Sensory receptors in the periodontal ligament of hamster incisors with special reference to the distribution, ultrastructure and three-dimensional reconstruction of Ruffini endings.

The innervation of the periodontal ligament in hamster incisors was investigated by means of immunohistochemistry for nervous-specific proteins and electron microscopy. The lingual periodontal ligament was found to be exclusively innervated by Ruffini endings which appeared to be most developed in this species among rodents; the labial periodontal ligament lacked them. The Ruffini endings occupied the alveolar half of the periodontal ligament, being intertwined with transverse collagen fibers. In electron microscopy, the Ruffini endings displayed expanded axon terminals filled with large-sized mitochondria. Three-dimensional reconstructions of the Ruffini endings at the electron microscopic level revealed complicated shapes for the axon terminals and a characteristic relationship with the associated terminal Schwann cells. The axon terminals were plate- or knob-shaped, the former being predominant. Each axon terminal was covered by thick Schwann sheaths derived from more than two terminal Schwann cells whose cell bodies were located apart from the axon terminals and contained a developed Golgi apparatus and rough endoplasmic reticulum. On the other hand, each terminal Schwann cell simultaneously extended their cytoplasmic processes to several axon terminals just like astrocytes. The thick Schwann sheath, for the most part, was covered by a multiple layer of basal lamina. These findings have aided us in understanding the entire structure of the periodontal Ruffini endings.