Fenestration in the myelin sheath of nerve fibers of the shrimp: A novel node of excitation for saltatory conduction

Giant nerve fibers of the shrimp family Penaeidae conduct impulses at the velocity highest among all animal species (∼210 m/s; highest in mammals = 120 m/s). We examined these giant and other small nerve fibers morphologically using a differential interference contrast microscope as well as an electron microscope, and found a very specialized form of excitable membrane that functions as a node for saltatory conduction of the impulse. This node appeared under the light microscope as a characteristic pattern of concentrically aligned rings in a very small spot of the myelin sheath. The diameter of the innermost ring of the node was about 5 μm, and the distance between these nodes was as long as 12 mm. Via an electron microscope, these nodes were characterized by a complete lack of the myelin sheath, forming a fenestration that has a tight junction with an axonal membrane. Voltage clamp measurements by a sucrose gap technique demonstrated that the axonal membrane at these fenestration nodes is exclusively excitable and that the large submyelinic space is a unique conductive pathway for loop currents for saltatory conduction through such fenestration nodes. © 1996 John Wiley & Sons, Inc.     

Function of tetraspan proteins in the myelin sheath

During the past few years, significant advances have been made in elucidating the mechanisms by which point mutations and altered gene dosages in tetraspan genes cause neurological disease. In addition, several new myelin tetraspans have been identified that are involved in adhesion, molecular trafficking, growth regulation, and migration of oligodendrocytes and Schwann cells.     

Localization and irregular distribution of Na,K-ATPase in myelin sheath from rat sciatic nerve

Sodium, potassium adenosine triphosphatase (Na,K-ATPase) is a membrane-bound enzyme that maintains the Na(+) and K(+) gradients used in the nervous system for generation and transmission of bioelectricity. Recently, its activity has also been demonstrated during nerve regeneration. The present study was undertaken to investigate the ultrastructural localization and distribution of Na,K-ATPase in peripheral nerve fibers. Small blocks of the sciatic nerves of male Wistar rats weighing 250-300g were excised, divided into two groups, and incubated with and without substrate, the para-nitrophenyl phosphate (pNPP). The material was processed for transmission electron microscopy, and the ultra-thin sections were examined in a Philips CM 100 electron microscope. The deposits of reaction product were localized mainly on the axolemma, on axoplasmic profiles, and irregularly dispersed on the myelin sheath, but not in the unmyelinated axons. In the axonal membrane, the precipitates were regularly distributed on the cytoplasmic side. These results together with published data warrant further studies for the diagnosis and treatment of neuropathies with compromised Na,K-ATPase activity.     

Membranous bodies in the myelin sheath of Schwann cells

Electron microscopy of muscle biopsies of patients with paralytic scoliosis and tuberculosis of spines reveals membranous bodies in the myelin sheath of Schwann cells. On the basis of this morphological pattern and indirect biochemical evidence, it is suggested that these bodies were formed by abnormal myelin metabolism.In this communication we report the presence, in large numbers of membranous bodies in the myelin sheath of Schwann cells in patients with paralytic scoliosis and with tuberculosis of lower cervical and upper dorsal spines.     

Structural properties of proteins specific to the myelin sheath

Summary. The myelin sheath is an insulating membrane layer surrounding myelinated axons in vertebrates, which is formed when the plasma membrane of an oligodendrocyte or a Schwann cell wraps itself around the axon. A large fraction of the total protein in this membrane layer is comprised of only a small number of individual proteins, which have certain intriguing structural properties. The myelin proteins are implicated in a number of neurological diseases, including, for example, autoimmune diseases and peripheral neuropathies. In this review, the structural properties of a number of myelin-specific proteins are described. Author’s address: Dr. Petri Kursula, Department of Biochemistry, University of Oulu, FIN-90014 Oulu, Finland     

On the biogenesis of the myelin sheath: Cognate polarized trafficking pathways in oligodendrocytes

Oligodendrocytes, the myelinating cells of the central nervous system, are capable of transporting vast quantities of proteins and of lipids, in particular galactosphingolipids, to the myelin sheath. The sheath is continuous with the plasma membrane of the oligodendrocyte, but the composition of both membrane domains differs substantially. Given its high glycosphingolipid and cholesterol content the myelin sheath bears similarity to the lipid composition of the apical domain of a polarized cell. The question thus arises whether myelin components, like typical apical membrane proteins are transported by an apical-like trafficking mechanism to the sheath, involving a raft-mediated mechanism. Indeed, the evidence indicates the presence of cognate apical and basolateral pathways in oligodendrocytes. However, all major myelin proteins do not participate in this pathway, and remarkably apical-like trafficking seems to be restricted to the oligodendrocyte cell body. In this review, we summarize the evidence on the existence of different trafficking pathways in the oligodendrocyte, and discuss possible mechanisms separating the oligodendrocyte's membrane domains.     

On the biogenesis of the myelin sheath: cognate polarized trafficking pathways in oligodendrocytes

Oligodendrocytes, the myelinating cells of the central nervous system, are capable of transporting vast quantities of proteins and of lipids, in particular galactosphingolipids, to the myelin sheath. The sheath is continuous with the plasma membrane of the oligodendrocyte, but the composition of both membrane domains differs substantially. Given its high glycosphingolipid and cholesterol content the myelin sheath bears similarity to the lipid composition of the apical domain of a polarized cell. The question thus arises whether myelin components, like typical apical membrane proteins are transported by an apical-like trafficking mechanism to the sheath, involving a 'raft'-mediated mechanism. Indeed, the evidence indicates the presence of cognate apical and basolateral pathways in oligodendrocytes. However, all major myelin proteins do not participate in this pathway, and remarkably apical-like trafficking seems to be restricted to the oligodendrocyte cell body. In this review, we summarize the evidence on the existence of different trafficking pathways in the oligodendrocyte, and discuss possible mechanisms separating the oligodendrocyte's membrane domains.     

Phagocytosis of degenerating myelin in transected feline optic nerve: an immunohistochemical study

The phagocytic activity of neuroglial cells in adult feline degenerating optic nerve was investigated by immunocytochemistry at both light and electron microscopy levels. Degeneration was initiated by unilateral eye enucleation and the segment distal to the transection showing true Wallerian degeneration was examined. Following enucleation, twelve adult domestic cats were examined over a period of seven to 215 days. All cases showed slow clearance of myelin debris and absence of proliferating monocytes throughout the post-enucleation period. All phagocytic cells present were neuroglial cells, and many of these cells expressed oligodendroglial antigens. These findings demonstrate the persistence of an active population of oligodendrocytes that might play an additional functional role during Wallerian degeneration of feline optic nerve.     

Phagocytosis of degenerating myelin in transected feline optic nerve: an immunohistochemical study

The phagocytic activity of neuroglial cells in adult feline degenerating optic nerve was investigated by immunocytochemistry at both light and electron microscopy levels. Degeneration was initiated by unilateral eye enucleation and the segment distal to the transection showing true Wallerian degeneration was examined. Following enucleation, twelve adult domestic cats were examined over a period of seven to 215 days. All cases showed slow clearance of myelin debris and absence of proliferating monocytes throughout the post-enucleation period. All phagocytic cells present were neuroglial cells, and many of these cells expressed oligodendroglial antigens. These findings demonstrate the persistence of an active population of oligodendrocytes that might play an additional functional role during Wallerian degeneration of feline optic nerve.