Morphological variation and cell adhesion of terminal Schwann cells-- a study by three-dimensional microanatomy and immunohistochemistry.

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Morphology and development of terminal Schwann cells associated with various sensory endings were analyzed both by three-dimensional microanatomy based on combined observation by scanning and electron microscopy, and by immunohistochemistry utilizing an antibody against the Schwann cell marker, S100β protein. Comparative observation of periodontal Ruffini endings and vibrissal lanceolate endings in rats and Meissner corpuscles in monkey palmar skn showed that terminal Schwann cells in these mechanoreceptors consistently issued main, lamellar processes to envelope axon endings. The main processes were provided with fine accessory projections specific to receptor types in morphology. The accessory projections of the terminal Schwann cells linked the different mechanoreceptors with surrounding tissues in distinct manners, suggesting that tissue deformations induced by external forces are transmitted to receptor sites of the axon terminals via the fine anchors. Maturation of terminal Schwann cells was observed in vibrissal lanceolate endings of 2 to 3 week-old rats. Immature Schwann cells, which were stellate in shape with fine processes radiating from the cell body, occurred in connective tissue surrounding lanceolate endings. These cells were gradually transformed into terminal Schwann cells, extending main processes along axon terminals. In the process of maturation, the great majority of the fine radial processes of the Schwann cels regressed, while the remainders became the anchoring, accessory projections. These findings indicate that the morphogenesis of terminal Schwann cells is induced by contact with axon terminals. Signaling substances and cell adhesion molecules involved in the cell interaction remain to be elucidated.