Biomedical subjects

J W Heath

Publications and source records attributed to J W Heath.

At least 19 recordsLinked to original sources

PubMed2002

Heritability and Y-chromosome influence in the jack male life history of chinook salmon (Oncorhynchus tshawytscha).

Jacking in chinook salmon (Oncorhynchus tshawytscha) is an alternative reproductive strategy in which males sexually mature at least 1 year before other members of their year class. We characterize the genetic component of this reproductive strategy using two approaches; hormonal phenotypic sex manipulation, and a half-sib breeding experiment. We 'masculinized' chinook salmon larvae with testosterone, reared them to first maturation, identified jacks and immature males based on phenotype, and genotyped all fish as male ('XY') or female ('XX') using PCR-based Y-chromosome markers. The XY males had a much higher incidence of jacking than the XX males (30.8% vs 9.9%). There was no difference in body weight, gonad weight, and plasma concentrations of testosterone and 17beta-estradiol between the two jack genotypes, although XY jacks did have a higher gonadosomatic index (GSI) than XX jacks. In the second experiment, we bred chinook salmon in two modified half-sib mating designs, and scored the number of jacks and immature fish at first maturation. Heritability of jacking was estimated using two ANOVA models: dams nested within sires, and sires nested within dams with one-half of the half-sib families common to the two models. The sire component of the additive genetic variance yielded a high heritability estimate and was significantly higher than the dam component (h(2)(sire) = 0.62 +/- 0.21; h(2)(dam) = -0.14 +/- 0.12). Our experiments both indicated a strong sex-linked component (Y-chromosome) to jacking in chinook salmon, although evidence for at least some autosomal contribution was also observed.

Analysis of Variance↗

PubMed2000

Abstinence education for urban youth.

Teen sexual problems in the U.S. are reaching enormous proportions. Attempts to prevent common problems, pregnancy and sexually transmitted diseases, are underway through the persistent efforts of community, health, academic and government organizations. Abstinence education is one of the current attempts. However, the lack of well defined theoretical frameworks and analyses of outcomes have limited progress in the study of abstinence education. This article describes a pilot program in abstinence-only education provided to six groups of young teens within an urban middle school. The framework for the program, cognitive social learning theory, is described and operationalized. Student pretest-posttest attitudes, open-ended written comments about the program and the researchers' anecdotes about behavioral changes in the students are the outcome measures. Positive attitudes about premarital abstinence increased for all six groups; for four of the six groups the increase was statistically significant.

Adolescent↗

PubMed1998

Alpha and beta subunits of CaM-kinase II are localized in different neurons in chick ciliary ganglion.

The ciliary ganglion of the chicken contains only two types of neurons. Using monoclonal antibodies against the alpha and the beta subunits of Ca2+/calmodulin-stimulated protein kinase II (CaMPK-II) we found that the alpha-subunit was localized to the choroid neurons while beta subunit was associated with the ciliary neurons. As both neurons receive their inputs from the oculomotor nerve, while their postganglionic axons leave via different nerves, the ciliary ganglion of the chicken is a neuronal system in which the functional differences between alpha and beta CaMPK-II homopolymers in the regulation of synaptic transmission can be investigated.

Animals↗

PubMed1995

Cryo-immunogold ultrastructural localization of laminin in adult rat peripheral nerve.

Elucidation of the functional roles of the extracellular matrix component laminin in adult peripheral nerve has been hindered by differing accounts of its ultrastructural localization. This is the first report applying the advantages of the cryo-immunogold technique to laminin localization in peripheral nerve. Laminin labelling was found over the basal lamina and possibly over the immediately subjacent Schwann cell plasma membrane, but specific labelling appeared to be absent from other membranes (including those of non-myelinated axon/Schwann cell clusters) and from endoneurial collagen fibrils. It would appear that the functional roles played by laminin in normal adult peripheral nerve are likely to be mediated via its localization in the basal lamina, rather than through a more widespread distribution within the endoneurium.

Animals↗

PubMed1995

Patterns of morphological variation within myelin internodes of normal peripheral nerve: quantitative analysis by confocal microscopy.

