[Morphological basis on periodontal Ruffini endings].

The periodontal ligament has a rich sensory nerve supply which serves as a sensory apparatus in addition to tooth support. The periodontal ligament contains nociceptors and low-threshold mechanoreceptors. Stimuli applied to teeth evoke various oral reflexes, which make smooth mastication possible via the periodontal mechanoreceptors. Recent morphological and physiological studies have revealed that Ruffini endings, categorized as low-threshold slowly adapting type II (SA II), are essential mechanoreceptors in the periodontal ligament. The periodontal Ruffini endings are ultrastructurally characterized by expanded axon terminals filled with a number of mitochondria and terminal or lamellar Schwann cells. The axon terminals of the periodontal Ruffini endings have finger-like projections, i.e. axonal spines, extending into the surrounding tissue to detect the deformation of collagen fibers. As histochemical marker enzymes for the periodontal Ruffini endings, the axon terminals and terminal Schwann cells are reactive for cytochrome oxidase activity and both acid phosphatase activity and non-specific cholinesterase activity, respectively. Many experimental studies also have revealed that periodontal Ruffini endings have high potential for neuroplasticity, confirmed by intense immunoreactivity for p75-NGFR and GAP-43. Mechanical stimuli due to tooth eruption and occlusion might be a prerequisite for the differentiation and maturation of the periodontal Ruffini endings. Further investigations are needed for clarifying the involvement of growth factors and the molecular mechanism of the development and regeneration processes of the Ruffini endings.