AXON-SCHWANN CELL RELATIONSHIPS IN NEUROPATHIES OF MUTANT MICE

The cellular immune mechanism after transfer of chemically extracted acellular nerve xenografts

Severe peripheral nerve defect by injuries causing functional loss require nerve grafting. Autograft has limitations for clinical use because it results in the creation of a new nerve injury and the generation of donor site morbidity. Based on these limitations, nerve allografts and xenografts provide a readily accessible alternative strategy. The aim of the present study was to observe the immune mechanism underlying the rejection of chemically extracted acellular nerve xenografts, and further evaluate immunogenicity of chemically treated acellular nerve grafts for clinical applications. A total of 160 BALB/c mice were randomly divided into a negative contrast group (NC, 40 mice), a fresh autograft group (AG, 40 mice), a fresh xenogeneic nerve group (FXN, 40 mice) and a chemically extracted acellular xenogeneic nerve group (CEXN, 40 mice). Various types of nerve grafts were implanted into the thigh muscle of BALB/C mice in the corresponding groups. At 3, 7, 14 and 28 days post-operation, the mice (10 mice from each group) were sacrificed and their spleens were extracted. The spleens were ground into paste. The erythrocytes and other cells were lysed using distilled water and the T lymphocytes were collected. Fluorescein isothiocyanate (FITC) -labeled monoclonal antibodies (CD3, CD4, CD8, CD25, IL-2, IFN-γ and TNF-α) were then added to the solution. The Fluorescence Activated Cell Sorting (FACS) was used to determine the positivity rate of the cells combined with the monoclonal antibodies above. No significant statistical differences were observed between the CEXN, NC and AG groups, so that no obvious immune rejections were observed among the chemically extracted acellular nerve xenografts.

Changes in T lymphocyte subsets and intracellular cytokines after transfer of chemically extracted acellular nerve allografts

The aim of the present study was to observe the immune mechanism underlying the rejection of chemically extracted acellular nerve allografts for use in clinical applications. A total of 128 BALB/c mice were randomly divided into a negative contrast group (NC, 32 mice), a fresh autograft group (AG, 32 mice), a fresh allogeneic nerve group (FN, 32 mice) and a chemically extracted acellular allogeneic nerve group (CEN, 32 mice). Various types of nerve grafts were implanted into the thigh muscle of BALB/C mice in the corresponding groups. At 3, 7, 14 and 28 days post-operation, the mice (8 cases from each group) were sacrificed and their spleens were extracted. The spleens were ground into paste. The erythrocytes and other cells were lysed using distilled water and the T lymphocytes were collected. Monoclonal antibodies (CD3, CD4, CD8, CD25, IL-2, IFN-γ and TNF-α) were then added to the solution. The Facial Action Coding System was used to determine the positive rates of the cells combined with the monoclonal antibodies above. No significant statistical differences were observed between the CEN, NC and AG groups. However, some data of the FN group were significantly higher than those of the other groups at the corresponding time. No obvious immune rejections were observed among the chemically extracted acellular nerve allografts compared with fresh nerve autograft.

Laminins and their receptors in Schwann cells and hereditary neuropathies

This review focuses on the influence of laminins, mediated through laminin receptors present on Schwann cells, on peripheral nerve development and pathology. Laminins influence multiple aspects of cell differentiation and tissue morphogenesis, including cell survival, proliferation, cytoskeletal rearrangements, and polarity. Peripheral nerves are no exception, as shown by the discovery that defective laminin signals contribute to the pathogenesis of diverse neuropathies such as merosin-deficient congenital muscular dystrophy and Charcot-Marie-Tooth 4F, neurofibromatosis, and leprosy. In the last 5 years, advanced molecular and cell biological techniques and conditional mutagenesis in mice began revealing the role of different laminins and receptors in developing nerves. In this way, we are starting to explain morphological and pathological observations beginning at the start of the last century. Here, we review these recent advances and show how the roles of laminins and their receptors are surprisingly varied in both time and place.

The case for a central nervous system (CNS) origin for the Schwann cells that remyelinate CNS axons following concurrent loss of oligodendrocytes and astrocytes

In certain experimental and naturally occurring pathological situations in the central nervous system (CNS), demyelinated axons are remyelinated by Schwann cells. It has always been assumed that these Schwann cells are derived from Schwann cells associated with peripheral nerves. However, it has become apparent that CNS precursors can give rise to Schwann cells in vitro and following transplantation into astrocyte-free areas of demyelination in vivo. This paper compares the behaviour of remyelinating Schwann cells following transplantation of peripheral nerve derived Schwann cells over, and into, astrocyte-depleted areas of demyelination to that which follows transplantation of CNS cells and that seen in normally remyelinating ethidium bromide induced demyelinating lesions. It concludes that while the examination of normally remyelinating lesions can not resolve the origin of the remyelinating Schwann cells, the results from transplantation studies provide strong evidence that the Schwann cells that remyelinate CNS axons are most likely generated from CNS precursors. In addition these studies also indicate that the precursors that give rise to these Schwann cells are the same cells that give rise to remyelinating oligodendrocytes.

