The correlation between vascular distribution and synovial structures was investigated using corrosion cast of the Synovial fold of the dog antebrachiocarpal joint (wrist joint) under scanning electron microscopy. Arterial branches arose from a main artery deep in the subsynovial layer, ascended toward the free margin of the fold, dividing into several branches, and finally formed a dense capillary network beneath the lining layer. Immunohistochemical study and a retrograde axonal tracing experiment revealed that almost all nerve fibers were associated with the arterial tree and that:(1) proximal parts of the arterial tree were innervated with neuropeptide Y (NPY)-containing noradrenergic sympathetic fibers, vasoactive intestinal peptidecontaining non-catecholaminergic sympathetic fibers from the stellate ganglion and substance P (SP)-and calcitonin gene-related peptide (C GRP )-containing sensory fibers from the C7-T1 segments of the dorsal root ganglia; (2) more distal parts of the arterial branches were associated with NPY-immunoreactive sympathetic fibers and SP-and C GRP-immunoreative sensory fibers; and (3) the most peripheral precapillary arterioles were accompanied by only SP-and CGRP-containing sensory fibers. These results indicate that synovial blood flow is regulated by at least three different nerve systems which possess regional differences.

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