Number Of Myelin Lamellae Research Articles

Assessment of Ultrastructural Neuroplasticity Parameters After In Utero Transduction of the Developing Mouse Brain and Spinal Cord.

The present study combines in utero transduction with transmission electron microscopy (TEM) aiming at a precise morphometrical analysis of ultrastructural parameters in unambiguously identified topographical structures, affected by a protein of interest that is introduced into the organism via viral transfer. This combined approach allows for a smooth transition from macrostructural to ultrastructural identification by following topographical navigation maps in a tissue atlas. High-resolution electron microscopy of the in-utero-transduced tissue reveals the fine ultrastructure of the neuropil and its plasticity parameters, such as cross-sectioned synaptic bouton areas, the number of synaptic vesicles and mitochondria within a bouton profile, the length of synaptic contacts, cross-sectioned axonal areas, the thickness of myelin sheaths, the number of myelin lamellae, and cross-sectioned areas of mitochondria profiles. The analysis of these parameters reveals essential insights into changes of ultrastructural plasticity in the areas of the nervous system that are affected by the viral transfer of the genetic construct. This combined method can not only be used for studying the direct effect of genetically engineered biomolecules and/or drugs on neuronal plasticity but also opens the possibility to study the in utero rescue of neuronal plasticity (e.g., in the context of neurodegenerative diseases).

The TAM receptor TYRO3 is a critical regulator of myelin thickness in the central nervous system.

Multiple sclerosis (MS) is an autoimmune, demyelinating disease of the central nervous system (CNS). Major deficits arise in MS patients due to an inability to repair damaged myelin sheaths following CNS insult, resulting in prolonged axonal exposure and neurodegeneration. The TAM receptors (Tyro3, Axl, and Mertk) have been implicated in MS susceptibility, demyelination and remyelination. Previously, we have shown that Tyro3 regulates developmental myelination and myelin thickness within the optic nerve and rostral region of the corpus callosum (CC) of adult mice. In this study we have verified and extended our previous findings via a comprehensive analysis of axonal ensheathment and myelin thickness in the CC of unchallenged mice, following demyelination and during myelin repair. We show that the loss of the Tyro3 receptor correlates with significantly thinner myelin sheaths in both unchallenged mice and during remyelination, particularly in larger caliber axons. The hypomyelinated phenotype observed in the absence of Tyro3 occurs independently of any influence upon oligodendrocyte precursor cell (OPC) maturation, or density of oligodendrocytes (OLs) or microglia. Rather, the primary effect of Tyro3 is upon the radial expansion of myelin. The loss of Tyro3 leads to a reduction in the number of myelin lamellae on axons, and is therefore most likely a key component of the regulatory mechanism by which oligodendrocytes match myelin production to axonal diameter.

Developmental loss of NT‐3 in vivo results in reduced levels of myelin‐specific proteins, a reduced extent of myelination and increased apoptosis of Schwann cells

This work investigates the role of NT-3 in peripheral myelination. Recent articles, based in vitro, propose that NT-3 acting through its high-affinity receptor TrkC may act to inhibit myelin formation by enhancing Schwann cell motility and/or migration. Here, we investigate this hypothesis in vivo by examining myelination formation in NT-3 mutant mice. On the day of birth, soon after the onset of myelination, axons showed normal ensheathment by Schwann cells, no change in the proportion of axons which had begun to myelinate, and no change in either myelin thickness or number of myelin lamellae. However in postnatal day 21 mice, when myelination is substantially complete, we observed an unexpected reduction in mRNA and protein levels for MAG and P(0), and in myelin thickness. This is the opposite result to that predicted from previous in vitro studies, where removal of an inhibitory NT-3 signal would have been expected to enhance myelination. These results suggest that, in vivo, the importance of NT-3 as a major support factor for Schwann cells (Meier et al., (1999) J Neurosci 19:3847-3859) over-rides its potential role as an myelin inhibitor, with the net effect that loss of NT-3 results in degradation of Schwann cell functions, including myelination. In support of this idea, Schwann cells of NT-3 null mutants showed increased expression of activated caspase-3. Finally, we observed significant reduction in width of the Schwann cell periaxonal collar in NT-3 mutant animals suggesting that loss of NT-3 and resulting reduction in MAG levels may alter signaling at the axon-glial interface.

