THE STRUCTURE, INNERVATION AND LOCATION OF ARTERIOVENOUS ANASTOMOSES IN THE EQUINE FOOT

In the foot of the horse, arteriovenous anastomoses (AVAs) of epitheli oid type occurred in the dermis of the coronary band, in the coronary and terminal papillae, in neurovascular bundles and at the entrance to and along the length of the dermal laminae. A particular feature of t he epithelioid segment of AVAs in the horse, compared with that of oth er species, was the height and surface complexity of many of the endot helial cells. They extended into the lumen, forming undercut and tunne l-like areas which correlated with the characteristic surface marking of AVAs observed in vascular casts. The number of cell organelles, inc luding the concentration of vesicles in the luminal cytoplasm, suggest ed cells with a high metabolic activity. The luminal surface possessed numerous microvilli and long cytoplasmic cell processes which appeare d to surround material in the lumen. The innervation of AVAs was more dense than that of the arteries and consisted of adrenergic and peptid ergic nerves. Noradrenaline- and neuropeptide Y-containing nerves were identified as the vasoconstrictor components of the nerve supply and occurred along arteries and formed dense plexuses around AVAs. Calcito nin gene-related peptide, substance P and vasoactive intestinal polype ptide are vasodilators and were present in single nerve fibres which a ccompanied arteries and AVAs along the length or the dermal laminae. I n this study the distribution, density and innervation of AVAs in the equine foot are correlated with their proposed role in the development of acute laminitis. The release of vasoactive peptides from diseased organs remote from the foot may induce inappropriate prolonged dilatat ion of AVAs and thus contribute to the laminar ischaemia of acute lami nitis.