scholarly journals The Structure of Myelin Sheaths in the Central Nervous System of Xenopus laevis (Daudin)

1960 ◽  
Vol 7 (1) ◽  
pp. 121-126 ◽  
Author(s):  
A. Peters
Keyword(s):  
Xenopus Laevis ◽  
Myelin Sheath ◽  
Cytoplasmic Process ◽  
Nerve Fibres ◽  
Myelinated Nerve ◽  
Outer Line ◽  
Myelin Sheaths ◽  
Dense Line ◽  
Central Myelin ◽  
The Central Nervous System

The structure of myelinated nerve fibres has been studied in the spinal cord and optic nerve of the tadpoles of Xenopus laevis. Potassium permanganate-fixed material was examined with the electron microscope. The myelin sheath itself is made up of spirally arranged lamellae in which the intraperiod and dense lines alternate. Inside the myelin sheath an inner cytoplasmic process surrounds the axon and where the external surfaces of its bounding membrane come together an internal mesaxon is formed. The intraperiod line begins within the mesaxon and the dense line usually begins in the same region by apposition of the cytoplasmic surfaces of the membrane. The width of each lamella is 140 A. The outer line in the sheath is the dense line and this terminates in a tongue where the cytoplasmic surfaces of the myelin-forming glial cell separate. Thus, central myelin in Xenopus tadpoles is arranged in the same way as peripheral myelin, the only difference being that in central fibres, cytoplasm on the outside of the sheath is confined to that present in the tongue. For this reason adjacent central sheaths come into apposition without any intervening material being present. When this occurs an intraperiod line is formed between them.

Observations on the Chemical Composition of Myelin and the Smallest Size of Myelinated Nerve-Fibres in the Central Nervous System

1948 ◽  
Vol s3-89 (5) ◽  
pp. 89-102
Author(s):  
A. BRODAL ◽  
R. G. HARRISON
Keyword(s):  
Central Nervous System ◽  
Nervous System ◽  
Chemical Composition ◽  
Nile Blue ◽  
Nerve Fibres ◽  
Myelinated Nerve ◽  
Myelin Sheaths ◽  
The Central Nervous System

Baker's (1946) acid haematein and pyridine-extraction control tests, claimed to be specific for phospholipines (Baker, 1947), have been applied to various parts of the central nervous system of rats and man. The sudan black method for the detection of lipoids and the nile blue method for the staining of acidic lipoids have also been used. The findings are in agreement with older statements in the literature that myelin contains a considerable amount of phospholipines. It was impossible to determine whether galactolipines or neutral lipoids are also present. In the acid haematein-stained sections finer fibres were seen than when other stains for myelin sheaths are employed. Fibres with a diameter of 0.5 µ or even somewhat less were stained in various parts of the central nervous system of rats. It is regarded as probable from these findings that fibres down to 0.5 µ or even smaller possess a lipoid investment. These observations lend support to the now commonly accepted view that the distinction between myelinated and so-called unmyelinated fibres is arbitrary. Some observations are made, however, which indicate that the presence of truly unmyelinated fibres cannot be excluded.

scholarly journals The thickness of the myelin sheaths of normal and regenerating peripheral nerve fibres

1948 ◽  
Vol 135 (880) ◽  
pp. 323-357 ◽  
Keyword(s):  
Myelin Sheath ◽  
Fibre Diameter ◽  
Distal Stump ◽  
Axon Diameter ◽  
Sheath Thickness ◽  
Nerve Fibres ◽  
Myelinated Nerve ◽  
Absolute Increase ◽  
Myelin Sheaths ◽  
Myelin Sheath Thickness

An investigation has been made of the relation between axon diameter, fibre diameter, and myelin sheath thickness in myelinated nerve fibres from the peroneal nerve of the rabbit. Fibres were measured: ( a ) in the normal nerve, and ( b ) during regeneration following nerve crushing. In the normal nerve, fibres ranging in diameter from 1 to 20 μ , were present. These had axons with diameters from 0·5 μ in the smallest, to about 15 μ in the largest fibres. The 0·5 μ axons had myelin sheaths about 0·5 μ thick, while the largest axons had sheaths about 2·5 μ thick. Between these extremes the curve relating sheath thickness to axon diameter was at first steep, and then more gradual in slope. In regenerating nerve fibres 15 mm. proximal to the site of lesion the axons decline steadily in diameter during the first period of regeneration (100 days). Thereafter they increase in size, although even after 300 days the largest axons have not regained their normal diameter. The axons, 10 mm. below the lesion, extremely thin at their first appearance, increase steadily in diameter, until, 300 days after injury they equal in diameter the axons of the proximal stump. The decrease in diameter of axons proximally is accompanied by an absolute increase in their myelin sheath thickness. This takes place first of all without any alteration in the total diameter of the fibres concerned, the increase in thickness of the myelin compensating the decrease in axon diameter. Between 60 and 100 days after injury, however, continuing axon decrease is accompanied by a decrease in the total diameter of the fibres. Between 100 and 200 days an increase in total fibre diameter parallels the increase in axon diameter, the myelin sheaths remaining abnormally thick. Between 200 and 300 days after injury the continued increase in axon diameter causes a diminution in myelin sheath thickness. Even at 300 days after injury, however, the sheaths are still thicker than in normal nerve. In the distal stump the myelin sheaths, when first developed, are thicker than those of fibres of corresponding diameter in normal nerve. Subsequently they grow still thicker, reaching their maximum thickness about 200 days after nerve injury. Between 200 and 300 days a slight diminution in sheath thickness occurs since the axons continue to increase while the total diameter remains the same. However, even at 300 days the sheaths are still thicker than normal. The bearing of these results on the nature of the forces maintaining the structure of nerve fibres is discussed.

