Imaging myelinated nerve fibres by confocal fluorescence microscopy: individual fibres in whole nerve trunks traced through multiple consecutive internodes

Raman Probes for In Situ Molecular Analyses of Peripheral Nerve Myelination

The myelinating activity of living Schwann cells in coculture with neuronal cells was examined in situ in a Raman microprobe spectroscope. The Raman label-free approach revealed vibrational fingerprints directly related to the activity of Schwann cells' metabolites and identified molecular species peculiar to myelinating cells. The identified chemical species included antioxidants, such as hypotaurine and glutathione, and compartmentalized water, in addition to sphingolipids, phospholipids, and nucleoside triphosphates also present in neuronal and nonmyelinating Schwann cells. Raman maps at specific frequencies could be collected, which clearly visualized the myelinating action of Schwann cells and located the demyelinated ones. An important finding was the spectroscopic visualization of confined water in the myelin structure, which exhibited a quite pronounced Raman signal at ∼3470 cm^-1. This peculiar signal, whose spatial location precisely corresponded to a low-frequency fingerprint of hypotaurine, was absent in unmyelinating cells and in bulk water. Raman enhancement was attributed to frustration in the hydrogen-bond network as induced by interactions with lipids in the myelin sheaths. According to a generally accepted morphological model of myelin, an explanation was offered of the peculiar Raman scattering of water confined in intraperiod lines, according to an ordered hydrogen bonding structure. The possibility of concurrently mapping antioxidant molecules and compartmentalized water structure with high spectral accuracy and microscopic spatial resolution enables probing myelinating activity and might play a key-role in future studies of neuronal pathologies. Compatible with life, Raman microprobe spectroscopy with the newly discovered probes could be suitable for developing advanced strategies in the reconstruction of injured nerves and nerve terminals at neuromuscular junctions.

Three-dimensional architecture of human diabetic peripheral nerves revealed by X-ray phase contrast holographic nanotomography

A deeper knowledge of the architecture of the peripheral nerve with three-dimensional (3D) imaging of the nerve tissue at the sub-cellular scale may contribute to unravel the pathophysiology of neuropathy. Here we demonstrate the feasibility of X-ray phase contrast holographic nanotomography to enable 3D imaging of nerves at high resolution, while covering a relatively large tissue volume. We show various subcomponents of human peripheral nerves in biopsies from patients with type 1 and 2 diabetes and in a healthy subject. Together with well-organized, parallel myelinated nerve fibres we show regenerative clusters with twisted nerve fibres, a sprouted axon from a node of Ranvier and other specific details. A novel 3D construction (with movie created) of a node of Ranvier with end segment of a degenerated axon and sprout of a regenerated one is captured. Many of these architectural elements are not described in the literature. Thus, X-ray phase contrast holographic nanotomography enables identifying specific morphological structures in 3D in peripheral nerve biopsies from a healthy subject and from patients with type 1 and 2 diabetes.

Peripheral nerve regeneration and intraneural revascularization

Peripheral nerves are particularly vulnerable to injuries and are involved in numerous pathologies for which specific treatments are lacking. This review summarizes the pathophysiological features of the most common traumatic nerve injury in humans and the different animal models used in nerve regeneration studies. The current knowledge concerning Wallerian degeneration and nerve regrowth is then described. Finally, the involvement of intraneural vascularization in these processes is addressed. As intraneural vascularization has been poorly studied, histological experiments were carried out from rat sciatic nerves damaged by a glycerol injection. The results, taken together with the data from literature, suggest that revascularization plays an important role in peripheral nerve regeneration and must therefore be studied more carefully.

