Gs. Molyneux et al., THE STRUCTURE, INNERVATION AND LOCATION OF ARTERIOVENOUS ANASTOMOSES IN THE EQUINE FOOT, Equine veterinary journal, 26(4), 1994, pp. 305-312
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.