The Ruffini endings and associated cells in the periodontal ligament of rat incisors were investigated by means of immunohistochemistry for glia-specific S-100 protein and electron microscopy. Numerous Ruffini endings, which were immunoreactive for S-100 protein as well as for neurofilament protein, were distributed in the alveolus-related part of the lingual periodontal ligament. In electron microscopy, the Ruffini endings displayed expanded axoplasmic spines filled with a large number of mitochondria and neurofilaments; some of the spines directly contacted the surrounding collagen fibers via fingerlike projections. The axoplasmic spines and Schwann sheath, for the most part, were covered alternately by single or multiple layers of the basal lamina.

Several rounded cells showing S-100 immunoreactivity occurred in the vicinity of the Ruffini endings. The rounded cells associated with Ruffini endings possessed a kidney-shaped nucleus and enveloped the axoplasmic spines with their cytoplasmic processes. From these morphological features, the cells in question were identified as the K-cells described by Everts et al. (1977). These K-cells developed Golgi apparatus and rough endoplasmic reticulum, suggesting active synthesis of proteins. Immunohistochemistry at the electron microscopic level revealed an intense immunoreactivity for S-100 protein in the cytoplasm of the K-cell and led to a conclusion that the K-cells were terminal Schwann cells associated with Ruffini endings, presumably corresponding to the lamellar cells in the inner bulb of sensory corpuscles.