Expression Pattern of Myelin-Related Apolipoprotein D in Human Multiple Sclerosis Lesions

Apolipoprotein D (Apo D) is a key molecule in the lipid transport during homeostasis and repair processes in normal and pathological conditions of the nervous system with a putative neuroprotective effect. In the last decades, huge experimental efforts have been made to know the exact mechanism of action of Apo D, even though, it remains an open question. In this regard, studies in mammals and flies have suggested that Apo D seems to act through a variety of cellular mechanisms related with its ability to selectively bind different lipid ligands. For instance, this apolipoprotein is required to myelin compaction, it participates in axon regeneration/remyelination, and it can control the magnitude and timing of the inflammatory response after injury, promoting myelin clearance, and regulating the number of immune cells recruited to the damaged area. These, among others, are some of the reasons to study Apo D in multiple sclerosis (MS) pathology, where it could be particularly important since the autoimmune reaction against oligodendrocytes (OLGs) and myelin is generally assumed as the most plausible cause of this pathology. The aim of this work was to investigate the Apo D expression pattern in MS lesions, including active and inactive demyelinating plaques, and also remyelinating ones. Human brain tissues with inflammatory demyelination consistent with MS were used to quantify Apo D immunosignal in different lesions. Our results show a clear decrease of Apo D expression in all sclerosis plaques, being lower in the inactive than in active areas but recovers in the remyelination ones. Apo D is mainly produced by the matured OLGs of white matter and is located in cell processes surrounding the myelin sheath. All these data seem to indicate an important role of Apo D in myelination/remyelination processes as a molecule with a neuroprotective potential, and may serve as a good starting point for its study in MS.

Label-free imaging of Schwann cell myelination by third harmonic generation microscopy

Understanding the dynamic axon-glial cell interaction underlying myelination is hampered by the lack of suitable imaging techniques. Here we demonstrate third harmonic generation microscopy (THGM) for label-free imaging of myelinating Schwann cells in live culture and ex vivo and in vivo tissue. A 3D structure was acquired for a variety of compact and noncompact myelin domains, including juxtaparanodes, Schmidt-Lanterman incisures, and Cajal bands. Other subcellular features of Schwann cells that escape traditional optical microscopies were also visualized. We tested THGM for morphometry of compact myelin. Unlike current methods based on electron microscopy, g-ratio could be determined along an extended length of myelinated fiber in the physiological condition. The precision of THGM-based g-ratio estimation was corroborated in mouse models of hypomyelination. Finally, we demonstrated the feasibility of THGM to monitor morphological changes of myelin during postnatal development and degeneration. The outstanding capabilities of THGM may be useful for elucidation of the mechanism of myelin formation and pathogenesis.

High-resolution fluorescence microscopy of myelin without exogenous probes

Myelin is a critical element of the central and peripheral nervous systems of all higher vertebrates. Any disturbance in the integrity of the myelin sheath interferes with the axon's ability to conduct action potentials. Thus, the study of myelin structure and biochemistry is critically important. Accurate and even staining of myelin is often difficult because of its lipid-rich nature and multiple tight membrane wraps, hindering penetration of immunoprobes. Here we show a method of visualizing myelin that is fast, inexpensive and reliable using the cross-linking fixative glutaraldehyde that produces strong, broad-spectrum auto-fluorescence in fixed tissue. Traditionally, effort is generally aimed at eliminating this auto-fluorescence. However, we show that this intrinsic signal, which is very photostable and particularly strong in glutaraldehyde-fixed myelin, can be exploited to visualize this structure to produce very detailed images of myelin morphology. We imaged fixed rodent tissues from the central and peripheral nervous systems using spectral confocal microscopy to acquire high-resolution 3-dimensional images spanning the visual range of wavelengths (400-750 nm). Mathematical post-processing allows accurate and unequivocal separation of broadband auto-fluorescence from exogenous fluorescent probes such as DAPI and fluorescently-tagged secondary antibodies. We additionally show the feasibility of immunohistochemistry with antigen retrieval, which allows co-localization of proteins of interest together with detailed myelin morphology. The lysolecithin model of de- and remyelination is shown as an example of a practical application of this technique, which can be routinely applied when high-resolution microscopy of central or peripheral myelinated tracts is required.

