Fibre size in the left and right recurrent laryngeal nerves (RLNs) in the man

Some morphological data of the right and left human RLNs were evaluated with the aim of verifying possible differences in the fibre composition of the two nerves. The following parameters were evaluated in the right and left RLNs of five human cases: 1) the maximum diameter of the fibres; 2) the axon diameter and area; 3) the myelin sheath area obtained substracting the axon area from the total area of each fibre. The obtained data were plotted on histograms for each case: moreover, histograms of all fibres of both left and right nerves of all five cases were made. The results show that the values of the maximum diameter of the fibres and of the myelin sheath area are always greater in a statistically significant way in the left RLNs than in the right RLNs. On the other hand the axon diameter is nearly the same in the nerves of both sides. These data suggest that the greater calibre of the myelin sheath in the fibres of the left inferior laryngeal nerve can be responsible of the faster conduction speed in this nerve. This fact might explain the simultaneous arrival of the impulses to the laryngeal muscles of the two sides in spite to the different length of the two nerves.     

Morphometric studies of peripheral nerves and spinal ganglion cells in streptozotocin diabetic rats

Morphometric studies of sural nerves and dorsal root ganglia cells L5 were performed in 8 diabetic rats 35 or 44d after the administration of streptozotocin and in 8 controls. Morphometry of photographed semithin sections was realized after whole body glutaraldehyde perfusion with semiautomatic MOP AM 02 and MOP Videoplan. The peripheral nerve showed no decrease of the total nerve area, number of nerve fibres or of myelinated area. Parameters area of fibres decreased in diabetic animals caused by reduction of myelin sheath thickness. There were no changes of mean neuron volume and maximal cell diameter in the dorsal root ganglia. Whereas in experimental short-term diabetes the peripheral nerves demonstrate morphologic changes correlating to biochemical abnormalities, no such correlations can be observed in the perikarya. It is suggested, that primary SCHWANN cell lesion is responsible for the observed myelin thickness reduction.     

Experimental Study of Low Dose Ultrashortwave Promoting Nerve Regeneration after Acellular Nerve Allografts Repairing the Sciatic Nerve Gap of Rats

Objectives To observe the effect of ultrashortwave (USW) therapy on nerve regeneration after acellular nerve allografts(ANA) repairing the sciatic nerve gap of rats and discuss its acting mechanisms. Methods Sixteen Wistar rats weighing 180–220 g were randomly divided into four groups with four rats in each group: normal control group; acellular group (ANA, treated by hypotonic-chemical detergent, was applied for bridging a 10 mm-long sciatic nerve defect); USW group (After 24 h of ANA repairing the sciatic nerve gap, low dose USW was administrated for 7 min, once a day, 20 times a course of treatment, three courses of treatment in all); and autografts group. 12 weeks after operation, a series of examinations was performed, including electrophysiological methods, the restoring rate of tibialis anterior muscle wet weight, histopathological observation (myelinated nerve number, myelin sheath thickness, and axon diameter), vascular endothelial growth factor (VEGF) mRNA expression of spinal cord, and muscle at injury site, and analyzed statistically. Results Compared to acellular nerve allografts alone, USW therapy can increase nerve conductive velocity, the restoring rate of tibialis anterior muscle wet weight, myelinated nerve number, axon diameter, VEGF mRNA expression of spinal cord, and muscle at injury site, the difference is significant. There were no differences between USW group and autografts group except myelin sheath thickness. Conclusions USW therapy can promote nerve axon regeneration and Schwann cells proliferation after ANA repairing the sciatic nerve gap of rats, the upregulation of VEGF mRNA expression of spinal cord and muscle may play an important role.     