The myelinated axon is dependent on the myelinating cell for support and maintenance: molecules involved

The myelin-forming cells, oligodendrocytes and Schwann cells, extend processes that spirally wrap axons and provide the insulation that allows rapid saltatory conduction. Recent data suggest a further role for the myelin-forming cells in axonal support and maintenance. This Mini-Review summarises some of the data that support this view and highlights the molecules involved.

CHEMOKINE EXPRESSION IN NERVE ALLOGRAFTS

OBJECTIVE

Chemokines (chemoattractant cytokines) play a major role in trafficking of cells to areas of inflammation. Infiltration of allograft tissues by immunocompetent cells is critical for rejection of donor tissues. The role of chemokines in nerve allograft rejection is not clear. We hypothesized that chemokines are responsible for attracting macrophages and T lymphocytes into nerve allograft tissue, initiating the graft rejection process.

METHODS

Lewis rats received 4-cm-long peroneal nerve allografts and isografts from ACI and Lewis rats, respectively. Twelve hours to 10 days after transplantation, grafts were removed and total cellular ribonucleic acid was extracted. Intragraft gene expression of several chemokines (cytokine-induced neutrophil chemoattractant, macrophage inflammatory protein [MIP]-2, monocyte chemoattractant protein-1, MIP-1 alpha, and regulated upon activation normal T-cell expressed and secreted [RANTES]) were analyzed by reverse transcription-polymerase chain reaction.

RESULTS

The cytokine-induced neutrophil chemoattractant was expressed in allografts and isografts at early time points (12 h to 6 d). Monocyte chemoattractant protein-1 messenger ribonucleic acid expression was similarly high in both isografts and allografts from 12 hours until 8 days after transplantation. MIP-1 alpha, MIP-2, and RANTES were expressed only in allografts. Kinetics of the neutrophil (MIP-2) and macrophage (MIP-1 alpha) chemokines revealed an early onset (12-24 h), a plateau from 1 to 4 days, and expression abruptly declining by Day 6. The lymphocyte chemoattractant RANTES had delayed kinetics, with a rise at Day 3, a peak at Day 4, and a gradual decline.

CONCLUSION

Induction of specific chemokine genes precedes nerve allograft infiltration by immunocompetent cells. MIP-1 alpha, MIP-2, and RANTES may be responsible for recruiting macrophages, granulocytes, and lymphocytes, respectively, to the rejecting allograft. In future studies, blockade of these specific chemokines or their receptors may prove to delay or prevent nerve allograft rejection.

Secondary axon atrophy and neurological dysfunction in demyelinating neuropathies

PURPOSE OF THE REVIEW

Secondary axonal atrophy is common in most if not all demyelinating neuropathies and is likely responsible for the majority of clinical symptoms. We review clinical, electrophysiological and morphological evidence for secondary axonal atrophy in demyelinating neuropathies and summarize recent hypotheses on possible pathomechanisms.

RECENT FINDINGS

Elucidation of genetic defects responsible for hereditary demyelinating neuropathies and insights into axon-Schwann cell interactions have allowed longitudinal studies of genetically defined demyelinating neuropathies and research into the pathomechanism of secondary axonal atrophy.

SUMMARY

There is ample clinical electrophysiological and electropathological evidence that secondary axonal atrophy is found in hereditary and demyelinating neuropathies. Recognizing secondary axonal atrophy as a main cause for clinical disability in demyelinating neuropathies is important for the clinician and may reveal a therapeutic target common to all different forms of demyelinating neuropathies.

Migration of cells into and out of peripheral nerve isografts in the peripheral and central nervous systems of the adult mouse

Peripheral nerve (PN) isografts provide a favourable environment for axon regeneration after peripheral and central nervous system (CNS) injury, but definitive information on the extent of cellular intermixing between donor and host tissues is lacking. We wished to compare migration patterns in fresh and predegenerate PN grafts, and also compare the extent of cell migration after transplantation to peripheral nervous system (PNS) versus CNS. To discern how host and donor cells interact after PN transplantation, sciatic nerve segments were transplanted from inbred adult mice into PN defects (PN-PN grafts) or into lesioned cerebral cortex of opposite gender siblings. Migrating male cells were identified using a Y-chromosome-specific probe and in situ hybridization methods, and characterized immunohistochemically. The extent of donor and host cellular intermixing was similar in fresh and predegenerate PN-PN isografts. There was substantial intermixing of donor and host cells by 8 days. Many host cells migrating into epineurial regions of grafts were immunopositive for F4/80 (macrophages). The endoneurium of grafted PN was also colonized by host cells; some were F4/80+ but many were immunostained with S-100 (Schwann cell marker). Donor S-100+ Schwann cells rapidly migrated out into proximal and distal host PN and by 12 weeks were found at least 2 mm from the grafts. Endoneurial microvessels in grafts were mostly donor-derived. By comparison, in male PN grafts to female CNS, even after 6 weeks few donor cells had migrated out into surrounding host cortex, despite the observation that almost all grafts contained regenerating axons and were thus attached to host CNS tissue.

