Development of myelinated nerve fibers in the sixth cranial nerve of the rat: a quantitative electron microscope study

Kir4.1 is specifically expressed and active in non-myelinating Schwann cells

Non-myelinating Schwann cells (NMSC) play important roles in peripheral nervous system formation and function. However, the molecular identity of these cells remains poorly defined. We provide evidence that Kir4.1, an inward-rectifying K+ channel encoded by the KCNJ10 gene, is specifically expressed and active in NMSC. Immunostaining revealed that Kir4.1 is present in terminal/perisynaptic SCs (TPSC), synaptic glia at neuromuscular junctions (NMJ), but not in myelinating SCs (MSC) of adult mice. To further examine the expression pattern of Kir4.1, we generated BAC transgenic Kir4.1-CreERT2 mice and crossed them to the tdTomato reporter line. Activation of CreERT2 with tamoxifen after the completion of myelination onset led to robust expression of tdTomato in NMSC, including Remak Schwann cells (RSC) along peripheral nerves and TPSC, but not in MSC. In contrast, activating CreERT2 before and during the onset of myelination led to tdTomato expression in NMSC and MSC. These observations suggest that immature SC express Kir4.1, and its expression is then downregulated selectively in myelin-forming SC. In support, we found that while activating CreERT2 induces tdTomato expression in immature SC, it fails to induce tdTomato in MSC associated with sensory axons in culture. NMSC derived from neonatal sciatic nerve were shown to express Kir4.1 and exhibit barium-sensitive inwardly rectifying macroscopic K+ currents. Thus, this study identified Kir4.1 as a potential modulator of immature SC and NMSC function. Additionally, it established a novel transgenic mouse line to introduce or delete genes in NMSC.

The Larval Zebrafish Vestibular System Is a Promising Model to Understand the Role of Myelin in Neural Circuits

Myelin is classically known for its role in facilitating nerve conduction. However, recent work casts myelin as a key player in both proper neuronal circuit development and function. With this expanding role comes a demand for new approaches to characterize and perturb myelin in the context of tractable neural circuits as they mature. Here we argue that the simplicity, strong conservation, and clinical relevance of the vestibular system offer a way forward. Further, the tractability of the larval zebrafish affords a uniquely powerful means to test open hypotheses of myelin's role in normal development and disordered vestibular circuits. We end by identifying key open questions in myelin neurobiology that the zebrafish vestibular system is particularly well-suited to address.

Schwann cells and deleted in colorectal carcinoma direct regenerating motor axons towards their original path

After complete nerve transection, a major challenge for regenerating peripheral axons is to traverse the injury site and navigate toward their original trajectory. Denervated Schwann cells distal to the lesion site secrete factors promoting axonal growth and serve as an axonal substrate, yet whether Schwann cells also actively direct axons toward their original trajectory is unclear. Using live-cell imaging in zebrafish, we visualize for the first time how in response to nerve transection distal Schwann cells change morphology as axons fragment, and how Schwann cell morphology reverses once regenerating growth cones have crossed the injury site and have grown along distal Schwann cells outlining the original nerve path. In mutants lacking Schwann cells, regenerating growth cones extend at rates comparable with wild type yet frequently fail to cross the injury site and instead stray along aberrant trajectories. Providing growth-permissive yet Schwann cell-less scaffolds across the injury site was insufficient to direct regenerating growth cones toward the original path, providing compelling evidence that denervated Schwann cells actively direct regenerating axons across the injury site toward their original trajectory. To identify signals that guide regenerating axons in vivo, we examined mutants lacking the deleted in colorectal carcinoma (DCC) guidance receptor. In these dcc mutants, a significant fraction of regenerating motor axons extended along aberrant trajectories, similar to what we observe in mutants lacking Schwann cells. Thus, Schwann cell and dcc-mediated guidance are critical early during regeneration to direct growth cones across the transection gap and onto their original axonal trajectory.