The relationship between developing oligodendrocyte units and maturing axons during myelinogenesis in the anterior medullary velum of neonatal rats

Myelinogenesis was investigated in whole-mounted anterior medullary vela from rats aged postnatal day (P) 10–12, using double immunofluorescence labelling with Rip and anti-neurofilament 200 (NF200) antibodies, to identify oligodendrocytes and axons, respectively. A number of discrete phases of maturation of oligodendrocyte units were recognised. (1) Promyelinating oligodendrocytes co-expressed Rip and Myelin basic Protein and formed axonal associations, prior to ensheathment. (2) Transitional oligodendrocytes contained both ensheathing and non-ensheating processes. (3) Myelinating oligodendrocytes were established after a period of remodelling (in which non-ensheathing processes were lost), appearing as oligodendrocyte unit morphological phenotypes with a definitive number of incipient myelin sheaths. (4) Maturation of myelinating oligodendrocytes was defined as the establishment of internodal sheath lengths and the redistrubution of myelin basic protein from the cell somata and radial processes into the myelin sheaths only. Myelination was probably related to the maturational state of the axons, since it was initiated when the latter had attained a critical diameter of between ～0.2 and 0.4 μm, coincident with the expression of NF200. Oligodendrocyte differentiation and myelination of the AMV were asynchronous and multifocal, and at P10: (1) axons which were destined to be of the largest calibre in the adult AMV were already myelinated by early developing oligodendrocytes, whilst those which were destined to be the smallest calibre in the adult were unmyelinated, but ultimately became ensheathed by late developing oligoendrocytes; (2) axons were sequentially ensheathed by early developing myelinating oligodendrocytes and late developing promyelinating oligodendrocytes; (3) all axons were small calibre; (4) oligodendrocyte units exhibited polymorphism. Thus, the development of oligodendrocyte morphological phenotypes was not related solely to either the physical dimension of axon calibre at the time of ensheathment, nor oligodendrocyte birth dates.     

Developmental changes at the node and paranode in human sural nerves: morphometric and fine-structural evaluation

Developmental alterations of paranodal fiber segments have not been investigated systematically in human nerve fibers at the light- and electron-microscopic level. We have therefore analyzed developmental changes in the fine structure of the paranode in 43 human sural nerves during the axonal growth period up to 5 years of age, and during the subsequent myelin development up to 20 years and thereafter. The nodal, internodal, and paranodal axon diameters reach their adult values at 4–5 years of age. The ratio between internodal and paranodal axon diameters remains constant at 1.8–2.0. Despite a considerable increase in myelin sheath thickness, the length of the paranodal myelin sheath attachment zone at the axon does not increase correspondingly, because of attenuation, separation from the axolemma, and piling up of myelin loops in the paranode. Separation of variable numbers of terminal myelin loops from the underlying axolemma results in the formation of bracelets of Nageotte, whereas the transverse bands of these loops disappear. The adaptation of the paranodal myelin sheath to axonal expansion during development probably occurs by uneven gliding of the paranodal myelin loops simultaneously with internodal slippage of myelin lamellae. Since mechanically stabilizing structures (tight junctions and desmosomes between adjacent paranodal myelin processes; transverse bands between myelin loops and paranodal axolemma) are unevenly arranged, especially during rapid axonal growth, paranodal axonal growth with simultaneous adaptation of the myelin sheath is probably discontinuous with time.Presented in part at the 10th Biennial Meeting of the Peripheral Nerve Study Group at Arden House, Harriman, New York, USA, June 30th–July 3rd, 1991, and as a doctoral thesis (M. Bertram) at the RWTH Aachen in 1991     

Differential Ultrastructural Localization of Myelin Basic Protein, Myelin/Oligodendroglial Glycoprotein, and 2'',3''-Cyclic Nucleotide 3''-Phosphodiesterase in the CNS of Adult Rats

In a light and electron microscopic immunocytochemical study we have examined the distribution of myelin basic protein (MBP), 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNP), and myelin/oligodendroglial glycoprotein (MOG) within CNS myelin sheaths and oligodendrocytes of adult Sprague-Dawley rats. Ultrastructural immunocytochemistry allowed quantitative analysis of antigen density in different myelin and oligodendrocyte zones: MBP was detectable in high density over the whole myelin sheath, but not in regions of loops, somata, or the oligodendrocyte plasma membrane. CNP reactivity was highest at the myelin/axon interface, and found in lower concentration over the outer lamellae of myelin sheaths, at the cytoplasmic face of oligodendrocyte membranes, and throughout the compact myelin. MOG was preferentially detected at the extracellular surface of myelin sheaths and oligodendrocytes and in only low amounts in the lamellae of compacted myelin and the myelin/axon border zone. Our studies, thus, indicate further the presence of different molecular domains in compact myelin, which may be functionally relevant for the integrity and maintenance of the myelin sheath.     

