blood vessels.
• There is evidence of catecholamine production and functional response in tenocytes.
• The sympathetic innervation might contribute to an autocrine/paracrine system.

The purpose of this study was to delineate the pattern of sympathetic innervation in the suprasesamoidean region of the deep digital flexor tendon (DDFT) in horses using immunohistochemical staining (IHC) for tyrosine hydroxylase (TH) and alpha-1 adrenergic receptor (α1-AR). Fourteen forelimbs were collected from 10 horses. Longitudinal sections of the suprasesamoidean region of healthy DDFTs were harvested. Most of the sympathetic innervation was found to be in the walls of blood vessels. The tendon tissue proper was sparsely innervated, with a lesser degree of innervation within the dorsal fibrocartilage. Increased α1-AR immunostaining was also detected in walls of blood vessels and in spindle cells of fibrocartilage. Both α1-AR and TH immunostaining were detected in tenocytes. These findings support the presence of autocrine/paracrine catecholaminergic signalling in equine tendon tissue.