Neurite beading is sufficient to decrease the apparent diffusion coefficient after ischemic stroke

Diffusion-weighted MRI (DWI) is a sensitive and reliable marker of cerebral ischemia. Within minutes of an ischemic event in the brain, the microscopic motion of water molecules measured with DWI, termed the apparent diffusion coefficient (ADC), decreases within the infarcted region. However, although the change is related to cell swelling, the precise pathological mechanism remains elusive. We show that focal enlargement and constriction, or beading, in axons and dendrites are sufficient to substantially decrease ADC. We first derived a biophysical model of neurite beading, and we show that the beaded morphology allows a larger volume to be encompassed within an equivalent surface area and is, therefore, a consequence of osmotic imbalance after ischemia. The DWI experiment simulated within the model revealed that intracellular ADC decreased by 79% in beaded neurites compared with the unbeaded form. To validate the model experimentally, excised rat sciatic nerves were subjected to stretching, which induced beading but did not cause a bulk shift of water into the axon (i.e., swelling). Beading-induced changes in cell-membrane morphology were sufficient to significantly hinder water mobility and thereby decrease ADC, and the experimental measurements were in excellent agreement with the simulated values. This is a demonstration that neurite beading accurately captures the diffusion changes measured in vivo. The results significantly advance the specificity of DWI in ischemia and other acute neurological injuries and will greatly aid the development of treatment strategies to monitor and repair damaged brain in both clinical and experimental settings.

Visualization of degenerating axons in a dysmyelinating mouse mutant with axonal loss

Mice homozygously deficient for the myelin component P0 show loss of axons in peripheral nerves. In order to investigate the morphological characteristics of degenerating axons, we crossbred the myelin mutants with a transgenic mouse line expressing yellow fluorescent protein (YFP) in a small proportion of neurons. Peripheral nerves of the double mutants were prepared into small fiber bundles and investigated by fluorescence microscopy. We could identify the tips of degenerating axon as bulb-like structures. Additionally, by electron microscopy, these structures were characterized as axoplasmic extensions containing numerous membraneous compartments. By immunoelectron microscopy, the degenerating end bulbs were in contact with ensheathing Schwann cells that contained YFP-immunoreactivity possibly reflecting phagocytosis of axon material by these cells. Immunohistochemistry using antibodies against macrophages revealed that YFP-positive bulbs, but also other axonal swellings, were often associated with macrophages supporting our previous findings that myelin-related axonal loss is partially mediated by these cells.

C-fiber (Remak) bundles contain both isolectin B4-binding and calcitonin gene-related peptide-positive axons

Unmyelinated nerve fibers (Remak bundles) in the rodent sciatic nerve typically contain multiple axons. This study asked whether C-fiber bundles contain axons arising from more than one type of neuron. Most small neurons of the lumbar dorsal root ganglion (DRG) are either glial cell line-derived neurotrophic factor dependent or nerve growth factor dependent, binding either isolectin B4 (IB4) or antibodies to calcitonin gene-related peptide (CGRP), respectively. Injection of IB4-conjugated horseradish peroxidase into a lumbar DRG resulted in intense labeling of IB4 axons, with very low background. Visualized by confocal fluorescence, IB4-binding and CGRP-positive nerve fibers originating from different DRG neurons came together and remained closely parallel over long distances, suggesting that these two types of axon occupy the same Remak bundle. With double-labeling immunogold electron microscopy (EM), we confirmed that IB4 and CGRP axons were distinct and were found together in single Remak bundles. Previous studies indicate that some DRG neurons express both CGRP and IB4 binding. To ensure that our immunogold results were not a consequence of coexpression, we studied large populations of unmyelinated axons by using quantitative single-label EM. Tetramethylbenzidine, a chromogen with strong intrinsic signal amplification of IB4-horseradish peroxidase, labeled as many as 52% of unmyelinated axons in the dorsal root. Concomitantly, 97% of the Remak bundles with more than one axon contained at least one IB4-labeled axon. Probabilistic modeling using binomial distribution functions rejected the hypothesis that IB4 axons segregate into IB4-specific bundles (P < 0.00001). We conclude that most Remak bundle Schwann cells simultaneously support diverse axon types with different growth factor dependences.