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

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.     

Prenatal exposure to low levels of carbon monoxide alters sciatic nerve myelination in rat offspring

Prenatal exposure to low concentrations of carbon monoxide (CO, 75 and 150 ppm from day 0 to day 20 of gestation), resulting in maternal blood HbCO concentrations equivalent to those maintained by human cigarette smokers, leads to subtle myelin alterations in the sciatic nerve of male rat offspring. The rapid growth spurt in pup body weight was related to the period of maximal increase in myelin sheath thickness in both control and CO-exposed animals. A significant reduction in myelin sheath thickness of sciatic nerve fibers, paralleled by changes in the frequency distribution, occurred in both 40- and 90-day-old rats exposed in utero to CO (75 and 150 ppm). Myelin deficit observed in 75 and 150 ppm CO-exposed animals showed up only after the major spurt in myelination but not early during development. The subtle myelin alterations observed in CO-exposed offspring were not accompanied by changes in developmental pattern of axon diameters and did not result in a gross impairment of motor activity. These results suggest that the myelination process is selectively targeted by a prenatal exposure model simulating the CO exposure observed in human cigarette smokers.     

FURTHER OBSERVATIONS ON THE STRUCTURE OF MYELIN SHEATHS IN 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.     

Centrifugal decrement in diameter of myelinated afferent fibres innervating frog taste organ

1. Regional changes in the diameter of single myelinated afferent nerve fibres innervating the taste disc of the fungiform papillae on the bullfrog tongue were investigated morphologically and functionally. 2. The diameter of myelinated afferents in the medial lingual branch of the glossopharyngeal nerve averaged 8.4 microns at the proximal end of the tongue and gradually decreased at the rate of 0.8 micron/cm length of the fibres as they ran in the apical direction of the tongue. 3. The conduction velocity of single myelinated afferent fibres within the tongue decreased gradually as they ran peripherally. 4. Electrophysiological inspection of neural connections between the fungiform papillae suggests that a gradual centrifugal decrease in the diameter of a single myelinated afferent fibre is not due to multiple bifurcations of the fibre at various sites within the tongue, but due to a natural gradual decrease in the thickness of the myelin sheath and the diameter of axon.     

缺氧缺血诱导对原代新生大鼠前脑混合细胞髓鞘形成和细胞骨架的影响

目的:该研究探讨了缺氧缺血诱导对新生大鼠前脑混合细胞髓鞘形成和细胞骨架的影响。方法:在原代培养的新生大鼠前脑混合细胞中,应用免疫组化染色髓鞘碱性蛋白(MBP)和劳克坚牢蓝(LFB)染色检测髓鞘和轴突的发育情况;Western印迹分析NF155蛋白的表达情况;免疫荧光分析参与细胞骨架调节的ERM-Rho GTP酶家族相关蛋白表达。结果:缺氧缺血对混合细胞的进一步分化和成熟起到抑制作用,MBP染色阳性率降低57%,髓鞘和轴突染色阳性率降低74%;NF155蛋白表达降低51%;Rho GTP酶家族成员Rac1、Cdc42分别降低81%和75%。结论:缺氧缺血使细胞突起的形成和骨架重组受到阻碍,继而影响髓鞘的发育和成熟,这一过程与ERM-Rho GTP酶细胞骨架通路有关。     

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

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.     