Detection of host and donor cells in sex-mismatched rat nerve allografts using RT-PCR for a Y chromosome (H-Y) marker

The donor and host source of support cells, such as Schwann cells, in nerve allograft segments have been the subject of debate. The objective of the present study was to assess the utility of a molecular technique that probes for a Y chromosome expressed gene (H-Y) in distinguishing host from donor tissue in sex-mismatched nerve allograft segments. Forty-two Lewis rats received bilateral syngeneic Lewis or allogeneic ACI rat peroneal nerve grafts, with or without cyclosporin A (CsA) treatment. At different times thereafter animals were sacrificed and samples were harvested. We transplanted males and females reciprocally, to study both survival of donor cells (persisting H-Y mRNA in male grafts by transcription polymerase chain reaction (RT-PCR), and graft infiltration by host cells (detectable H-Y mRNA in female grafts). A kinetic analysis revealed a progressive loss of viable donor cells (loss of H-Y mRNA signal) from allografts, beginning 2-3 weeks, and culminating at 4 weeks, with little detectable H-Y in the absence of CsA treatment. CsA treatment led to prolonged survival of allograft cells, confirmed by detectable H-Y mRNA. By studying female grafts in male rats we could confirm that loss of viable donor tissue in allografts was accompanied by infiltration of host (H-Y mRNA positive) cells, whereas no H-Y mRNA signal was seen in males receiving autografts from females or in immunosuppressed allograft segments. These data suggest that reverse RT-PCR analysis for a Y chromosome gene product can be a valuable tool to assess the origin of viable cells in sex-mismatched nerve allotransplantation tissue.

Successful nerve regeneration and persistence of donor cells after a limited course of immunosuppression in rat peripheral nerve allografts

BACKGROUND

The origin of Schwann cells and effect of a limited course of immunosuppression using cyclosporine (CsA) were examined in rat peripheral nerve allotransplants.

METHODS

Phenotypes of Schwann cells in groups without, with continuing, and with limited (12 weeks) CsA treatment were examined immunohistochemically in allogeneically and syngeneically transplanted animals from 4 to 36 weeks after transplantation.

RESULTS

In the group receiving no CsA, little nerve regeneration was obtained; donor Schwann cells were rejected and replaced by recipient cells. In continuing and limited-course CsA groups, successful nerve regeneration was achieved at postoperative week 36, as was also observed in the syngeneic group. Schwann cells in the continuing CsA group remained donor-derived. In the limited-course CsA group, graft rejection and loss of function occurred after the withdrawal of CsA, and donor Schwann cells were replaced by recipient cells in the part of the graft where rejection had been complete. However, many donor Schwann cells remained at week 36, when the rejection response subsided.

CONCLUSION

Possible clinical use of a limited course of immunosuppression was supported by this demonstration of long term persistence of donor Schwann cells.

Myelin mutants: model systems for the study of normal and abnormal myelination

Spontaneous mutations that perturb myelination occur in a range of species including man, and together with engineered mutations have been used to study disease, normal myelination and axon/glial inter-relationships. Only a minority of the currently defined mutations have an apparently simple pathogenesis due to lack of a functional protein. Mutations in the myelin basic protein gene lead to a lack of protein, resulting in changes in the structure of myelin, which can be rescued by transgenic complementation. The pathogenesis of autosomal dominant and X-linked mutations affecting either oligodendrocytes or Schwann cells is more complex. Point mutations may act in a dominant negative manner and gene dosage is clearly linked to phenotypic change. Mutations in regulatory genes, such as those encoding transcription factors, can also disturb myelination by selected cell types. Other less-well studied and unexpected consequences of myelin mutations, such as seizures in mutations affecting genes expressed in Schwann cells and axonal changes associated with dysmyelination, are also considered. With the major developments in gene mapping and cloning it is now relevant to study mutations in a variety of species with the real prospect of defining their molecular basis. Examples are given of unusual, but potentially useful, uncharacterized mutations in dog and bovine.

Prolonged survival of transected nerve fibres in C57BL/Ola mice is an intrinsic characteristic of the axon

Transected axons in C57BL/Ola mice survive for extraordinary lengths of time as compared to those of normal rodents. The biological difference in the substrain that confers the phenotype of prolonged axonal survival is unknown. Previous studies suggest that 'defect' to be a property of the nervous system itself, rather than one of haematogenous cells. Neuronal or non-neuronal elements could be responsible for this phenotype. This study was undertaken to determine whether Schwann cells, the most numerous of the non-neuronal cells intrinsic to the peripheral nerve, are responsible for delayed degeneration of transected axons. We created sciatic nerve chimeras by transplanting nerve segments between standard C57BL/6 and C57BL/Ola mice, allowing regeneration of host axons through the grafts containing donor Schwann cells. These nerves were then transected and the time course of axonal degeneration was observed. The results show that fast or slow degeneration is a property conferred by the host, and therefore cannot be ascribed to the Schwann cells. Similarly, transected C57BL/Ola axons in explanted dorsal root ganglia cultures survived longer than transected axons from standard mice. Taken together these results indicate that the responsible abnormality is intrinsic to the C57BL/Ola axon.