Primary nerve repair by muscle autografts prepared with local anesthetic

We evaluated the usefulness of muscle autografts obtained immediately after graft preparation with lidocaine injections for primary nerve repair. The right sciatic nerve of adult Wistar rats was sectioned, and muscle grafts obtained 15 min or 24 h after lidocaine injection were used to repair a gap 1.5 cm long. Axon and fiber diameters, as well as myelin thickness, decreased to similar extents for grafts of both time intervals. The G-ratios in the distal stumps of both groups were not different from controls, indicating that regenerated axons had a proper level of myelination. The ultrastructural appearance of the neuromuscular junctions was similar to that of normal samples. These results indicate that there are no restrictions to the use of a muscle graft for primary nerve repair, immediately after lidocaine injection, since the nerve regeneration was comparable to that observed with this type of graft used 24 h after being prepared.

Quantitative and qualitative analysis of Wallerian degeneration using restricted axonal labelling in YFP-H mice

We investigated the usefulness of YFP-H transgenic mice [Neuron 28 (2000) 41] which express yellow fluorescent protein (YFP) in a restricted subset of neurons to study Wallerian degeneration in the PNS. Quantification of YFP positive axons and myelin basic protein (MBP) immunocytochemistry revealed that YFP was randomly distributed to approximately 3% of myelinated motor and sensory fibres. Axotomy-induced Wallerian degeneration appeared as fragmentation of fluorescent signals in individual YFP positive axons with a morphology and timing similar to Wallerian degeneration observed by more traditional methods. In YFP-H transgenic mice co-expressing a high dosage of WldS, a chimeric gene that protects from Wallerian degeneration [Nat Neurosci. 4 (2001) 1199], axonal fragmentation in distal tibial nerves after sciatic nerve axotomy was approximately 10 times delayed. Considerable retardations of Wallerian degeneration using the same transgenic expression system were also observed in cultures of nerve explants, enabling in vitro real-time imaging of axonal fragmentation. Remarkably, single YFP-labelled axons could be traced in peripheral nerves for unusually long distances of up to 2.9 cm exploiting confocal fluorescence imaging. Altogether transgenic YFP-H mice prove to be a valuable tool to study mechanisms of Wallerian degeneration in vivo and in vitro.

Acetylcholine receptors and nerve terminal distribution at the neuromuscular junction of non-obese diabetic mice

Skeletal muscle is one of the main targets of the metabolic alterations in diabetes, in which protein synthesis is markedly reduced followed by increased proteolysis. Ultrastructural and functional changes in the presynaptic compartment of the neuromuscular junction (NMJ) have been demonstrated, but little attention has been paid to the proteins in the postsynaptic muscle fiber membrane. In the present work, we studied the changes in acetylcholine receptors (AChRs) and nerve terminal distribution in the NMJ of non-obese diabetic (NOD) mice. The sternomastoid muscles of adult female NOD mice were double-labeled for AChR and nerve terminal observation by fluorescence and reflected light confocal microscopy. In 62.4% of the diabetic endplates, AChR branches broke apart into receptor islands that stained less than in the normal mice. These patches had regular junctional folds. At most of the endplates studied, the nerve terminals colocalized with AChRs, and sprouts were seen in 10% of the diabetic endplates. The intramuscular nerve branches and axons in the nerve to the sternomastoid muscle showed no degenerative disorders. These results suggest that metabolic alterations in the diabetic muscle fiber can affect the distribution and expression of molecules, such as AChRs, in the postsynaptic membrane of the neuromuscular junction.

Assessment of nerve ultrastructure by fibre-optic confocal microscopy

Fibre-optic technology combined with confocality produces a microscope capable of optical thin sectioning. In this original study, tibial nerves have been stained in a rat model with a vital dye, 4-(4-diethylaminostyryl)-N-methylpyridinium iodide, and analysed by fibre-optic confocal microscopy to produce detailed images of nerve ultrastructure. Schwann cells, nodes of Ranvier and longitudinal myelinated sheaths enclosing axons were clearly visible. Single axons appeared as brightly staining longitudinal structures. This allowed easy tracing of multiple signal axons within the nerve tissue. An accurate measurement of internodal lengths was easily accomplished. This technique is comparable to current histological techniques, but does not require biopsy, thin sectioning or tissue fixing. This study offers a standard for further in vivo microscopy, including the possibility of monitoring the progression of nerve regeneration following microsurgical neurorraphy.