In vivo nerve-macrophage interactions following peripheral nerve injury

In vertebrates, the peripheral nervous system has retained its regenerative capacity, enabling severed axons to reconnect with their original synaptic targets. While it is well documented that a favorable environment is critical for nerve regeneration, the complex cellular interactions between injured nerves with cells in their environment, as well as the functional significance of these interactions, have not been determined in vivo and in real time. Here we provide the first minute-by-minute account of cellular interactions between laser transected motor nerves and macrophages in live intact zebrafish. We show that macrophages arrive at the lesion site long before axon fragmentation, much earlier than previously thought. Moreover, we find that axon fragmentation triggers macrophage invasion into the nerve to engulf axonal debris, and that delaying nerve fragmentation in a Wld(s) model does not alter macrophage recruitment but induces a previously unknown 'nerve scanning' behavior, suggesting that macrophage recruitment and subsequent nerve invasion are controlled by separate mechanisms. Finally, we demonstrate that macrophage recruitment, thought to be dependent on Schwann cell-derived signals, occurs independently of Schwann cells. Thus, live cell imaging defines novel cellular and functional interactions between injured nerves and immune cells.

New insights into signaling during myelination in zebrafish

Myelin is a vertebrate adaptation that allows for the rapid propagation of action potentials along axons. Specialized glial cells-oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS)-form myelin by repeatedly wrapping axon segments. Debilitating diseases result from the disruption of myelin, including multiple sclerosis and Charcot-Marie-Tooth peripheral neuropathies. The process of myelination involves extensive communication between glial cells and the associated neurons. The past few years have seen important progress in understanding the molecular basis of the signals that coordinate the development of these fascinating cells. This review highlights recent advances in myelination deriving from studies in the zebrafish model system, with a primary focus on the PNS. While Neuregulin1-ErbB signaling has long been known to play important roles in peripheral myelin development, work in zebrafish has elucidated its roles in Schwann cell migration and radial sorting of axons in vivo. Forward genetic screens in zebrafish have also uncovered new genes required for development of myelinated axons, including gpr126, which encodes a G-protein coupled receptor required for Schwann cells to progress beyond the promyelinating stage. In addition, work in zebrafish uncovered new roles for Schwann cells themselves, including in regulating the boundary between the PNS and CNS and positioning a nerve after its initial outgrowth.

Congenital hypomyelinating neuropathy with lethal conduction failure in mice carrying the Egr2 I268N mutation

Mouse models of human disease are helpful for understanding the pathogenesis of the disorder and ultimately for testing potential therapeutic agents. Here, we describe the engineering and characterization of a mouse carrying the I268N mutation in Egr2, observed in patients with recessively inherited Charcot-Marie-Tooth (CMT) disease type 4E, which is predicted to alter the ability of Egr2 to interact with the Nab transcriptional coregulatory proteins. Mice homozygous for Egr2(I268N) develop a congenital hypomyelinating neuropathy similar to their human counterparts. Egr2(I268N) is expressed at normal levels in developing nerve but is unable to interact with Nab proteins or to properly activate transcription of target genes critical for proper peripheral myelin development. Interestingly, Egr2(I268N/I268N) mutant mice maintain normal weight and have only mild tremor until 2 weeks after birth, at which point they rapidly develop worsening weakness and uniformly die within several days. Nerve electrophysiology revealed conduction block, and neuromuscular junctions showed marked terminal sprouting similar to that seen in animals with pharmacologically induced blockade of action potentials or neuromuscular transmission. These studies describe a unique animal model of CMT, whereby weakness is due to conduction block or neuromuscular junction failure rather than secondary axon loss and demonstrate that the Egr2-Nab complex is critical for proper peripheral nerve myelination.

Structural and Ultrastructural Study of the Intracranial Portion of the Oculomotor, Trochlear and Abducent Nerves in Dog

The right intracranial portion of the oculomotor, trochlear and abducent nerves were removed from six adult German shepherd dogs and analysed by light and electron microscopy. In all cases the nerve sectional area was calculated. Unmyelinated and myelinated fibres were analysed and number, diameter and cross-sectional area were calculated. In myelinated fibres, also calculated were the corresponding axon area and diameter, and myelin sheath thickness. The mean number of myelinated fibres was 8543.50 +/- 1231.85 being the unmyelinated 1402 +/- 241.58 in the oculomotor nerve; 1509 +/- 223.17 and 287.67 +/- 72.28 in the trochlear nerve and 2473.00 +/- 211.41 and 231.25 +/- 92.67 respectively in the abducent. The mean diameter was 10.23 +/- 0.68 microm in myelinated and 0.43 +/- 0.21 for unmyelinated in oculomotor nerve, 10.53 +/- 0.55 microm and 0.33 +/- 0.04 for the trochlear, and 10.45 +/- 1.27 microm and 0.47 +/- 0.09 in the abducent nerve respectively. This study reveals that oculomotor, trochlear and abducent nerves of the dog show structural and ultra-structural features similar to the same nerves in other species.