Comparison of regeneration across nerve allografts with temporary or continuous cyclosporin A immunosuppression

The efficacy of short-term immunosuppression in a nerve allograft model was examined by comparing regeneration across peripheral nerve allografts with either temporary (12 weeks) or continuous (30 weeks) cyclosporin A treatment. One-hundred fifty Lewis rats received 2-cm nerve grafts from allogeneic ACI or syngeneic Lewis rat donors and were allocated to the following groups: allogeneic grafts and continuous cyclosporin A, with 18 weeks (20 rats) or 30 weeks (20 rats) of survival after graft placement; allogeneic grafts and temporary cyclosporin A, with 12 weeks (10 rats), 18 weeks (20 rats), or 30 weeks (20 rats) of survival; and control rats with allogeneic and syngeneic grafts, no cyclosporin A, with 12, 18, or 30 weeks (10 rats each) of survival. Functional regeneration across the nerve grafts was serially assessed with walking-track analysis. Endpoint evaluations included electrophysiological, histological, and morphometric studies. Both walking-track and electrophysiological function reached a plateau at a significantly worse level in nonimmunosuppressed allograft recipients than in syngeneic or treated allograft recipients. The group with temporary therapy experienced significant worsening in both motor and electrophysiological function at Week 18, 6 weeks after cyclosporin A withdrawal, compared to the group with continuous treatment. At Week 30, motor and electrophysiological function in the temporary-treatment group recovered to levels similar to those of the syngeneic and continuous cyclosporin A groups. Histological assessment of the graft segments from the temporary cyclosporin A group at 18 weeks showed evidence of rejection, with mononuclear cell infiltration and demyelination; morphometric evaluation demonstrated significantly decreased numbers of nerve fibers in the distal host segment. These histological and morphometric changes were no longer present in the nerves from the temporarily immunosuppressed rats at Week 30. Withdrawal of immunosuppression after successful regeneration through nerve allografts results in short-term graft rejection. Eventual restoration of graft histological and function parameters is comparable to continuously immunosuppressed rats. Temporary immunosuppression of nerve allograft recipients is feasible.

Schwann cell proliferation following lysolecithin-induced demyelination

Schwann cell division, meticulously regulated throughout development, occurs at an extremely low level in normal adult nerves. Loss of the myelin sheath in disease results in active proliferation of Schwann cells. The dividing cells are usually thought to be the Schwann cells of the demylinated fibres and their daughters. In this study we asked if other populations of Schwann cells might also divide following focal monophasic demyelination, and if the proliferating Schwann cells would be found only in the foci of demyelination. [3H]thymidine incorporation was examined by autoradiography at intervals after topical application of lysolecithin (lysophosphatidyl choline) to rat sciatic nerves. The postlabelling intervals were set to identify premitotic cells, cells shortly after mitosis (perimitotic cells) and postmitotic cells, as well as to provide cumulative labelling over 3 days. The affected nerves had three distinct zones. The first was a zone of nearly complete demyelination immediately beneath the perineurium. The subjacent zone was normal morphologically except for numerous supernumerary Schwann cells, displacement of some Schwann cell perikarya, ultrastructural changes in a few myelinated fibres, and rare demyelinated and remyelinated fibres. The third zone, beneath the first two, was normal. In the focus of demyelination there were large numbers of Schwann cells in S phase on days 4 and 6. These cells included premyelinating Schwann cells that were contacting or ensheathing demyelinated axons or collateral axonal sprouts. The subjacent region also contained dividing Schwann cells, most of which were Schwann cells of unmyelinated Remak fibres. In addition, occasional Schwann cells of thickly myelinated fibres (fibres that had not previously undergone demyelination) were labelled by the premitotic schedule; most of these fibres had morphological abnormalities in the Schwann cell perikaryon or myelin sheath. In many, the perikaryon of the Schwann cell was beginning to separate from the rest of the Schwann cell cytoplasm and the myelin sheath. These changes suggested that these fibres were destined to undergo subsequent demyelination, a hypothesis supported by the absence of any normal myelinated fibres with labelled Schwann cell nuclei in nerves removed 1 week after labelling. Thus, this model provided no evidence for division by Schwann cells that continued to maintain myelin sheaths. Taken together, these results suggest that there is a 'surround' of Schwann cell proliferation around foci of demyelination; in this surround multiple populations of Schwann cells are recruited to proliferate, including Schwann cells of intact unmyelinated fibres. Structurally normal unmyelinated fibres appear to provide an unexpected source of new Schwann cells in nerve disease.

