A TOPOGRAPHICAL AND ULTRASTRUCTURAL-STUDY OF SENSORY TRIGEMINAL NERVE-ENDINGS IN THE RAT TEMPOROMANDIBULAR-JOINT AS DEMONSTRATED BY ANTEROGRADE TRANSPORT OF WHEAT-GERM AGGLUTININ-HORSERADISH PEROXIDASE (WGA-HRP)

To extend our previous light microscopic observations concerning the d istribution of trigeminal sensory nerves in the synovium of the rat te mporomandibular joint, we investigated the detailed distribution and f ine structure of sensory nerve endings at the light and electron micro scopic level by the anterograde transport method using wheat germ aggl utinin-horseradish peroxidase (WGA-HRP) injected into the trigeminal g anglion. At the light microscopic level, HRP-labeled nerve fibers were observed in the joint capsule and peripheral portion of the disc. The anterior portion of the disc was more densely innervated than the pos terior portion, while no nerves were found in the central portion. At the electron microscopic level, HRP reaction products were observed in tra-axonally in the thinly myelinated (A delta) and unmyelinated (C) a xons in the anterior portion of the joint capsule, and were also local ized in the extracellular space surrounding the unmyelinated fibers an d terminals. In the subsynovial layer of the synovial membrane, the ma jority of labeled axons located near blood vessels or among the collag enous fibrils were covered by Schwann cell sheaths, although some nake d axon terminals without sheaths were also found. These unsheathed ter minals contained mitochondria, small clear vesicles, and large granula r vesicles, and were close to the synovial A and/or B cells near the j oint cavity. The minimum distance between the terminals and synovial c ells was 75 nm. This is the first demonstration of trigeminal sensory nerve terminals close to synovial lining cells or joint cavity and sug gests that neuropeptides such as substance P may be released close to the synovial lining cells or joint cavity.