Electrical activity and structural correlates of giant nerve fibers in Kuruma shrimp (Penaeus japonicus)

Morphology and recordings of electrical activity of Kuruma shrimp (Penaeus japonicus) giant medullated nerve fibers were carried out. A pair of giant fibers with external diameter of about 120 μ and 10 μ in myelin thickness were found in the ventral nerve cord. The diameter of the axon is about 10 μ. Thus there is a wide gap between the axon and the external myelin sheath. Each axon is doubly coated directly by Schwann cells and indirectly by the myelin sheath layer which is produced by those Schwann cells. Impulse conduction velocities of these giant fibers showed a range between 90–210 m/sec at about 22°C. Large action potentials (up to 113 mV, rise time of 0.16–0.3 msec, maximum rate of rise of 650–1250 V/sec, half decay time of 0.2–0.3 msec, maximum rate of fall of 250–450 V/sec and total duration of less than 1.5 msec) could be obtained by inserting microelectrodes or by longitudinal insertion of 25 μ diameter capillary electrodes into the gap but no DC-potential difference was observed across the myelin sheath. Transmyelin electrical parameters were very favorable for fast impulse conduction: myelin resistance of 3 × 10⁴ Ω cm²; time constant of 0.38 msec; myelin capacitance of 1.35 × 10^?8 F/cm²; gap fluid resistivity of 23 Ω cm. The existence of nodes of Ranvier could not be demonstrated morphologically, but electrophysiological evidence suggests that a type of saltatory conduction occurs in these giant fibers.     

SEDIMENTATION ANALYSIS OF RIBONUCLEIC ACID EXTRACTED FROM ISOLATED MAUTHNER NERVE FIBRE COMPONENTS

Abstract— Segments of spinal cords from goldfish or from carp were incubated in vitro in the presence of RNA precursors for varying periods of time. Mauthner nerve fibres were isolated from the fresh unfixed tissue, or, for the separate analyses of axon and myelin sheath, from the fixed spinal cord.
The myelinated Mauthner fibre isolated from the incubated spinal cord showed RNA synthesis. A considerable part of the material sedimented at 4 S , but part of the nucleic acid was recovered at higher sedimentation values, up to 30 S. Newly synthesized RNA was extracted from the isolated myelin sheath as well as from the axon. Isolated myelinated Mauthner fibres were also incubated in vitro with RNA precursors. In this case incorporation occurred exclusively in material sedimenting at 4 S or lower. The turnover rate for RNA from the fibre was of a higher order than that of the bulk of RNA from the spinal cord. The findings of RNA synthesis in these tissue components lacking nuclei could possibly be explained as owing to mitochondria.
Studies by electron microscopy demonstrated the extent of purity of the isolated components and it was found that contamination was so small as to make it unlikely that the RNA investigated originated in contaminating tissue.     

Morphological and morphometric studies of early myelination in the cervical dorsal funiculus of the newborn mouse spinal cord

The early myelination of the dorsal funiculus at the level of the 4th cervical spinal cord was ultrastructurally studied in the one-day-old mouse. It was found that the fibers were mainly unmyelinated. However, some early myelinated fibers were scattered among unmyelinated fibers. In the initial stage of the myelination, the axon was partially contracted by a piece of cytoplasmic process of the oligodendroglial cell. The two lips of the oligodendroglial process then extended and converged, enwrapping the axon completely and forming the first contact point. With the anchorage of that contact point, the two lips of the process became elongated and enfolded by each other, and produced the internal and external tongues of the future myelin sheath. More contact points were formed at a regular interval by the regional fusion of the two external surface layers of the opposed cytoplasmic membranes of adjacent tongue processes. With the advanced bidirectional spiralization of the two tongue processes, many contact points were found between the adjacent lamellae of the concentrically arranged oligodendroglial process; simultaneously, the cleft between the neighboring contact points disappeared and formed the initial sites of the intraperiod line. During the early myelination, one single axon ensheathed concentrically by two different oligodendroglial processes as well as several axons enwrapped by a continuous spiral myelin sheath of one oligodendroglial cell were frequently observed. The cross-sectional areas of unmyelinated axons varied from 0.01 to 0.2 micron 2, with a median of 0.07 micron 2; whereas, that of promyelinated axons ranged from 0.09 to 1.4 micron 2, with a median at 0.61 micron 2. These data support the suggestion that the axon calibre is a critical factor for the initiation of central myelination.     