Schwann cell-neuronal interactions in the rat involve nerve growth factor

To gain some insight into possible functions of nerve growth factor (NGF), we suppressed the endogenous levels of NGF in newborn rats by subcutaneous injections (3 microliters/g body weight) of rabbit antibodies to purified mouse beta-NGF (ANTI-NGF). Fiber and axonal areas and perimeters were measured for unmyelinated and myelinated sensory fibers in T9 dorsal roots (DR) in three groups of animals: 1) ANTI-NGF treated littermates, 2) preimmune sera treated littermates (PREIMM), and 3) untreated littermates (UNTR). In some rats, fibers in ventral roots (VR) were measured and, in other rats, sensory processes in peripheral nerves (PN) were measured following radical ventral rhizotomy. The only outer area and perimeter measurements that were statistically different were those in the ventral root (P less than 0.013 and P less than 0.043, respectively). However, myelin thickness was significantly thinner in the dorsal roots of the ANTI-NGF group than in the dorsal roots of the UNTR and PREIMM groups (P less than 0.000009 and P less than 10(-6), respectively). Myelin thickness in the ventral roots of the ANTI-NGF group was also statistically thinner than that in the UNTR group (P less than 0.001). There were no statistically significant differences when comparing the UNTR group to the PREIMM group. In the peripheral nerves studied, there was no significant change in the myelin thickness between the ANTI-NGF and UNTR groups of animals. These results indicate that Schwann cell-neuronal interactions are altered by the inactivation of NGF, and that 1) the central processes of sensory fibers are affected and not the peripheral processes and 2) motor fiber myelination is altered.(ABSTRACT TRUNCATED AT 250 WORDS)

The effects of intrauterine growth retardation on the structural development of cranial nerves (optic, trochlear) in fetal sheep

A quantitative morphometric study of the development of myelinated fibres in the optic and trochlear nerves has been made in growth-retarded fetal sheep at 140 days gestation (term = 146 days). Intrauterine growth retardation was induced as a result of the reduction of placental mass, by prior removal of placentation sites in six ewes. In the optic nerve (central nervous system) the mean diameter of myelinated fibres was not significantly reduced but the thickness of the myelin sheath relative to axon diameter was disproportionately reduced. In the trochlear nerve (peripheral nervous system) there was a significant reduction of 23% (P less than 0.01) in the mean diameter of myelinated fibres; however the normal axon:myelin ratio was maintained. The total number of myelinated fibres in the trochlear nerve did not differ between the normal and growth-retarded group, indicating that there was not a greater than normal incidence of cell death during intrauterine growth retardation in the nucleus of the trochlear nerve. The differential effect of intrauterine growth retardation on myelination in the central and peripheral nervous systems suggests that chronic intrauterine deprivation affects oligodendrocyte activity but does not markedly affect the capacity of Schwann cells to produce myelin.

Conditioning lesion effects on rat sciatic nerve regeneration are influenced by electrical stimulation delivered to denervated muscles

The rate of regeneration and the initial delay of the fastest growing fibers of the rat sciatic nerve were electrophysiologically evaluated after a freeze at mid-thigh. A prior section or a prior freeze at the ankle level increased the rate of regeneration and decreased the initial delay with different magnitudes. These phenomena are named 'conditioning lesion effects'. A daily electrical stimulation transcutaneously delivered to the foot sole muscles from the day following their denervation by the prior lesion did not modify the increased rate of regeneration but prevented the decrease of the initial delay whatever the type of the prior lesion. Therefore, the initiation of earlier sprouting of the parent axons seems to be specifically controlled by a signal associated with muscle denervation properties.

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.

Inherited neuromuscular diseases in the mouse. A review of the literature

There are several neuromuscular disorders affecting the human being. Most of these are poorly understood and lack and effective treatment. Due to the limitation of experimental manipulation in "anima nobili", inherited neuromuscular diseases in laboratory animals constitute a valuable source of scientific information. Amongst several animal species affected by neuromuscular disorders the house mouse is of particular interest because of its small size, short pregnancy and low costs of maintanence. In the present review 20 murine mutants with diseases affecting peripheral nerves, skeletal muscles and motor end-plates are tabulated. Genetic, clinical and pathological aspects are discussed aiming to provide information about these mutants which might be of great interest as animal models for human neuromuscular diseases.

The distribution of (Na+ + K+)ATPase is continuous along the axolemma of unensheathed axons from spinal roots of 'dystrophic' mice

(Na+ + K+)ATPase-like immunoreactivity along the axolemma of sensory and motor neurons and the plasmalemma of Schwann cells from spinal roots of dystrophic mice (129 ReJ Dy/Dy) was determined using polyclonal antibodies specific for guinea pig renal (Na+ + K+)ATPase (GP-17), along with polyclonal (439-2) and monoclonal (9A5) antibodies specific for rat renal (Na+ + K+)ATPase. In normal and dystrophic mice, (Na+ + K+)ATPase-like immunoreactivity was observed along the axolemma at nodes of Ranvier using GP-17 and 439-2, each of which binds to isozymes of (Na+ + K+)ATPase composed of the alpha and alpha + forms of the catalytic subunit. Staining was not seen along the nodal axolemma with 9A5, a preparation that binds to the alpha form of the catalytic subunit. The terminal processes and microvilli of Schwann cells were stained using all three antibody probes. The axolemma of unensheathed axons in dystrophic mice was continuously and uniformly labelled with GP-17 and 439-2, but not 9A5. Concentrations of (Na+ + K+)ATPase-like immunoreactivity along Schwann cell processes were observed most often in areas adjacent to unensheathed axolemma. At heminodes, staining abruptly decreased along Schwann cell processes in areas that were separated from the unensheathed axolemma by other intervening Schwann cell processes. It was concluded from these data that in dystrophic mice (Na+ + K+)ATPase is uniformly distributed along unensheathed portions of axons without evidence of detectable focal concentrations of the enzyme, and that the catalytic subunit of (Na+ + K+)ATPase along unensheathed axons is distinct from the alpha form found in Schwann cells and other organs. In addition, (Na+ + K+)ATPase is concentrated along the plasmalemma of Schwann cells in regions of close apposition to axolemmal areas associated with large ionic fluxes.

