SCANNING AND TRANSMISSION ELECTRON-MICROSCOPY OF RUFFINI ENDINGS IN THE PERIODONTAL-LIGAMENT OF RAT INCISORS

The Ruffini organ is an arborized axon ending categorized as a low-thr eshold stretch receptor. We have previously shown that the lingual per iodontal ligament of rat incisors is densely innervated with Ruffini e ndings. In the present study, fine structures in the surface of the pe riodontal Ruffini endings and their topographical relationship with th e surrounding collagen fibers were observed by a combination of scanni ng and transmission electron microscopy to analyze the mechanism of th e stretch reception. The entire length of the branches of the Ruffini endings, excepting their terminal portions, corresponded well with tho se depicted by previous investigators in the following points: (1) the ir cylindrical appearance covered by Schwann cell processes; (2) the p resence of numerous axon fingers protruding through gaps in the Schwan n sheath and; (3) their isolation from collagen fibers by multilayered basal lamina. On the other hand, tips of the axon branches-together w ith their Schwann sheaths-became attenuated and projected into tight b undles of collagen, indicating their susceptibility to mechanical defo rmations of the surrounding tissue. Margins of the axon terminals were conspicuously ruffled with long tongue-like projections of Schwann ce lls. The Schwann cell tongues twined around collagen bundles in their distal portions, and associated closely with fine axon projections in their proximal portions, suggesting their involvement in the mechanica l transmission of stimuli to axon terminals. (C) 1997 Wiley-Liss, Inc.