PHASE A~D ELECTRON MICROSCOPIC STUDIES OF EXPERIMENTAL DEMYELINATION I. VARIATIONS IN MYELIN SHEATH CoNTOUR IN NoRMAL GUINEA PIG SciATIC

During an electron microscopic study of experimental demyelination, many variations in the contour of the myelin sheath were observed in normal guinea pig sciatic nerves. These variations consisted of loops and folds of compact myelin which indented the axoplasm or protruded into the Schwann cell cytoplasm. Depending on the plane of section they were seen as isolated ovoids of myelin within axoplasm or Schwann cell cytoplasm. Under the light microscope, these loops and ovoids closely resembled myelin forms seen in early demyelinative lesions and it was necessary to define their structure and distribution before proceeding with the pathological study. Variations in myelin sheath contour are not described in the classical histological studies of Ranvier (16), Nageotte (14), and Cajal (1). In their figures, the myelin sheath is shown as a smooth, axoplasm filled, cylindrical tube with interruptions at Schmidt-Lantermann incisures and the nodes of Ranvier. A slight indentation is present in the region of the nucleus. Juxtanodal irregularities of the myelin sheath are illustrated as preparative artifacts although other observers (12, 2) described round, osmophilic structures located between the myelin sheath and the Schwann cell membrane near the nucleus and elsewhere along the length of normal peripheral nerve fibers. Ridging of the myelin sheath adjacent to the node was noted by several investigators including Hess and Young (ll); one of their figures also shows a large fold of myelin indenting axoplasm near a node shown in longitudinal section, but it was not described or discussed. A few electron micrographs in several subsequent studies of peripheral nerve ultrastructure show similar myelin loops and folds (7, 18). However, their fine structure and distribution were not discussed. Hess and Lansing described an isolated ovoid with the lamellar structure of myelin within the Schwann cell cytoplasm of guinea pig sciatic nerve and tentatively identified it as an Elzholz