Knowledge of variations in the morphology of normal myelinated peripheral nerve fibres is fundamental to subsequent interpretation of neuropathology. It would be advantageous for structural analysis of normal variations to be based on entire myelin internodes, but acquisition of such data via the classic approach of nerve fibre teasing has been hindered by limitations in optical resolution and specimen preparation. This study addressed these limitations through a new confocal imaging method which permits detailed visualisation of individual myelinated fibres in intact peripheral nerve trunks, and quantitated previously unrecognised patterns of morphological variation within normal internodes. The study focused particularly on Schmidt-Lanterman incisures, the narrow cytoplasmic channels which traverse normal compact myelin and provide foci for disruption of the compact sheath in a number of peripheral neuropathies. Analysis was based on confocal fluorescence images of multiple sequential internodes, traced within posterior tibial nerve trunks of adult male Sprague-Dawley rats. The strength of relationships between internodal size variables (length, fibre diameter, myelin sheath thickness) and total number of incisures per internode were documented. Each internode was divided into 4 regions of equivalent length (regions 1-4), and variations in the distribution of incisures and Schwann cell nuclear location were evaluated. Regional variations were consistent, irrespective of differences in fibre diameter, myelin sheath thickness, and internodal length. Expressed in terms of proximodistal orientation, there was a unimodal distribution of incisures within internodes of this fibre population (diameter range 5-9 microns), with region 3 containing the highest number of incisures and region 4 the lowest (P < 0.05). The Schwann cell nucleus was located more frequently in region 3 than in region 2 (P < 0.01). Contrary to previous reports, an incisure was found in close association with the nucleus in at least 50% of internodes. Documentation of frequent incisure-nuclear association and consistent patterns of variation within internodes extends knowledge of the microanatomy of normal peripheral nerve, and may provide insight into the functional role of incisures. Demonstration of such patterns in normal nerve may contribute to the understanding of pathological change, for example progression of ovoid formation from midinternodal regions during wallerian degeneration.

Animals↗

PubMed1994

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

Current methods of morphological analysis do not permit detailed imaging of individual myelinated fibres over substantial lengths without disruption of neighbouring, potentially significant, cellular and extracellular relationships. We report a new method which overcomes this limitation by combining aldehyde-induced fluorescence with confocal microscopy. Myelin fluorescence was intense relative to that from other tissue components, enabling individual myelinated nerve fibres to be traced for distances of many millimeters in whole PNS nerve trunks. Image obtained with a Bio-Rad MRC-600 confocal laser scanning microscope clearly displayed features of PNS and CNS myelinated fibres including nodes of Ranvier; fibre diameter; sheath thickness and contour; branch points at nodes; as well as (in the PNS) Schmidt-Lanterman incisures and the position of Schwann cell nuclei. Direct comparisons using the same specimens (whole nerve trunks; also teased fibres) showed confocal imaging to be markedly superior to conventional fluorescence microscopy in terms of contrast, apparent resolution and resistance to photobleaching. Development of the fluorophore was examined systemically in sciatic nerves of young adult rats. In separate experiments, animals were perfused systemically using (1) 5% glutaraldehyde; (2) Karnovsky's solution; (3) 4% paraformaldehyde; buffered with either 0.1 M sodium phosphate or sodium cacodylate (pH 7.4). The concentration of glutaraldehyde in the fixative solution was the principal determinant of fluorescence intensity. Confocal imaging was achieved immediately following perfusion with 5% glutaraldehyde or Karnovsky's. Fluorescence intensity increased markedly during overnight storage in these fixatives and continued to increase during subsequent storage in buffer alone. The fluorophore was stable and resistant to fading during storage (15 months at least), enabling data collection over extended periods. To demonstrate application of the method in neuropathology, individual fibres in transected sciatic nerve trunks were traced through multiple successive internodes: Classical features of Wallerian degeneration (axonal swelling and debris; ovoid formation and incisure changes; variation among fibres in the extent of degeneration) were displayed. The method is compatible with subsequent ultrastructural examination and will complement existing methods of investigation of myelinated fibre anatomy and pathology, particularly where preservation of 3-dimensional relationships or elucidation of spatial gradients are required.

Animals↗

PubMed1994

Morphometric analysis of axons myelinated during adult life in the mouse superior cervical ganglion.

In experimental studies addressing the regulation of myelin formation and maintenance by Schwann cells, the sympathetic nervous system of young adult rodents has served a key role as an essentially nonmyelinated yet modifiable control tissue. Nevertheless there is clear evidence of substantial myelination in the superior cervical ganglion (SCG) of normal mice and rats of more advanced age. Against this background, interpretation of experimental outcomes in particular sympathetic tissues will require detailed quantitative control data taking account of animal age. To provide a baseline for future investigations on myelin remodelling, an ultrastructural morphometric study of myelinated fibres in the SCG was undertaken in 4 strains (QS, Balb/C, C57 and CBA) of adult male mice aged 32-72 wk. Numbers of myelinated fibres in SCG cross-sections varied substantially between individual animals, and the mean numbers for QS (132), Balb/C (165) and CBA (254) were significantly higher than that for C57 (32). Both axonal and fibre diameter were distributed unimodally (means for the 4 strains ranged from 2.3-2.4 microns and 3.2-3.6 microns respectively). Myelin spiral length was distributed unimodally and skewed to the right (range of means = 227-357 microns) and was significantly greater in QS mice as compared with the other 3 strains. While the mean g ratio (axonal diameter/fibre diameter) was significantly lower in QS mice than in the other 3 strains, the range for mean g ratio was 0.64-0.73, indicating that myelination had proceeded appropriately even though late in onset in this tissue. The index of circularity was high in all strains, both for axons (range of means = 0.80-0.88) and fibres (range of means = 0.84-0.89). The small axonal and fibre diameter and unimodal distribution are consistent with the characteristics of autonomic myelinated fibres and it is probable that most are postganglionic sympathetic fibres arising within the SCG. In terms of providing a sufficient population of myelinated fibres for future experimental studies, the QS, Balb/C and CBA strains would be preferable to C57 mice.