Identification of viral infection by confocal microscopy

Confocal microscopy is a valuable adjunct to electron microscopy in the fields of diagnostic and investigative virology. Confocal imaging can be used to examine large amounts of tissue stained by a variety of methods for evidence of viral infection. Areas thus identified can then be processed for ultrastructural study, allowing a highly focused search for viral pathogens. With the possible exception of the vibrating microtome, all of the equipment and reagents necessary for the preparation of specimens for confocal scanning are available in any well-stocked histology laboratory. Although originally developed to facilitate viral diagnosis by EM, the methods described herein can be applied to the ultrastructural study of any focal pathologic process.

Co-ordination of contractile activity in guinea-pig mesenteric lymphatics

1. Intraluminally perfused lymphatic vessels from the mesentery of the guinea-pig were examined in vitro to investigate their contractile activity and the co-ordination of this activity between adjacent lymphangions. 2. Lymphangions constricted at fairly regular intervals and exhibited 'refractory' periods of up to 3 s during which constrictions did not occur. 3. The contractile activity of adjacent lymphangions was highly co-ordinated. 4. The smooth muscle was found to be continuous between the adjacent lymphangions for the majority of valve regions examined morphologically (52 of 63 preparations). 5. Mechanical and electrical coupling between adjacent lymphangions was indicated, as some lymphangions underwent transient dilatations just prior to constriction, whereas direct electrophysiological measurements showed that the smooth muscle of most adjacent lymphangions was electrically coupled across the valve (15 out of 20 pairs of lymphangions). 6. It is concluded that perfused lymphangions of guinea-pig mesenteric lymphatic vessels rhythmically constrict, with the contractile activity of adjacent lymphangions highly co-ordinated. The findings also indicate that transmission of both mechanical and electrical signals between the adjacent lymphangions contribute to the co-ordination of their contractile activity.

Dual staining assessment of Schwann cell viability within whole peripheral nerves using calcein-AM and ethidium homodimer

A membrane permeant nucleic acid stain, ethidium homodimer was used in combination with calcein-AM to document the viability of Schwann cells (SCs) in whole nerves after cold storage assays. Segments of peripheral nerves were, (i) kept intact in buffer (viability controls), (ii) thawed after a cryopreservation, according to a protocol which has been previously shown to maintain the integrity of most nerve components [Ruwe and Trumble, J. Reconstr. Microsurg., 1990, 6: 239-244; Gauthier et al., In 3rd International Symposium on Axonal Regrowth in the Mammalian Spinal Cord and Peripheral Nerve, Deauville, France, 1995, p. 24, abstract], (iii) killed by chemical injury, or (iv) by successive freezing-thawing. Teased preparations of nerve fibers were prepared from the various types of nerve segments and incubated with calcein-AM and ethidium homodimer, which stain, respectively, living and dead cells. In control or cryopreserved nerves, staining with calcein-AM resulted in bright green fluorescence in the cytoplasm of SCs, with no red fluorescence of ethidium homodimer. In contrast, in killed nerve preparations, intense ethidium red fluorescence was observed in SC nuclei, with negligible green calcein cytoplasmic fluorescence. Thus, the combination of calcein-AM/ethidium homodimer appeared as an effective tool for assessing the viability of SCs and determine the quality of cold stored nerve preparations used in graft repair procedures. In addition, the generated fluorescence enabled clear visualization of myelinated fibers by confocal imaging.

Protein zero (P0)-deficient mice show myelin degeneration in peripheral nerves characteristic of inherited human neuropathies

Mutations in the human gene for the myelin recognition molecule protein zero (P0) give rise to severe and progressive forms of dominantly inherited peripheral neuropathies. We have previously reported that mice homozygous for a null mutation in P0 have severely hypomyelinated nerves ten weeks after birth. Here we show hypomyelination already exists at day four with subsequent demyelination and impaired nerve conduction. Furthermore, heterozygous mutants show normal myelination, but develop progressive demyelination after four months of age. Thus, the pathology of homo- and heterozygous P0 mutants resembles that of the severely affected Déjérine-Sottas and the more mildly affected Charcot-Marie-Tooth type 1B patients, respectively.