Morphometric analysis of the myelin-associated oligodendrocytic basic protein-deficient mouse reveals a possible role for myelin-associated oligodendrocytic basic protein in regulating axonal diameter

Myelin-associated oligodendrocytic basic protein is a member of the proteins constituting the central nervous system myelin. By morphometric analysis, we demonstrated that axons of myelin-associated oligodendrocytic basic protein-deficient mice had larger diameters and more myelin lamellae as compared to those of wild-type mice at the same age. It is known that the number of myelin lamellae increases linearly with axonal diameter, and that the rate of radial axonal growth is the factor controlling the rate of myelin formation. In line with these observations, we found that the regression line for axonal diameter and the number of myelin lamellae in myelin-associated oligodendrocytic basic protein-deficient mice appeared to be identical to that in wild-type mice, indicating that the increase in the number of myelin lamellae was the result of the increase in axonal diameter. Furthermore, we generated myelin basic protein/myelin-associated oligodendrocytic basic protein-double-deficient mice through mating myelin-associated oligodendrocytic basic protein-deficient mice with shiverer mice, an autosomal recessive mutant characterized by a lack of all isoforms of myelin basic protein. With these knock-out mice, we showed that axons of the double-deficient mice had larger diameters and smaller form factor, an index of the deformation of the fiber contour, in ensheathed fibers than those of shiverer mice, although there was no difference in axonal diameter of unmyelinated fibers between them. Taken together, myelin-associated oligodendrocytic basic protein seemed to play a role in controlling axonal diameter and in keeping axons round.

Small axons relative to number of myelin lamellae in Charcot-Marie-Tooth disease 1A with peripheral myelin protein 22 gene duplication.

In myelinated fibers of the sural nerves in Charcot-Marie-Tooth disease 1A (CMT 1A), with peripheral myelin protein (PMP) 22 gene duplication, it has been controversial whether axonal attenuation occurs or the myelin sheath thickens. Therefore, the relationship between the transverse axonal area and the number of myelin lamellae was morphometrically studied in myelinated fibers of the sural nerves in CMT 1A with PMP22 gene duplication to re-evaluate such relationships as are revealed in hereditary motor sensory neuropathy, type 1 without genetic diagnosis. In electron microscopic studies both the axonal circumference and natural log (ln) axonal area were significantly (P < 0.01) smaller in the CMT 1A group (n = 8) than in the control group (n = 9). Myelin lamellae numbers, however, were similar in both groups. In a regression analysis relating ln axonal areas to the numbers of myelin lamellae, ln axonal area in the CMT 1A group was significantly (P < 0.05) smaller than in the control group in myelinated fibers with the numbers of myelin lamellae equal to or over 40. Therefore, it was concluded that the axonal area was smaller in large-diameter myelinated fibers in the CMT 1A group than in the control group. We speculated on the existence of axonal attenuation due to the impaired effect of Schwann cells on myelinated axons in CMT 1A with PMP22 gene duplication.

Morphometry of Axon Cytoskeleton at Internodal Regions of Rat Sciatic Nerve during Aging

Background: Nerve endings undergo a lifespan morphofunctional modulation which is reported to be markedly impaired with aging. Neurone structural remodelling is in charge of processes occurring in the nerve cell soma, however the axonal transportation of organelles and molecules by cytoskeletal elements plays a very important role in the morphological rearrangements taking place at peripheral compartments. Objective: To assess the involvement of axonal ultrastructure in the reported age-related decline of slow axoplasm flow mechanisms, we carried out a morphometric study of axon cytoskeleton in aging. Methods: Female Wistar rats (3, 12 and 30 months of age) were anesthetized and perfused with saline followed by a fixation solution (glutaraldehyde 5% + formalin 2% in 0.1 cacodylate buffer pH 7.4). The excised sciatic nerves were processed according to conventional electron microsopic procedures. Axons sectioned perpendicularly to their longitudinal axis at the internodal region (mean axoplasm area: 18.25–26.5 μm²) were sampled by a systematic random procedure. The overall number of neurofilaments (No.Nfs) and microtubules (No.Mts) per total axoplasm area analysed, the numeric density (number/μm² of axoplasm area) of neurofilaments (NaNfs) and microtubules (NaMts), the myelin thickness, the number of myelin lamellae and the R proportion [No.Nfs/(No.Nfs + No.Mts)] were the parameters measured by computer-assisted semiautomatic methods. Results: No.Nfs, NaNfs, myelin thickness and the number of myelin lamellae did not change between 12 and 30 months of age, while a significant increase of these parameters was found in a comparison with younger rats. No.Mts and NaMts were significantly increased at 12 vs. 3 as well as at 30 vs. 12 months of age, respectively. R proportion did not show any difference due to age. Conclusions: The present findings support that the dynamic condition of the axonal cytoskeleton appears to be preserved at a high extent in aging. Thus, the intra-axonal defective spacing of cytoskeletal elements (e.g. neurofilaments), rather than their number, is proposed to contribute to the decline of the slow axonal transport of organelles and molecules reported in aging.