Pathogenesis of X-linked Charcot-Marie-Tooth disease: differential effects of two mutations in connexin 32

X-linked Charcot-Marie-Tooth disease is an inherited peripheral neuropathy arising in patients with mutations in the gene encoding connexin 32 (Cx32). Cx32 is expressed at the paranodes and Schmidt-Lantermann incisures of myelinating Schwann cells in which it is believed to form a reflexive pathway between the abaxonal and adaxonal cytoplasmic domains. Patients with the Val181Ala (V181A) mutation have a severe peripheral neuropathy. Experiments using a nude mouse xenograft system show that Schwann cells expressing only this mutant form of Cx32 are profoundly impaired in their ability to support the earliest stages of regeneration of myelinated fibers. Coupling between paired Xenopus oocytes expressing V181A is reduced compared with the coupling between oocytes expressing wild-type human Cx32 (32WT), and protein levels assayed by Western blot are substantially lower. Immunocytochemisty shows that Neuro2a cells expressing the V181A mutant have very few gap junction plaques compared with cells expressing 32WT; Cx32 protein levels are lower in these cells than in those expressing 32WT. Because failure of normal regeneration is evident before formation of myelin, loss of function of Cx32 may impact on the function of precursors of the myelinating Schwann cell before the formation of the hypothesized reflexive pathway. The Glu102Gly (E102G) mutation leads to a milder phenotype. Early regeneration is normal in grafts with Schwann cells expressing the E102G mutant. The only abnormality detected in the behavior of its channel is increased sensitivity to acidification-induced closure, a property that may lead to reduced gap junction coupling during periods of metabolic stress. This restricted functional abnormality may explain the relatively mild phenotype seen in the xenograft model and in E102G patients.

Evidence for impaired axonal regeneration in PMP22 duplication: studies in nerve xenografts

Whether axonal regeneration in Charcot-Marie-Tooth (CMT) neuropathies is impaired has not been addressed in detail. Our studies in nude mice harboring xenografts from patients with different primary Schwann cell (SC) genetic defects suggested an intimate association between the onset of myelination and impairment in the growth capacity of nude mice axons engulfed by the mutant SCs. To assess the effects of peripheral myelin protein 22 (PMP22) gene duplication on the regeneration process, we conducted morphometric studies to generate temporal growth profiles of myelinated axons within the xenografts obtained from CMT1A patients and from healthy controls. Axon size distribution histograms in controls at different time intervals revealed that size differentiation of myelinated fibers within the grafts is established as early as 2 weeks, and that the temporal pattern of myelination of different sized axons has striking similarities to myelination during development. In PMP22 duplication grafts, the onset of myelination is delayed and the regeneration capacity of all fiber sizes is impaired. This defect, however, is most pronounced for the large diameter axons. In addition, significant large fiber loss occurred after 12 weeks with a concomitant new cycle of regeneration of small size axons. These studies show that the PMP22 duplication in SCs have profound effects on the regeneration process, which might be a contributing factor to preferential distal axonal loss.

Rapid method for culturing embryonic neuron-glial cell cocultures

A streamlined, simple technique for primary cell culture from E17 rat tissue is presented. In an attempt to standardize culturing methods for all neuronal cell types in the embryo, we evaluated a commercial medium without serum and used similar times for trypsinization and tested different surfaces for plating. In 1 day, using one method and a single medium, it is possible to produce robust E17 cultures of dorsal root ganglia (DRG), cerebellum, and enteric plexi. Allowing the endogenous glial cells to repopulate the cultures saves time compared with existing techniques, in which glial cells are added to cultures first treated with antimitotic agents. It also ensures that all the cells present in vivo will be present in the culture. Myelination commences after approximately 2 weeks in culture for dissociated DRG and 3-4 weeks in cerebellar cultures. In enteric cultures, glial wrapping of the enteric neurons is seen after 3 weeks (2 weeks in ascorbic acid), suggesting that basal lamina production is important even for glial ensheathment in the enteric nervous system. No overgrowth of fibroblasts or other nonneuronal cells was noted in any cultures, and myelination of the peripheral nervous system and central nervous system cultures was very robust.