An electron microscope study of quantitative relationships between axon and Schwann cell sheath in myelinated fibres of peripheral nerves

The quantitative relationships between the cross-sectional area of the Schwann cell sheath (myelin included) and that of its related axon were studied by electron microscopy in the nerve fibres of the spinal roots of lizard (Lacerta muralis). In both ventral and dorsal roots the cross-sectional area of the Schwann cell sheath (myelin included) was found to be directly proportional to that of its related axon (correlation coefficients between 0.88 and 0.92). The ratio between the cross-sectional area of the Schwann cell sheath (myelin included) and that of its related axon tends to diminish as the cross-sectional area of the latter increases. Thus, under normal conditions, in myelinated fibres of the spinal roots of the lizard a quantitative balance exists between the nerve tissue and its associated glial tissue. This result agrees with those previously obtained in the spinal ganglia of the lizard, gecko, cat and rabbit. Some of the mechanisms probably involved in the control of the quantitative balance between nerve tissue and its associated glial tissue in peripheral nerves are presented and discussed.

Neuronal growth factors produced by adult peripheral nerve after injury

Dorsal root ganglion neurons from embryonic rats, co-cultured with endoneurial explants from transected, adult rat sciatic nerve, extended neurites in the absence of exogenous nerve growth factor (NGF). The effect was seen with endoneurial explants from normal adult sciatic nerves or from nerves which had been permanently transected up to 51 days prior to explantation. The rate of outgrowth decreased at 5 and 7 days and reached a minimum at 14 days after transection. A second phase of increased neurite-promoting activity appeared in 28-, 35-, 41- and 51-day posttransection tissue. The early phase, but not the late phase, was partially inhibited by antisera to NGF.

A quantitative study of developing axons and glia following altered gliogenesis in rat optic nerve

Axonal and glial cell development within rat optic nerve in which gliogenesis was altered by systemic injection of 5-azacytidine (5-AZ) was examined by quantitative electron microscopy. In neonatal (0-2 days) rat optic nerves, all fibers are premyelinated, and they exhibit a fairly uniform diameter (approximately 0.22 micron). These fibers occupy approximately 55% of the optic nerve volume. At this early age, glia within the optic nerve consist only of cells of astrocytic lineage and progenitor cells. These glia occupy approximately 28% of the optic nerve volume, and there are approximately 80 glial cells/optic nerve cross section. In 14-day-old normal optic nerves, myelinated and ensheathed fibers comprise approximately 17% and 9%, respectively, of the total number of axons. Mean axonal diameter of myelinated fibers is approximately 0.75 micron, while mean diameter for ensheathed axons is approximately 0.50 micron. By volume, these fibers occupy approximately 25% of the nerve, which is similar to the volume occupied by premyelinated axons in these nerves. At 14 days of age, there are approximately 300 glial cells/optic nerve transverse section, and these glia occupy approximately 37% of the volume in normal optic nerve. Oligodendroglia represent approximately 40% of total glial cells present, while astroglia and progenitor cell each comprise approximately 30% of the cells. In optic nerves from 14-day-old rats treated with 5-AZ, few myelinated fibers are present and the number of oligodendroglia is markedly reduced. Axonal diameter of premyelinated fibers is similar to that of age-matched controls. Myelinated and ensheathed fibers comprise approximately 2% of the total fibers present in 5-AZ-treated optic nerves, with the remaining fibers being premyelinated. The few myelinated and ensheathed fibers present in 5-AZ-treated optic nerves display similar axonal diameters to corresponding fibers from age-matched control tissue. Glial cells occupy approximately 40% of the nerve volume, and there are approximately 200 glia/nerve cross section in 5-AZ-treated rats. Astroglia comprise approximately 63% of the total glial cells, while approximately 12% of the cells are oligodendroglia. These results demonstrate that 5-AZ is a potent inhibitor of oligodendrogliogenesis, with a concomitant marked reduction in the number of myelinated fibers.

A quantitative assessment of myelin sheaths in the peripheral nerves of dystrophic, quaking, and trembler mutants

If myelin sheaths are relatively thin for axon caliber, this is generally taken as a sign of insufficient myelin formation. However, recent studies have shown that sheath thickness relates not only to axon caliber; the relative length of the internode is also important. Foreshortened internodes have slightly thinner sheaths than long internodes of the same fiber caliber (Friede and Bischhausen 1982). In the present study we compared sheath thickness with internode geometry in the sciatic fibers of three murine mutants, the Dystrophic, Quaking and Trembler mice, using a new computer-assisted method. A quantitative correspondence was found between abnormally thin sheaths and internode foreshortening. The magnitude of the changes was the same as that found previously in normal and regenerated fiber populations. The data show that the geometric proportions of internodes cannot be ignored when assessing sheath thickness, and they also shed some new light on the mechanisms which produce abnormally thin sheaths.