Trapidil-mediated inhibition of CNS remyelination results from reduced numbers and impaired differentiation of oligodendrocytes.

In a previous study, we described the inhibitory effects of the growth factor-antagonist, trapidil, on spontaneously occurring oligodendrocyte remyelination in the rat spinal cord following lysolecithin-induced demyelination [30]. The objective of the present study was to further investigate the mechanisms of trapidil-mediated impairment of remyelination and thus obtain greater insight into the steps at which growth factors may be involved in remyelination. To this end, an ultrastructural analysis of the cellular composition of lesions from control and trapidil-treated animals was undertaken. Demyelination was created in the dorsal funiculus of 6-week-old female rats by the injection of 1.0 microliter of 1% lysolecithin. The animals received daily intraperitoneal injections of trapidil (80 mg/kg) or saline for 21 days, beginning on the day of lesion induction. Quantitative electron microscopic examination of lesions from both groups of animals showed that trapidil-treated lesions had reduced numbers of oligodendrocytes (P = 0.02) with a higher relative proportion of immature phenotypes, but increased numbers of microglia (P = 0.0009) and dystrophic axons (P0.02). In addition, the numbers of myelin lamellae around remyelinated axons were fewer in trapidil-treated animals. These results suggest that trapidil-mediated impairment of CNS remyelination is due to a blockage of growth factor-mediated proliferation and/or recruitment of remyelinating cells. Furthermore, the presence of oligodendrocytes with a more immature phenotype and the decreased thickness of the myelin sheaths of remyelination in the trapidil-treated animals indicate an impairment of growth factor-mediated differentiation.

Regeneration of myelinated fiber after crush injury is retarded in sciatic nerves of mutant Japanese quails deficient in neurofilaments.

Regeneration of myelinated fibers in the sciatic nerve 2 weeks after crush injury was studied morphometrically in mutant Japanese quails deficient in neurofilaments and in normal quails (controls). There were fewer regenerated myelinated fibers per nerve at 10 mm (R1) and 20 mm (R2) distal to the crush site in mutants than in controls (P < 0.05). Both median and maximum diameters were smaller (P < 0.01) in mutants than in controls. On electron microscopy, transverse axonal area and axonal circumference were smaller (P < 0.001) at both R1 and R2 in mutants than in controls. The number of myelin lamellae was less (P < 0.01) in mutants than in controls at R1, but was similar at R2. There were fewer myelin lamellae in relation to axonal area in mutants than in controls at R1 (P < 0.0001) and R2 (P = 0.0032). The results indicate a retardation of both radial growth of axons and myelination around axons of the same size in mutants compared with controls. Such retardation may be explained by the deficiency of neurofilaments and the altered relationship between Schwann cell and axon in the mutant.

The effect of brain-derived neurotrophic factor on regeneration of nerve fibers after crush injury--morphometric evaluation

Brain-derived neurotrophic factor (BDNF) is produced in Schwann cells and fibroblasts in the peripheral nerve, and is considered to play an important role in survival, maintenance, development and repair of the peripheral neuron. In this study, the effect of human recombinant BDNF on the regeneration of nerve fibers following a crush injury to the sciatic nerves of Sprague-Dawley rats was evaluated. In the experimental group, 20 mg/kg of BDNF was injected subcutaneously three times a week for 4 weeks in seven rats. In the control group, only the dissolving solution used in the experimental group was injected similarly in seven rats as in the experimental group. The various morphometric parameters were evaluated in the peroneal, sural and tibial nerves and the nerve to soleus muscle distal to the crush site. There were no statistically significant differences in the change of body weight, the improvement of the width between the first and fifth toes on the side of the crush, the density of total myelinated fibers (numbers per mm2), the total number of myelinated fibers (number per nerve), maximum and median diameters of myelinated fibers, the density of unmyelinated fibers, and the electron microscopic parameters of axon and myelin sheath. On the other hand, the number of myelin lamellae in relation to the transverse axonal area was significantly greater in the experimental group than in the control group. Therefore, BDNF may promote the myelination. However, such an effect on myelination seems not to be clinically significant, because such an effect was not demonstrated in other morphometric evaluations reflecting the myelination condition. Therefore, taking all the data obtained in this study into consideration, we concluded that there was no definite evidence that BDNF promoted the regeneration of nerve fibers at least under these experimental conditions.