Schwann cell caveolin-1 expression increases during myelination and decreases after axotomy

The caveolins are a family of related proteins that form the structural framework of caveolae. They have been implicated in the regulation of signal transduction, cell cycle control, and cellular transport processes, particularly cholesterol trafficking. Caveolin-1 is expressed by a variety of cell types, including Schwann cells, although its expression is greatest in differentiated cell types, such as endothelial cells and adipocytes. In the present work, we characterize caveolin-1 expression both during rat sciatic nerve development and after axotomy. Schwann cells express little caveolin-1 on postnatal days 1 and 6. By P30, myelinating Schwann cells express caveolin-1, which is localized in the outer/abaxonal myelin membranes as well as intracellularly. After axotomy, Schwann cell caveolin-1 expression in the distal nerve stump decreases as Schwann cells revert to a premyelinating (p75-positive) phenotype; residual caveolin-1 within the nerve largely localizes to myelin debris and infiltrating macrophages. We speculate that caveolin-1 plays a role in the biology of myelinating Schwann cells.

TGFbeta1 modulates the phenotype of Schwann cells at the transcriptional level

We have examined the effects of transforming growth factor beta1 (TGFbeta1) on gene expression in cultured rat Schwann cells (SCs). TGFbeta1 decreased the steady-state mRNA levels of several genes that are expressed by myelinating SCs but had varied effects on the mRNA levels of NCAM, L1, GAP-43, and p75-genes that are expressed by denervated and nonmyelinating SCs. TGFbeta1 antagonized the effects of forskolin on the mRNA levels of the transcription factors Oct-6/tst-1/SCIP and Krox20. Transcriptional run-off analysis demonstrated that the effects of TGFbeta1 on gene expression occur at least in part at the level of transcription. Thus, TGFbeta1 suppresses the expression of genes that characterize the different phenotypes of SCs, and these changes occur at least in part at a transcriptional level.

Nerve fiber composition of the intracranial portion of the oculomotor, trochlear, and abducens nerves in the sheep

In the present investigation, the fiber content and the diameter spectra of the intracranial portion of the three oculomotor nerves (oculomotor, trochlear, and abducens nerves) were analysed in sheep by light and electron microscopy. It was determined that up to 14.98% of fibers in the oculomotor nerve, 17.01% in the trochlear nerve, and 11.87% in the abducens nerve were unmyelinated. The myelinated fibers showed a bimodal distribution in their size spectrum in all three nerves, with a majority of large myelinated axons, but a considerable proportion of small myelinated fibers, as well. The sensory function of the unmyelinated fibers present in the three oculomotor nerves is discussed also on the basis of our previous morphofunctional investigations.

Diffusional anisotropy in cranial nerves with maturation: quantitative evaluation with diffusion MR imaging in rats

PURPOSE

To investigate the correlation between diffusional anisotropy and developmental changes in anatomy, which include myelination, in central and peripheral nerves in an animal model by using quantitative diffusion magnetic resonance (MR) imaging and electron microscopy.

MATERIALS AND METHODS

In vivo transverse and longitudinal apparent diffusion coefficients (ADCs) of the optic and trigeminal nerves in 2-10-week-old rats were measured with MR imaging. Then the animals were sacrificed at each time point, and transverse and longitudinal sections of optic and trigeminal nerves were studied with electron microscopy.

RESULTS

In the optic nerve, the ADC parallel to the neurofibers increased with development and increased contemporaneously with myelination, while the ADC perpendicular to the nerve did not change. This resulted in a significant increase in diffusional anisotropy. There were no significant changes in ADCs in either direction in the trigeminal nerve. Longitudinal sections of optic nerve showed a marked change in the orientation of each fiber. As development proceeded, the axons, which initially followed tortuous courses, assumed straighter and more parallel orientations. Trigeminal nerves displayed straight parallel courses at 2 weeks that did not change over the study period.

CONCLUSION

Changes in fiber anatomy in maturation from tortuous to straighter and more parallel orientation can account for changes in longitudinal ADC and in diffusional anisotropy.

