Atypical Neural Sheaths Formed by M^|^uuml;ller Cells in Chicken Retina

Abstract

N chicken optic nerve the intraorbital part was found to consist of bundles of myelinated nerve fibers, the myelin sheaths of which were formed by oligodendroglia. At the optic nerve head, only thick nerve fibers were myelinated. When the thick nerve fibers were followed into the optic nerve fiber layer of the retina, they were found to lose the typical myelin sheaths and become newly enclosed by atypical sheaths. The atypical sheaths were formed by Müller cells, and were found to consist of a single membranous structure which spirally wrapped up thick axons. This membranous structure was formed by fusion of the cytoplasmic faces of the plasma membrane of Müller cells as a result of loss of cytoplasm, like the formation of the periodic lines of typical myelin. However, the outer faces of the plasma membrane rarely fused with each other, so that intraperiodic lines were absent.