Hypomyelination in the peripheral nervous system of shiverer mice and in shiverer in equilibrium normal chimaera

In shiverer mice, the P1 component of myelin basic protein (MBP) is deficient in both the central nervous system (CNS) and peripheral nervous system (PNS) but compact myelin is more grossly defective in the CNS. In the PNS, myelin exhibits a normal periodic structure, and although examples of subtle abnormalities of shiverer Schwann cell ultrastructure have been described previously, myelin thickness has been reported as unremarkable when observed by light microscopy. We report a quantitative investigation of the myelin sheath thickness of shiverer Schwann cells in which a mild but apparently consistent hypomyelination of axons ensheathed by shiverer Schwann cells was observed. This abnormality was expressed both in the peripheral nerves of a homozygous shiverer mouse and in the shiverer Schwann cells populating the mosaic nerves of a mature shiverer in equilibrium normal mouse chimaera. In addition, multiple interlamellar gaps was found to be a highly consistent feature of shiverer myelin. These observations extend the description of the peripheral nerve defects expressed in shiverer mice and further define these abnormalities as direct consequences of the shiverer Schwann cells' intrinsic genotype. In light of these results, a significant role for P1 in the formation and/or maintenance of normal myelin in the PNS is suggested.

Limited remyelination of CNS axons by Schwann cells transplanted into the sub-arachnoid space

Areas of primary demyelination which did not subsequently remyelinate spontaneously were prepared in the cat spinal cord by injecting small volumes of ethidium bromide into tissue which had previously been exposed to 40 Grays of X-irradiation. Autologous peripheral nerve tissue was placed in the sub-arachnoid space over such lesions, either at the time of injecting ethidium bromide, or at 14 days or 28 days after injecting ethidium bromide. The extent of Schwann cell remyelination was assessed 28 days after transplantation. In no case were all the demyelinated axons remyelinated; rather, remyelination was limited to axons near to blood vessels. It was concluded that Schwann cells migrated from the transplanted tissue into the lesion via the perivascular space and that they failed to remyelinate the bulk of demyelinated axons because of an absence within the CNS of suitable extracellular matrix.

Defective Schwann cell function in canine inherited hypertrophic neuropathy

Segments of peripheral nerve from dogs with canine inherited hypertrophic neuropathy ( CIHN ) were transplanted to the transected sciatic nerve of immuno incompetent mice. Regenerating mouse axons penetrated the grafts and were myelinated by dog Schwann cells. In grafts collected 3 or more months after transplantation, filamentous or granular material, identical to that occurring in nerves of affected dogs, accumulated in myelinating Schwann cells. Demyelinated fibers were only rarely found in grafted segments of affected nerve. Neither filamentous accumulations nor demyelination were observed in grafts of control canine nerve. These results indicate that CIHN is associated with a defect in Schwann cell function, and the abnormal accumulations of filaments suggest that the defect may be in the cytoskeleton. The rarity of demyelination in grafts suggests that some factor in addition to the Schwann cell defect is required to precipitate myelin destruction.

Hereditary motor and sensory neuropathy of demyelinating and remyelinating type in children. Ultrastructural and morphometric studies on sural nerve biopsy specimens from ten sporadic cases

Ten autosomal recessive/sporadic cases of hereditary motor and sensory neuropathy type I (HMSN I), nine of which originated from the northern part of Sweden, were included in the study. Parents were free from neurologic symptoms. Motor and sensory conduction velocity was normal when recorded, i.e., in 19 and 17 parents, respectively. Sural nerve biopsies from the ten cases revealed a varying degree of onion bulb formation. In eight of the cases the onion bulbs consisted of abundant basement membranes, whereas the Schwann cells were few and sometimes lacking. There were in some cases considerable differences between separate fascicles as to the loss of myelinated nerve fibers. In the six biopsies in which teasing was performed signs of present and previous demyelination were noticed. Numerous internodal segments were abnormally thin with reference to their length. In many such segments there were marked local thickenings of the nerve fiber. In cross sections the probable counterparts to these thickenings were nerve fibers with unduly thick myelin sheaths and complex folding of the myelin. Ultrastructural axonal changes were seen in the majority of the cases. The pathogenetic and diagnostic implications of the present findings are discussed.

Effects of crush injury on the abnormalities in the spinal roots and peripheral nerves of dystrophic mice

Lumbosacral spinal roots and peroneal nerves in dystrophic and control mice were crushed and allowed to regenerate. Six weeks after crush injury, the dystrophic roots no longer showed the typical groups of unensheathed axons that characterize the uncrushed roots. Thus, the location of this ensheathment defect in the spinal roots cannot be the exclusive mechanism responsible for its development. Crush injury and regeneration also tended to correct a second abnormality in the peripheral nervous system of dystrophic mice: the discontinuities in the Schwann cell basal laminas. Because the regenerated nerves contained increased amounts of collagen, the results of this study support the evidence from tissue culture experiments that the extracellular matrix may be involved in the pathogenesis of these disorders. However, the outcome of the present in vivo experiments indicates that genetically normal fibroblasts are not required for this change to occur.