Promyelinating Schwann cells express Tst-1/SCIP/Oct-6

Tst-1/SCIP/Oct-6, a POU domain transcription factor, is transiently expressed by developing Schwann cells and is required for their normal development into a myelinating phenotype. In tst-1/scip/oct-6-null sciatic nerves, Schwann cells are transiently arrested at the "promyelinating" stage, when they have a one-to-one relationship with an axon but before they have elaborated a myelin sheath. To determine when Schwann cells express Tst-1/SCIP/Oct-6, we examined beta-galactosidase (beta-gal) expression in heterozygous tst-1/scip/oct-6 mice, in which one copy of the tst-1/scip/oct-6 gene has been replaced with the LacZ gene. beta-Gal expression from the LacZ gene seems to parallel Tst-1/SCIP/Oct-6 expression from the endogenous tst-1/scip/oct-6 gene in developing and regenerating sciatic nerves. Furthermore, electron microscopic examination of 5bromo-4-chloro-3-indolyl-beta-D-galactopyranoside- (X-gal) and halogenated indolyl-beta-D-galactoside- (Bluo-gal) stained nerves showed that promyelinating Schwann cells express the highest levels of beta-gal, both in developing and in regenerating nerves. Thus, the expression of beta-gal, a surrogate marker of Tst-1/SCIP/Oct-6, peaks at the same stage of Schwann cell development at which development is arrested in tst-1/scip/oct-6-null mice, indicating that Tst-1/SCIP/Oct-6 has a critical role in promyelinating Schwann cells.

Signals that initiate myelination in the developing mammalian nervous system

The myelination of axons by oligodendrocytes in the central nervous system and Schwann cells in the peripheral nervous system is essential for the establishment of saltatory conduction. In the absence or destruction of the myelin sheath, as seen in demyelinating diseases, impulse conduction is impeded resulting in severe sensory and motor deficits. Axon myelination is the culmination of a sequence of events that begins with the differentiation of glial cells and proceeds to the transcription and translation of myelin genes, the elaboration of a myelin sheath, and the recognition and ensheathment of axons. This review examines the regulatory mechanisms for each of these steps and compares and contrasts the role of the axon in initiating myelination in the central and peripheral nervous system.

Regenerative axonal sprouting in the cat trochlear nerve

Following peripheral trochlear nerve axotomy in the cat, the normal number of myelinated axons is restored despite significant motor neuron death, suggesting regulation of the number of myelinated axons in the regenerated nerve. In this study we used light and electron microscopy to examine the production and maintenance of axonal sprouts at different locations in the nerve and at different postoperative intervals. Despite proliferative sprouting and an overproduction of nonmyelinated axons in the regenerating trochlear nerve, the number of myelinated axons was strictly regulated. Only approximately 1,000 regenerated axons were eventually remyelinated, but many nonmyelinated axons were still present 6-8 months postaxotomy. Regenerated axons were remyelinated in a proximal-to-distal direction between 3 and 4 weeks postaxotomy. We also examined the maturation of regenerated myelinated axons by measuring axon diameter and myelin index (an expression of myelin thickness). Mean myelinated axon diameter remained significantly below normal in long-term regenerated nerves. Mean myelin index was not different from normal at 4 weeks postaxotomy but was significantly decreased at long postoperative intervals, reflecting a slightly thicker myelin sheath relative to the axon diameter. This relative increase in mean myelin thickness could serve to restore normal conduction velocity despite the decrease in mean axon diameter. We suggest that the regulation of the number of myelinated axons at the normal number despite cell death and the increase in mean myelin thickness may both be compensatory mechanisms that function to restore preoperative conditions and maximize functional recovery.

Myelin protein expression in dissociated superior cervical ganglia and dorsal root ganglia cultures

Schwann cells of the adult rat superior cervical ganglia (SCG) synthesize negligible levels of the major myelin glycoprotein, P0, in vivo. This suggests that the sympathetic axons of the SCG are deficient in one of more components involved in the regulation of myelin protein expression. Here we have compared the ability of neurites from neonatal rat SCG and embryonic rat dorsal root ganglia (DRG) to induce Schwann cell expression of myelin proteins after growth in culture using a serum-free medium. Steady-state P0 mRNA levels in the SCG and DRG culture paradigms were determined with a sensitive polymerase chain reaction (PCR) assay that amplified cDNA produced by reverse transcription of mRNA. This semiquantitative assay showed a linear response to increasing amounts of P0 and actin mRNA and required substantially less cellular RNA than typical hybridization techniques. Using the PCR assay, we found that SCG cultures contained significantly lower amounts of P0 mRNA than did DRG cultures. To further confirm that SCG cultures have negligible expression of myelin proteins, immunoblot analyses were done to examine the steady-state levels of both P0 and myelin basic protein. While nonmyelinating DRG cultures had readily detectable amounts of these myelin-specific proteins, neither could be demonstrated in the SCG cultures. The data indicate that SCG neurites lack one or more signals needed to induce myelin protein expression. Employing SCG and DRG cultures in comparative biochemical studies should prove useful in identifying the axonal molecule(s) involved in the regulation of myelin protein expression.