Intracranial portion of the trochlear nerve and dorsal oblique muscle composition in dog: a structural and ultrastructural study

In the present investigation the right intracranial portion of the trochlear nerves and dorsal oblique muscle of the right ocular globe were removed from six adult dogs and analyzed by light and electron microscopy. Unmyelinated fibers were observed in the analyzed nerves. The number, diameter, area, and density of myelinated fibers were determined, as were corresponding axon area and diameter and myelin sheath thickness. Frequency histograms of myelin sheath thickness and fiber size show a bimodal distribution with a similar proportion of large and small fibers. Muscle samples were taken from the central portion of the muscle belly, subsequently frozen, cut, and stained with m-ATPase at pH 4.6. Fibers were classified as Type 1 or Type 2 according to their reaction to the m-ATPase and detailed morphologic and morphometric studies were made. The muscles showed two clearly distinct layers, a central layer and a peripheral layer, chiefly composed of Type 2 fibers. The fibers in the central layer were larger in size than those in the peripheral layer.     

Myelinated fibers of the mouse spinal cord after a 30-day space flight

Myelinated fibers and myelin-forming cells in the spinal cord at the L3–L5 level were studied in C57BL/6N mice that had spent 30 days in space. Signs of destruction of myelin in different areas of white matter, reduction of the thickness of myelin sheath and axon diameter, decreased number of myelin-forming cells were detected in “flight” mice. The stay of mice in space during 30 days had a negative impact on the structure of myelinated fibers and caused reduced expression of the markers myelin-forming cells. These findings can complement the pathogenetic picture of the development of hypogravity motor syndrome.     

A dual role of erbB2 in myelination and in expansion of the schwann cell precursor pool

Neuregulin-1 provides an important axonally derived signal for the survival and growth of developing Schwann cells, which is transmitted by the ErbB2/ErbB3 receptor tyrosine kinases. Null mutations of the neuregulin-1, erbB2, or erbB3 mouse genes cause severe deficits in early Schwann cell development. Here, we employ Cre-loxP technology to introduce erbB2 mutations late in Schwann cell development, using a Krox20-cre allele. Cre-mediated erbB2 ablation occurs perinatally in peripheral nerves, but already at E11 within spinal roots. The mutant mice exhibit a widespread peripheral neuropathy characterized by abnormally thin myelin sheaths, containing fewer myelin wraps. In addition, in spinal roots the Schwann cell precursor pool is not correctly established. Thus, the Neuregulin signaling system functions during multiple stages of Schwann cell development and is essential for correct myelination. The thickness of the myelin sheath is determined by the axon diameter, and we suggest that trophic signals provided by the nerve determine the number of times a Schwann cell wraps an axon.     

A STRUCTURAL ANALYSIS OF THE MYELIN SHEATH IN 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.     

PROTEIN SYNTHESIS IN THE ISOLATED MAUTHNER NERVE FIBRE OF GOLDFISH

Abstract— Mauthner nerve fibres isolated from the spinal cord of goldfish were incubated, in the presence of radioactive amino acids for varying periods of time. It was found that the Mauthner fibre synthesizes proteins in the absence of cell nuclei. Amino acid incorporation showed sensitivity to puromycin and to acetoxycycloheximide but resistance to chloramphenicol. Only slight inhibition was caused by actinomycin-D. The contribution of the denuded axon to the total protein synthesis was about 30 per cent per unit length Mauthner fibre. The remaining activity was due to the myelin sheath compartment. Fractionation experiments showed that the incorporation in the sheath was due to components other than the myelin lamellae. The subcellular distribution of newly synthesized proteins in the isolated and incubated Mauthner fibre was compared to that found in the incubated spinal cord. The results strongly suggested the existence in the Mauthner fibre of a primary microsomal, rather than a mitochondrial, protein synthesizing system.