scholarly journals THE ULTRASTRUCTURE OF ADULT VERTEBRATE PERIPHERAL MYELINATED NERVE FIBERS IN RELATION TO MYELINOGENESIS

1955 ◽  
Vol 1 (4) ◽  
pp. 271-278 ◽  
Author(s):  
J. David Robertson
Keyword(s):  
Electron Microscope ◽  
Schwann Cell ◽  
Cell Surface ◽  
Peripheral Nerve ◽  
Myelin Sheath ◽  
Nerve Fibers ◽  
Myelinated Nerve ◽  
Thin Sections ◽  
Double Membrane ◽  
Myelinated Nerve Fibers

Adult chameleon myelinated peripheral nerve fibers have been studied with the electron microscope in thin sections. The outer lamella of the myelin sheath has been found to be connected as a double membrane to the surface of the Schwann cell. The inner lamella is connected as a similar double membrane with the double axon-Schwann membrane. The relations of these double connecting membranes suggest that the layered myelin structure is composed of a double membrane which is closely wound about the axon as a helix. These findings support the new theory of myelinogenesis proposed recently by Geren. The possible significance of these results with respect to cell surface membranes and cytoplasmic double membranes is discussed.

scholarly journals THE SUBMICROSCOPIC ORGANIZATION OF AXON MATERIAL ISOLATED FROM MYELIN NERVE FIBERS

1953 ◽  
Vol 98 (3) ◽  
pp. 269-276 ◽  
Author(s):  
E. De Robertis ◽  
C. M. Franchi
Keyword(s):  
Electron Microscopy ◽  
Electron Microscope ◽  
Phase Contrast ◽  
Nerve Fibers ◽  
Myelinated Nerve ◽  
Myelinated Nerve Fibers ◽  
Dense Granules ◽  
Small Clusters ◽  
Membranous Material ◽  
The Way

A technique has been developed for the extrusion of axon material from myelinated nerve fibers. This material is then compressed and prepared for observation with the electron microscope. All the stages of preparation and purification of the axon material can be checked microscopically and in the present paper they are illustrated with phase contrast photomicrographs. Observation with the electron microscope of the compressed axons showed the presence of the following components: granules, fibrils, and a membranous material. Only the larger granules could be seen with the ordinary microscope. A considerable number of dense granules were observed. Of these the largest resemble typical mitochondria of 250 mµ by 900 mµ. In addition rows or small clusters of dense granules ranging in diameter from 250 to 90 mµ were present. In several specimens fragments of a membrane 120 to 140 A thick and intimately connected with the axon were found. The entire axon appeared to be constituted of a large bundle of parallel tightly packed fibrils among which the granules are interspersed. The fibrils are of indefinite length and generally smooth. They are rather labile structures, less resistant in the rat than in the toad nerve. They varied between 100 and 400 A in diameter and in some cases disintegrated into very fine filaments (less than 100 A thick). The significance is discussed of the submicroscopic structures revealed by electron microscopy of the material prepared in the way described.

scholarly journals A TIME-SEQUENCE AUTORADIOGRAPHIC STUDY OF THE IN VIVO INCORPORATION OF [1, 2-3H]CHOLESTEROL INTO PERIPHERAL NERVE MYELIN

1973 ◽  
Vol 58 (1) ◽  
pp. 42-53 ◽  
Author(s):  
Frank A. Rawlins
Keyword(s):  
Schwann Cell ◽  
Peripheral Nerve ◽  
Myelin Sheath ◽  
Central Zone ◽  
Autoradiographic Study ◽  
Nerve Fibers ◽  
Time Sequence ◽  
Time Interval ◽  
Electron Microscope Autoradiography

A time-sequence study of the incorporation and distribution of cholesterol in peripheral nerve myelin was carried out by electron microscope autoradiography. [1,2-3H]Cholesterol was injected into 10-day old mice and the sciatic nerves were dissected out at 10, 20, 40, 60, 90, 120, and 180 min after the injection. 20 min after injection the higher densities of grains due to the presence of [3H]cholesterol were confined to the outer and inner edges of the myelin sheath. Practically no cholesterol was detected in the midzone of the myelin sheath. 1 ½ h after injection, cholesterol showed a wider distribution within the myelin sheath, the higher densities of grains occurring over the two peripheral myelin bands, each approximately 3,100 Å wide. Cholesterol was also present in the center of the myelin sheath but to a considerably lesser extent. 3 h after injection cholesterol appeared homogeneously distributed within the myelin sheath. Schwann cell and axon compartments were also labeled at each time interval studied beginning 20 min postinjection. These observations indicate that preformed cholesterol enters myelin first and almost simultaneously through the inner and outer edges of the sheath; only after 90 min does the density of labeled cholesterol in the central zone of myelin reach the same density as that in the outer and inner zones. These findings suggest that cholesterol used by the nerve fibers in the formation and maintenance of the myelin sheath enters the lamellae from the Schwann cell cytoplasm and from the axon. The possibility of a bidirectional movement of molecules, i.e. from the Schwann cell to the axon and from the axon to the Schwann cell through the myelin sheath, is noted. The results are discussed in the light of recent observations on the exchange, reutilization, and transaxonal movement of cholesterol.