scholarly journals A STRUCTURAL ANALYSIS OF THE MYELIN SHEATH IN THE CENTRAL NERVOUS SYSTEM

1967 ◽  
Vol 34 (2) ◽  
pp. 555-567 ◽  
Author(s):  
Asao Hirano ◽  
Herbert M. Dembitzer
Keyword(s):  
Central Nervous System ◽  
Nervous System ◽  
Electron Microscope ◽  
Myelin Sheath ◽  
Cerebral White Matter ◽  
Experimental Conditions ◽  
Myelin Sheaths ◽  
Basic Theme ◽  
Cell Processes ◽  
The Central Nervous System

The cerebral white matter of rats subjected to a variety of noxious experimental conditions was examined in the electron microscope. Several unusual configurations of the myelin sheath are identified in addition to the usual configuration. These variations include the presence of (a) formed organelles within the inner and outer loops, (b) isolated islands of cytoplasm in unfused portions of the major dense lines, (c) apparently unconnected cell processes between the sheath and the axon, and (d) concentric, double myelin sheaths. A generalized model of the myelin sheath based on a hypothetical unrolling of the sheath is described. It consists of a shovel-shaped myelin sheet surrounded by a continuous thickened rim of cytoplasm. Most of the unusual myelin configurations are explained as simple variations on this basic theme. With the help of this model, an explanation of the formation of the myelin sheath is offered. This explanation involves the concept that myelin formation can occur at all cytoplasmic areas adjacent to the myelin proper and that adjacent myelin lamellae can move in relation to each other.

scholarly journals THE FORMATION AND STRUCTURE OF MYELIN SHEATHS IN THE CENTRAL NERVOUS SYSTEM

1960 ◽  
Vol 8 (2) ◽  
pp. 431-446 ◽  
Author(s):  
A. Peters
Keyword(s):  
Central Nervous System ◽  
Nervous System ◽  
Electron Microscope ◽  
Xenopus Laevis ◽  
Peripheral Nervous System ◽  
Nerve Fibre ◽  
Potassium Permanganate ◽  
Myelin Sheaths ◽  
A Cell ◽  
The Central Nervous System

The development and structure of myelin sheaths have been studied in the optic nerves of rats and of Xenopus laevis tadpoles. Both potassium permanganate- and osmium-fixed material was examined with the electron microscope. In the first stage of myelinogenesis the nerve fibre is surrounded by a cell process which envelops it and forms a mesaxon. The mesaxon then elongates into a loose spiral from which the cytoplasm is later excluded, so that compact myelin is formed. This process is similar to myelinogenesis in the peripheral nervous system, although in central fibres the cytoplasm on the outside of the myelin is confined in a tongue-like process to a fraction of the circumference, leaving the remainder of the sheath uncovered, so that contacts are possible between adjacent myelin sheaths. The structure of nodes in the central nervous system has been described and it is suggested that the oligodendrocytes may be the myelin-forming cells.

scholarly journals FURTHER OBSERVATIONS ON THE STRUCTURE OF MYELIN SHEATHS IN THE CENTRAL NERVOUS SYSTEM

1964 ◽  
Vol 20 (2) ◽  
pp. 281-296 ◽  
Author(s):  
A. Peters
Keyword(s):  
Central Nervous System ◽  
Nervous System ◽  
Direct Evidence ◽  
Radial Component ◽  
Adult Rats ◽  
Direction Parallel ◽  
Myelin Sheaths ◽  
Adult Mice ◽  
Central Myelin ◽  
The Central Nervous System

Direct evidence has been presented to confirm the existence of a spiral in the myelin sheaths of the central nervous system. An account of some of the variations in structure of central myelin sheaths has been given and it has been shown that the radial component of myelin sheaths has the form of a series of rod-like thickenings of the intraperiod line. These thickenings extend along the intraperiod line in a direction parallel to the length of the axon. The relative position of the internal mesaxon and external tongue of cytoplasm has been determined in a number of transverse sections of sheaths from the optic nerves of adult mice, adult rats, and young rats. In about 75 per cent of the mature sheaths examined, these two structures were found within the same quadrant of the sheath, so that the cytoplasm of the external tongue process tends to lie directly outside that associated with the internal mesaxon. The frequency with which the internal mesaxon and external tongue lie within the same quadrant of the sheath increases both with the age of the animal and with the number of lamellae present within a sheath. The possible significance of these findings is discussed.

scholarly journals The Structure and Conduction Velocity of the Medullated Nerve Fibres of Prawns

1941 ◽  
Vol 18 (1) ◽  
pp. 50-54 ◽  
Author(s):  
W. HOLMES ◽  
R. J. PUMPHREY ◽  
J. Z. YOUNG
Keyword(s):  
Central Nervous System ◽  
Nervous System ◽  
Conduction Velocity ◽  
Myelin Sheath ◽  
Axon Diameter ◽  
Relative Thickness ◽  
Nodes Of Ranvier ◽  
Nerve Fibres ◽  
The Central Nervous System ◽  
Myelinated Fibres

1. The structure of the myelinated fibres of prawns is described, and the homologies of the nucleated sheath which lies between the axon and the fatty layer discussed. 2. The relative thickness of the myelin sheath increases with decrease in total diameter of the fibre along a curve similar in shape to that found in vertebrates and earthworms. 3. Nodes of Ranvier are found in the sheaths of most fibres of a diameter greater than about 13µ 4. The nodes are similar to those in vertebrate nerves in that the myelin sheath is interrupted at the node. 5. The conduction velocity of fibres in the central nervous system of axon diameter 26µ and total diameter 35µ is between 18 and 23 m. per sec., a rate faster than is found in the "unmyelinated" fibres of similar size in other crustacea.

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.

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