Vascularization of the hoof joint synovial membrane

The vascular supply of the synovial membrane was studied in the dorsal recessus of the hoof joint. This region is supplied by two arteries originating from an arterial ring that is formed by anastomosing dorsal branches out from the medial and lateral digital palmar artery at the level of the short pastern bone. The two arteries feed a vascular plexus in the border region between synovial membrane and fibrous layer of the joint capsule. Delicate arteries branch out from this plexus and run towards synovial folds and villi, there supplying a convolute of helical, tortuous and glomerular vessels (luminal diameter 8-10 μm). The intricate threedimensional arrangement of these vessels is shown in corrosion cast preparations. In tissue sections, corresponding vascular profiles are defined by two or three endothelial cells. Adventitial cells strongly stain for actin. Therefore, these vessels are classified as venules. The venules merge and form draining veins which empty into accompanying veins of the vascular plexus in the border region between synovial membrane and fibrous joint capsule. At this level, in addition, arteriovenous anastomoses (Hoyer-Grosser organs) were identified. The remarkably coiled, tortuous or glomerular venules within the synovial membrane most probably serve as a target for sensory neuropeptides (substance P, CGRP) to be released from nociceptive nerve terminals. Arteriovenous anastomoses are apt to control perfusion of these venules and consequently control production of synovial fluid.