Smaller axon and unaltered numbers of microtubules per axon in relation to number of myelin lamellae of myelinated fibers in the mutant quail deficient in neurofilaments

Smaller axon and unaltered numbers of microtubules per axon in relation to number of myelin lamellae of myelinated fibers in the mutant quail deficient in neurofilaments

Nodes of Ranvier and myelin sheath dimensions along exceptionally thin myelinated vertebrate PNS axons.

The trigeminal alveolar branch in the lower jaw of the cichlid Tilapia mariae was examined by light and electron microscopy on single and serial sections, and by light microscopy on teased fibre preparations. The principal purpose was to find out if the exceptionally thin myelinated axons (d < 1 micron) present in this nerve possess true nodes of Ranvier, and to determine the dimensions of their myelin sheaths. This necessitated analysis of the whole size range of myelinated fibres, with respect to nodal and internodal morphology. The results show that the exceptionally thin myelinated fibres exhibit primitive nodal regions, with patches of axolemmal undercoating, and few Schwann cell processes in the node gap. This contrasts with the more complex nodal organization seen in larger trigeminal alveolar branch fibres. For the whole population of myelinated fibres the number of myelin lamellae increases rectilinearly with axon diameter, and sheath length increases with fibre diameter according to a logarithmic expression. The myelin sheaths of the exceptionally thin trigeminal alveolar branch fibres are composed of 10-20 lamellae, and extend 35-50 microns along the axon. These results show that the structural complexity of nodal regions in the trigeminal alveolar branch decreases with decreasing fibre size, that the exceptionally thin myelinated trigeminal alveolar branch fibres possess primitive nodes and that they have very short myelin sheaths. Our crude theoretical calculations suggest that these fibres might be capable of saltatory conduction.

Polyglucosan body axonal enlargement increases myelin spiral length but not lamellar number

The area of the unrolled myelin sheet of internodes of myelinated fibers (MF) of peripheral nerve is thought to be determined by axonal caliber and internodal length. We studied the effect of a focal increase of axonal caliber due to the deposition of polyglucosan bodies (PGB), amylopectin-like glucose polymers, on number of myelin lamellae (NL), interlamellar distance (periodicity), and myelin spiral length (MSL) from a sural nerve biopsy specimen of a patient with chronic inflammatory demyelinating polyneuropathy. Axonal area, NL, periodicity, and MSL were estimated within internodes of MF above, at, and below PGB. The axon caliber at the level of the PGB was significantly ( P < 0.002) increased when the PGB was included. At the PGB, NL and their periodicity were not significantly different from those above or below the PGB. The MSL was significantly longer overlying the PGB than it was in the same internode above or below the PGB. Because slippage or stretching of the myelin sheath as well as movement of molecular constituents of myelin is not likely over large distances, localized biosynthesis and assembly of new myelin may explain this increase of MSL.

Congenital Hypomyelinating Polyneuropathy in Two Golden Retriever Littermates

Serial peripheral nerve biopsies from two golden retriever littermates with chronic neurologic disease were taken for morphologic and morphometric evaluation. Teased nerve preparations were difficult to interpret due to the lightness of myelin staining. Light and electron microscopic findings were characterized by the following: reduced number of myelinated axons, presence of myelinated sheaths inappropriately thin for the caliber of the fiber, poor myelin compaction, increased numbers of Schwann cell nuclei, increased concentration of neurofilaments in myelinated axons, many Schwann cells with voluminous cytoplasm, and increased perineurial collagen. Onion bulb formation was not seen. In contrast to control data, a poor correlation was seen between numbers of myelin lamellae (ML) and axonal circumference (AC). The frequency distribution of ML ranged from 5 to 55 lamellae in affected animals (mean, 28 lamellae) compared to 20 to 140 lamellae in controls (mean, 66 lamellae). The ML/AC ratio was significantly reduced (P less than 0.001) in nerves of affected dogs. Morphometric results indicated that fibers of all calibers were hypomyelinated.