Role of axons in the regulation of P0 biosynthesis by Schwann cells

The role of axons in the expression of the major myelin glycoprotein, P0, has been investigated using neuron/Schwann cell cultures. These cultures were either nonmyelinating or myelinating due to growth in defined medium or in medium containing serum and chick embryo extract, respectively. The neurons and Schwann cells used in the studies were derived from embryonic day 15 rat dorsal root ganglia (DRG), and the Schwann cells from these ganglia are shown not to synthesize appreciable levels of P0 prior to growth in culture. Myelinating cultures of Schwann cells and neurons grown together for 18-21 days synthesize P0 that is readily identified by immunoblotting. The nonmyelinating cultures, which do not assemble basal lamina, also synthesize P0 that is detectable by either [3H]mannose precursor incorporation or by immunoblotting. The steady-state level of P0 in the nonmyelinating cultures is less than that of the myelinating cultures, and the P0 that is synthesized by the former appears to be catabolized shortly after its biosynthesis. Since nonmyelinating Schwann cells synthesize P0 when in contact with neurites in vitro, we have examined the ability of such nonmyelinating cells to express the glycoprotein in vivo. Very little steady-state P0 is detected in immunoblots of the adult rat cervical sympathetic trunk (CST), a nerve in which approximately 99% of the axons are nonmyelinated. Similarly, the amounts of [3H]mannose and [3H]amino acids that are incorporated into newly synthesized P0 are much lower in the CST than in the adult sciatic nerve.(ABSTRACT TRUNCATED AT 250 WORDS)

Axon-myelin relationships in rat cranial nerves III, IV, and VI: a morphometric study of large- and small-fibre classes

The primary objectives of this study were to determine (1) if quantitative axon-myelin relationships are similar for large- and for small-fibre classes within individual nerves and (2) if the same axon-myelin relationships hold for equivalent fibre classes in closely similar nerves. The oculomotor, trochlear, and abducent nerves of the rat were examined since they each contain distinct large- and small-fibre classes and are similar in a wide range of anatomical and developmental respects. Accordingly, morphometric analyses of axon-myelin relationships were performed separately on large and small fibres of each of the three nerves. Within each nerve, the setting of the relationship between the two parameters was found to be different for the two fibre classes: Scatterplots relating sheath thickness to axon perimeter for large fibres were shifted upwards relative to those for small fibres. These differences were also reflected in the positions of the regression lines fitted to the plots and in the g-ratios. Significant differences were found between nerves in relation to their large fibres: Those of the abducent nerve had significantly thicker sheaths, those of the oculomotor nerve had significantly smaller axon perimeters, and the myelin sheath-axon perimeter relationship of the abducent nerve differed significantly from that of the other two. This study therefore shows that morphometric axon-myelin relationships may differ significantly between equivalent fibre classes of nerves that are closely similar in respect of morphological class, central origin, peripheral distribution, developmental environment, and function.

Nerve fibre morphometry: a comparison of techniques

This study compared different approaches to measuring nerve axon and fibre diameters and areas from transverse sections. A mock photomicrograph and mock tissue section, each with 100 identical, circular 'fibres' was constructed. Three measurement protocols were investigated: (A) circular approximation from minimum diameter; (B) circular approximation from the mean of orthogonal diameters; and (C) calculation of diameter and area from a digitized circumference. For each protocol, all 100 fibres on the photomicrograph were repeatedly measured using a digitizing tablet. Similarly, the fibres on the mock tissue section were measured using a digitizing tablet and microscope with camera lucida. The variance for these data was calculated. Protocols were compared on the basis of variability and the amount of digitizing time required. For diameter measurements, protocol B showed significantly lower variability than A or C (P less than 0.05), with only a modest increase in digitizing time over A. For area measurements, protocols B and C showed significantly lower variability than A (P less than 0.05), again with a modest increase in digitizing time. Measurements made using the microscope and camera lucida showed significantly lower variability than those made from the photomicrograph, but took more time. These data suggest that for diameter measurements, a mean of orthogonal diameters approach is best, and that for area measurements, a traced circumference approach is best as it is more flexible than the orthogonal diameter approach. While the microscope and camera lucida setup is more time-consuming to use, it eliminates the need for photomicrograph production.