The fine structure and morphological organization of non-myelinated nerve fibres

1956 ◽  
Vol 144 (917) ◽  
pp. 496-506 ◽  
Keyword(s):  
Fine Structure ◽  
Schwann Cell ◽  
Peripheral Nerve ◽  
Schwann Cells ◽  
Myelin Sheath ◽  
Peripheral Nerves ◽  
Nerve Fibres ◽  
Myelinated Nerve ◽  
Morphological Organization ◽  
Myelinated Fibres

The fine structure and morphological organization of non-myelinated nerve fibres were studied by ultra-thin sectioning and electron microscopy in peripheral nerves, autonomic nerves and dorsal roots. Several non-myelinated fibres share the cytoplasm of a Schwann cell. The Schwann cells of non-myelinated fibres form a syncytium. The fibres are incompletely sur­rounded by Schwann cell cytoplasm and are suspended in the cytoplasm by mesaxons formed by the plasma membranes of the Schwann cell. The various relationships of mesaxon and nerve fibre are described. Non-myelinated fibres which do not share a Schwann cell are seen very frequently in the sciatic nerve of a new-born mouse but become less common as myelination proceeds and are rare in adults. It is therefore suggested that in developing peripheral nerves, the non­ myelinated fibres that are destined to myelinate are not organized into groups within a single Schwann cell, even before their myelin sheath has appeared; they are, at least for the ages examined here, individuals in relation to a surrounding individual Schwann cell. It is also suggested that the non-myelinated fibres that will never acquire a myelin sheath are organized in a developing peripheral nerve in the same manner as in the adult nerve—several fibres sharing a single Schwann cell that is part of a syncytial system of Schwann cells. Thus, in a developing peripheral nerve, it appears that two types of non-myelinated fibres are present—one destined to myelinate and lying alone in its own Schwann cell and the other, destined to remain unmyelinated and sharing, along with other non-myelinated fibres of the same type, a Schwann cell. The significance of these observations is discussed in relation to the development of nerve fibres and possible physiological importance.

Membrane Specializations in Developing Nodes of Ranvier

1980 ◽  
Vol 38 ◽  
pp. 626-627
Author(s):  
J.H. Tao-Cheng ◽  
J. Rosenbluth
Keyword(s):  
Schwann Cell ◽  
Freeze Fracture ◽  
Structural Features ◽  
Nerve Fibers ◽  
Nodes Of Ranvier ◽  
Myelinated Nerve ◽  
Thin Sections ◽  
Frog Tadpole ◽  
Cell Layers ◽  
Mature Node

Mature myelinated nerve fibers exhibit distinctive structural features at nodes of Ranvier and the adjacent paranodal regions. In order to obtain information about the interrelationships between these specializations during development, thin sections and freeze-fracture replicas of immature peripheral nerve fibers from grass frog tadpole hind legs were examined during the period of myelinogenesis. Early in myelination axons are enwrapped individually by a few loose Schwann cell layers whose edges overhang each other forming "terminal loops" against the axolemma. Unlike those of the mature node, these loops are widely sepa-rated and irregularly spaced (Fig.l), and similarly the presumptive nodal region between successive developing myelin segments is usually much longer than adult nodes of Ranvier. The presumptive nodal axolemma may exhibit a cytoplasmic "undercoating." However, the density of this coating is highly variable. Usually it is much lower than at adult nodes, and in some cases the undercoating is not distinguishable. The outermost layers of the Schwann cell are usually the first to form axoglial junctional specializations character¬ized by the presence of "transverse bands" and ER cisternae applied to the junctional Schwann cell membrane. In some instances the outermost layer con¬tacts the axon over an extensive area and forms multiple small junctional specializations at widely separated intervals along the length of the axolemma.

scholarly journals ELECTRON MICROSCOPE STUDIES OF THE FORMATION OF NODES OF RANVIER IN MOUSE SCIATIC NERVES

1957 ◽  
Vol 3 (4) ◽  
pp. 589-598 ◽  
Author(s):  
Betty Geren Uzman ◽  
Genevieve Nogueira-Graf
Keyword(s):  
Electron Microscope ◽  
Schwann Cell ◽  
Cell Surface ◽  
Myelin Sheath ◽  
Three Dimensions ◽  
Nodes Of Ranvier ◽  
Internodal Length ◽  
Intimate Contact ◽  
General Picture ◽  
Sciatic Nerves

Observations with the electron microscope of longitudinal sections of the sciatic nerves of infant mice during the period of early myelin formation are described. These observations are interpreted in relation to previous studies of transverse sections, and a general picture of the formation of an internodal length of the myelin sheath in three dimensions is formulated. In general, an internodal length of myelin sheath is attained by the spiral wrapping of the infolded Schwann cell surface; the increase in length of the internode during maturation is at least partially explained by the increased length of axon covered by the overlapping of successive layers during the wrapping of the infolded Schwann cell surface; and the nodes of Ranvier refer to the structure complex at the junctions of adjacent non-syncytial Schwann cells. The fact that the mode of formation of myelin brings each of its layers into intimate contact with the axon surface at the nodes is emphasized because of the possible functional significance of this arrangement. The manner of origin of Schmidt-Lantermann clefts remains obscure. Certain isolated observations provide evidence for the possibility that occasional internodes of myelin may form from several small segments of myelin within a single Schwann cell.

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