Electron microscopic observations of the nucleus, glial dome, and meninges of the rat acoustic nerve

Abstract

Abstract The rat acoustic nerve is separated into central and peripheral portions by an astrocytic glial dome which is convex peripheralward. The long central portion is of typical central nervous system structure with narrow extracellular space (100–200 Å in width), oligodendrocytes and astrocytes. The glial dome is penetrated by acoustic nerve fibers at a node of Ranvier; the basal lamina of the astrocytes is reflected back over the peripheral Schwann cells at this site. Centrally, the myelin is thinner than peripherally. Acoustic nerve neurons, ranging in size from 25–60 μ, occur in the central portion of the nerve and may be divided into two groups based upon size and density of organelles: large and medium‐sized. All the neurons possess an eccentric nucleus and a peripheral clear zone in the perikaryon beneath which Nissl substance is aggregated, but the medium‐sized neurons have fewer organelles than the large cells. Dendrites and axons are similar in ultrastructure. While collagenous fibrils, fibroblasts, Schwann cells and extensive extracellular space occur in the peripheral portion of the nerve, no structure corresponding to perineurium or epineurium exists. Instead, dura mater surrounds the acoustic nerve within the modiolus and the pia mater encloses bundles of nerve fibers up to the modiolar foramina where it is reflected back as arachnoid mater. Nerve fibers traversing the modiolar foramina are devoid of a meningeal or perineurial covering; this condition also prevails in the osseous spiral lamina, although wisps of pia‐like cells enclose groups of ganglion cells and nerve fibers in the spiral tract. These findings may help to explain acoustic nerve involvement in pathological processes such as meningitis and encephalitis.