Region-specific appearance of myelin constituents in the developing rat spinal cord

The appearance of myelin-specific glycolipids and of myelin basic protein was studied with regard to the detailed anatomy of the rat cervical spinal cord. The expression of these constituents in particular fibre tracts and regions occurs at specific times of development between postnatal days 1 and 14. This mosaic-like appearance started in the ventral funiculus (day 1) followed by fasciculus cuneatus and ventro-lateral funiculus (day 2), and fasciculus gracilis and dorso-lateral funiculus (days 3 to 4). Cortico-spinal tract (day 11), Lissauer tract (day 14) and the commissures started to acquire myelin very late. In the grey matter, myelin constituents appeared around days 11 to 14 in a patchy pattern. These results support a concept of highly local interactions regulating oligodendrocyte differentiation. In addition, a general rostro-caudal gradient of myelin development exists in the spinal cord, which is independent of the ascending or descending nature of the fibre tracts. Appearance of myelin constitutents in the caudal spinal cord was not prevented by a neonatal transection at mid-thoracic levels.

Changes in the histograms of nerves resulting from growth and various modalities of damage. A computer simulation

A computer program simulates the changes in a nerve's histogram during normal growth and from various neuropathies. The program shows how a nerve's histogram will change from various percentages of fiber damage or from preferential damage to either thick or thin fibers, or from various degrees of fiber restitution, or from single-event or repetitive damage. In single-event damage, the main alteration is a preponderance of thin (regenerating) fibers. Patterns of selective fiber vulnerability are difficult to deduce from the shape of the histogram. Repetitive damage remodels the histogram to a broad unimodal fiber distribution at reduced mean caliber. Comparison of simulated changes with data from an experimental isoniazid neuropathy yielded a close match between observed changes and simulation.

P2, P1, and P0 myelin protein expression in developing rat sixth nerve: a quantitative immunocytochemical study

Myelination and the expression of myelin proteins P2, P1, and P0 were studied quantitatively in the rat sixth cranial nerve during development. The postnatal development and growth of all myelin sheaths in this nerve have been studied morphometrically in a companion paper. Epon-embedded blocks with closely matched topography in the transverse plane were selected from rats perfused at ages 1-4, 8, 15, and 20 days. From each block, serial semithin sections were cut, etched, and immunostained according to the peroxidase-antiperoxidase method with well-characterized polyclonal antisera that reacted specifically with P0 glycoprotein and the basic proteins P1 and P2. The immunoreactivities of individual myelin sheaths were measured by densitometry. Numbers of compact myelin lamellae, myelin spiral lengths, and axon diameters were determined on electronmicrographs of adjacent thin sections. At birth anti-P0 immunoreactivity was found on sheaths with two and more compact lamellae; neither P1 nor P2 immunoreactivity was observed. On day 2, myelin sheaths with five and eight lamellae were stained respectively by anti-P1 and anti-P2. On day 3 the percentages of myelin sheaths stained were substantially higher: P0 95%, P1 78%, P2 15%. By day 4, anti-P0 and anti-P1 immunoreactivity was present in 95% of myelin sheaths; 35% were stained by anti-P2. For P2, staining intensity and percentage of myelin sheaths stained continued to increase and by day 20, 85% were anti-P2-positive. The density of immunoreactivity was not uniform in all myelin sheaths. At young ages staining varied with all three proteins. The variability decreased as myelin sheaths thickened; it persisted longest for anti-P2. We conclude that the density and distribution of immunoreactivities of P0, P1, and P2 reflect their relative concentrations during myelin sheath development and growth. We attribute lack of detectable anti-P2 immunoreactivity in some small sheaths at 20 days to their early stage of myelination and also to limitations of the method. We infer from our observations that all myelin-forming Schwann cells express P2 basic protein.