Quantitative studies of the abnormal axon-Schwann cell relationship in the peripheral motor and sensory nerves of the dystrophic mouse

The nature and extent of abnormal axon-Schwann cell relationships in peripheral portions of dystrophic motor and sensory nerves were quantitatively evaluated between 1 and 9 months of age using teased fibres and electron micrographs. The results show that in the dystrophic (dy/dy) common peroneal (CPN) and tibial nerves (TN), and less in the dy/dy sural nerve (SN): (1) the number of Schwann cell nuclei associated with myelinated axons is increased with respect to normal; (2) the average internodal length is correspondingly reduced; (3) the average dystrophic internode elongates roughly in parallel with the average normal internode, and with the dystrophic limb; the longitudinal growth of the dystrophic limb is normal; (4) the variation of internodal length is greater than normal; it does not increase with age; (5) the incidence of the nodes of Ranvier which are wider than the normal 3 micrometers limit does not increase with age; and (6) the number of myelinated axons is reduced in the dy/dy CPN and TN but not in the dy/dy SN; it shows no change with age. These data indicate that: (1) in the dy/dy peripheral nerves (PNS) the abnormal axon-Schwann cell relationships and the reduced number of myelinated axons have been established prior to 1 month of age, thereafter progressive degenerative processes do not appear to take place, and (2) the dy/dy sensory nerves are less affected than the motor ones.

Differentiation of the nodal and internodal axolemma in the optic nerves of neonatal rats

Axon plasma membranes (axolemma) were studied by freeze-fracture electron microscopy at stages prior to and during myelination in the optic nerves of neonatal rats. In unensheathed axons, intramembranous particles associated with the internal (P) and external (E) leaflets of the axolemma increased in number before reaching a plateau (approximately 600/micron2 in both leaflets) at about 9 days postnatally. In newly myelinated fibres, by contrast, the distribution of particles was asymmetrical; fewer particles (approximately 200/micron2) were found on the E-face and greater numbers (approximately 1400/micron2) were present on the P-face, distributions similar to those observed in mature myelinated fibres. Node-like aggregations of particles were not found in unensheathed pre-myelinated axons nor were they present in axons presumed to be ensheathed by glial cytoplasm but not yet myelinated, although nodal specializations could be easily identified in fibres with only a few turns of compact myelin. These observations show first that there is a redistribution of particles in the P- and E-faces of the internodal axolemma coincident with the onset of myelination and secondly, that nodal specializations (represented by the increased densities of E-face particles) appear after ensheathment but before the formation of compact myelin in fibres of the rat optic nerve.

Inferior alveolar nerve regeneration and incisor pulpal reinnervation following intramandibular neurotomy in the cat

Regeneration of the inferior alveolar nerve and mandibular incisor pulpal reinnervation was qualitatively and quantitatively examined by electron microscopy 2 days--11 months after intramandibular neurotomy in young adult cats. Fifteen millimeters central to the proximal stump moderate atrophic alterations of myelinated axons were observed 1--2 months after surgery. By 4--11 months a principally normal picture had been restored. The proportion of unmyelinated axons was increased 2--4 months after operation but had normalized by 11 months. In the distal stump the first regenerating axons were observed at 2 weeks. The regenerated myelinated axons failed to re-establish the previous fibre size range and normal axo-glial relations did not appear. A seemingly stable morphological pattern was reached 4--11 months postoperatively. In the late survival period the proportion of unmyelinated axons was subnormal. In the incisor pulps virtually all axons disappeared after surgery. By two weeks pulpal reinnervation had begun. From two months on, a structurally largely normal pulpal axon population was present except for some persisting unmyelinated axon degeneration. The findings are consistent with previous physiological data and suggest that structural normalization at proximal and preterminal levels follows upon re-establishment of peripheral contacts.

Purification of mouse Schwann cells using neurite-induced proliferation in serum-free monolayer culture

We have recently reported that neonatal mouse dorsal root ganglionic Schwann cells will (i) survive and assume characteristic morphologies in a serum-free, fully defined cultured medium (N1 medium), (ii) proliferate extensively in the same N1 medium if neurons are also present and maintained by nerve growth factor, and (iii) display a strong proliferative response to serum even in the absence of neuronal elements, while also undergoing marked changes in their morphology and their associative behavior toward neurites. In this report, we present a detailed procedure, based upon these earlier observations, which yields purified cultures of either neurons plus associated Schwann cells or Schwann cells in the absence of neurons. The procedure utilizes the neuritic mitogen for selective expansion of Schwann cell numbers in serum-free primary cultures, and a secondary culture step involving neuronal removal and additional Schwann cell expansion using the serum mitogen. The procedure requires 9 days for the generation of 3-4 X 10(6) Schwann cells from 12 newborn mice (with a Schwann cell to neuron ration of 10) and an additional 6-7 days for the generation of a neuron-free secondary population of 40 X 10(6) Schwann cells with less than 3% contamination by identifiable ganglionic fibroblasts.