Morphometric alteration of rat myelinated fibers with aging.

The number, size, density and pathologic alterations of myelinated fibers (MF) of ventral and dorsal roots and of peroneal and sural nerves in groups of female Fischer 344 rats at 10, 20, and 30 months of age were evaluated to characterize nerve changes with old age. Except for minimal changes in the peroneal nerve, no statistically significant alteration in number of MF/nerve or in the fascicular area was associated with aging. The unaltered number was misleading since striking changes in MF size distribution, pathologic alterations of fibers and the presence of regeneration clusters suggested age-related degenerative and regenerative events. These changes were most dramatic in the ventral root, where myelin infolding, myelin separation from axon and ballooning, macrophagia and hyperplasia of Schwann cell nuclei were pronounced. Concomitant with these alterations, axonal atrophy (a reduction in caliber of axons relative to myelin spiral length or number of myelin lamellae) was demonstrated in MF of the ventral root in old age.

Effect of acclimation and fixation temperatures on the number of lamellae and periodicity of myelin in fibres of the optic nerve of goldfish.

Previous findings from our laboratory have shown that the optic nerves of goldfish acclimated to different temperatures differ considerably in their glycerophospholipid composition. This paper describes changes in the morphology of the nerve with different acclimation and fixation temperatures. Optic nerves of 5 and 25 degrees C acclimated fish were excised and fixed at the temperature of acclimation, or at the reverse temperature, and the morphology observed by electron microscopy. Under all temperature conditions considered there is a statistically significant linear relationship between the radius of the axon and the number of myelin lamellae. However, the temperature of acclimation and fixation both influence the regression coefficients for this relationship, the higher the acclimation temperature the lower the coefficient and the higher the fixation temperature the higher the coefficient. The periodicity of the myelin also alters with these temperatures, being greater in the 25 degrees C fish than in the 5 degrees C ones. Myelin sheath thickness is also significantly greater in the 25 degrees C fish. These results are discussed in relation to observed changes in glycerophospholipid composition and conduction velocities.

Equine laryngeal hemiplegia Part II. An electron microscopic study of peripheral nerve

The recurrent laryngeal nerves were examined by electron microscopy in five control, four subclinical and four clinical laryngeal hemiplegic horses. In addition, the peroneal nerve was examined in two horses in the latter group. The distally distributed loss of large myelinated fibres in the left recurrent laryngeal nerve seen by light microscopy was confirmed. In addition, active axonal pathology was found to be more evident than indicated by light microscopic investigations. The onion bulb formations observed indicated the repetitive nature of the damaging influence to nerve fibres. Although the pathological changes were most obvious in the distal left recurrent laryngeal nerve, alterations similar in type and distribution were present in other areas of the left and right nerves, and in the distal hindlimb nerves. The observation of fibres with inappropriately thick myelin sheaths relative to their axonal calibre, was confirmed statistically by determining the regressions of axis cylinder perimeter against the number of myelin lamellae. In conclusion, the peripheral nerve pathology of equine laryngeal hemiplegia was demonstrated to be a distally distributed loss of myelinated fibres, with considerable active axonal damage, in conjunction with axonal atrophy. These features suggest that this disease may be classified as a distal axonopathy.

Functional development of the visual system in normal and protein deprived rats. III: Recordings from adult optic nerve in vitro.

A persistent increase in the latencies of the visual evoked response recorded from the cortical surface of protein deprived adult rats was described recently (Sjöström et al. 1984). The morphological correlate to this alteration is unknown. Previous studies on malnourished rats have shown a reduction of axonal diameters and in the number of myelin lamellae in relation to axonal circumference, and hence the possibility of a decrease in fibre conduction velocity must be considered. In parallel study, we have established that changes in diameters and myelination of optic nerve fibres similar to those previously reported in malnourished rats are present in adult protein deprived (PD) rats (Conradi et al. 1985). In the present paper, recordings of optic nerves in vitro from adult normal (C) and protein deprived (PD) rats are described. The compound action potentials were very similar in the two groups. Three positive peaks were easily defined, probably corresponding to three functional groups of optic nerve fibers. No significant differences in amplitudes or conduction velocities for the three peaks were found between the C and PD rats. It is concluded that the increased latencies of the evoked response are not caused by a decrease in conduction velocity.

Axo-glial relations in the retina-optic nerve junction of the adult rat: electron-microscopic observations

The retina-optic nerve junction (ROJ) was examined by electron microscopy in adult rats, with particular emphasis on the unmyelinated-myelinated nerve fibre transition. Both single sections and serial sections were used. The non-retinal part of the ROJ is covered by an extensively folded glia limitans, facing the choroidea, sclera and pia mater. The blood vessels within the ROJ follow a transverse course and are surrounded by unusually wide perivascular spaces with a glia limitans-like outer delimitation. The endothelial cells exhibit numerous pinocytotic vesicles on their abluminal aspect. In the unmyelinated part of the ROJ the axons are embedded in an extensive meshwork of fibrous astrocytic processes. Some unmyelinated axons exhibit patches of axolemmal undercoating with externally associated astrocytic processes. Typical oligodendrocytes are not found, but a few small dark glial cells of unknown identity can be observed. Atypical ensheathment and myelination of axons at this level by ectopic Schwann cells occurred in one case. In the transition segment of the ROJ a pattern similar to that along dysmyelinated axons is observed, including aberrant axo-glial contacts, unusually thin and short myelin sheaths, intercalated unmyelinated segments, distorted myelin termination regions, bizarre paranodes and myelin termination regions without associated nodally differentiated axolemma. Neither sheath length nor number of myelin lamellae is related to axon diameter in the transition region. Axon diameter tends to be somewhat larger at myelinated than unmyelinated levels of the same axon. We suggest that the unusual axo-glial relations in this region are due to a deficient proliferation and differentiation of oligodendroglial cells, and that the pattern of glial ensheathment in the ROJ might be a consequence of the locally deficient blood-brain barrier.

Myelinated non-axonal neuronal elements in the feline olfactory bulb lack sites with a nodal structural differentiation

Myelinated dendrites in the external plexiform layer (EPL) of the feline olfactory bulb and myelinated axons in the lateral olfactory tract (LOT), were examined by transmission electron microscopy. The results show that the non-axonal myelin sheaths are extremely thin and short and that the number of myelin lamellae does not increase with increasing dendritic diamter. In myelinated LOT axons the sheaths tend to be thicker and the myelin lamellar number increases with axon diameter. Domains with node-like structural characteristics are not encountered along myelinated dendrites, neither between successive myelin sheaths nor where single sheaths terminate. The partly myelinated neuronal perikarya, which occur in the EPL, also lack node-like domains. In contrast, typical nodes are easily found in myelinated LOT axons. In the periglomerular region dendrites and neuronal perikarya are surrounded by non-compacted glial sheets. It is concluded that myelination and node formation are relatively independent events and that morphogenetic glial-neuronal interactions may give different results in different parts of the same neuron.

Myelination of the rat optic nerve during postnatal undernourishment and recovery: a morphometric analysis

Newborn rats were undernourished from the second postnatal day through 20 days of age and weaned to a diet of laboratory chow ad libitum. Optic nerve development was examined by various light and electron microscopic techniques at 14, 21, 35 and 63 days of age. The degree of undernourishment achieved resulted in body growth lag comparable to results obtained in our previous studies. Although cellularity (cells per photomicrograph area) of the oligodendroglia was unaffected, there was an apparent significant relative reduction in the total number of myelinated fibers by 21 days of age, as determined by light micrscopic sampling. Ratios of unmyelinated-to-myelinated fibers were thus estimated by electron microscopy, and results indicated an early increase in the ratio (14 days). Either as the result of catch up in a developmental lag, or as a result of possible restorative effects of rehabilitation, these differences were insignificant by 35 days of age. The relationship between axon circumference and numbers of myelin lamellae was determined by regression analysis, which revealed a significant reduction in numbers of lamellae over axons of all sizes at 14 days. By 21 days, only fibers in the size range of 1–2 μm of circumference showed a difference, and by 35 days there were no significant differences. These results all indicate that there is a significant myelin reduction in optic nerve of the undernourished rat.