Animals↗

PubMed1992

Structural maturation of synapses in the rat superior cervical ganglion continues beyond four weeks of age.

We have examined the morphology of preganglionic synapses in the rat superior cervical ganglion (SCG) at 10 days, 4 weeks and 1 year. Between 10 days and 4 weeks the mean thickness of the postsynaptic density (PSD) increased from 45.9 +/- 0.1 nm to 52.1 +/- 1.7 nm (P = 0.017), the mean length of the PSD (0.41 +/- 0.02 microns) did not change, and the distribution of synapses on the neuronal surface changed with a decrease in the proportion of somatic and an increase in the proportion of dendritic spine synapses. Since both synapse elimination and synapse formation are occurring during this period several mechanisms may contribute to these changes. However, between 4 weeks and 1 year, when there is no net change in the number of synapses, the mean length of the PSD increased to 0.53 +/- 0.02 microns (P = 0.001), there was no change in either the mean thickness of the PSD or the distribution of the synapses but the proportion of concave ('smile') synapses increased. A comparison with previous developmental studies of synapses in cerebral cortex of rat and chicken indicate that both the nature and the rate of synapse maturation can vary between different populations of synapses.

Aging↗

PubMed1992

Myelin sheath survival after guanethidine-induced axonal degeneration.

Membrane-membrane interactions between axons and Schwann cells are required for initial myelin formation in the peripheral nervous system. However, recent studies of double myelination in sympathetic nerve have indicated that myelin sheaths continue to exist after complete loss of axonal contact (Kidd, G. J., and J. W. Heath. 1988. J. Neurocytol. 17:245-261). This suggests that myelin maintenance may be regulated either by diffusible axonal factors or by nonaxonal mechanisms. To test these hypotheses, axons involved in double myelination in the rat superior cervical ganglion were destroyed by chronic guanethidine treatment. Guanethidine-induced sympathectomy resulted in a Wallerian-like pattern of myelin degeneration within 10 d. In doubly myelinated configurations the axon, inner myelin sheath (which lies in contact with the axon), and approximately 75% of outer myelin sheaths broke down by this time. Degenerating outer sheaths were not found at later periods. It is probably that outer sheaths that degenerated were only partially displaced from the axon at the commencement of guanethidine treatment. In contrast, analysis of serial sections showed that completely displaced outer internodes remained ultrastructurally intact. These internodes survived degeneration of the axon and inner sheath, and during the later time points (2-6 wk) they enclosed only connective tissue elements and reorganized Schwann cells/processes. Axonal regeneration was not observed within surviving outer internodes. We therefore conclude that myelin maintenance in the superior cervical ganglion is not dependent on direct axonal contact or diffusible axonal factors. In addition, physical association of Schwann cells with the degenerating axon may be an important factor in precipitating myelin breakdown during Wallerian degeneration.

Animals↗

PubMed1991

Myelin maintenance by Schwann cells in the absence of axons.

Formation and maintenance of myelin sheaths in the peripheral nervous system are regulated by unknown molecular interactions that are thought to depend upon physical contact between Schwann cells and axons. However, recent studies describing axons surrounded by two concentric myelin internodes in the superior cervical ganglion (SCG) of normal rodents have demonstrated that the outer myelin internodes are maintained without physical contact with the axon. To determine whether the centrally enclosed axon has a trophic effect in maintaining these remote outer internodes, we have produced axonal degeneration by surgical or chemical means. The results indicate that maintenance of myelin internodes totally displaced from axonal contact depends neither upon the presence of the axon nor on diffusible axonal factors. A further implication of these studies is that myelin breakdown during Wallerian degeneration is regulated by a positive signal which originates in degenerating nerves, rather than solely by loss of axonal trophic substances.

Animals↗

PubMed1991

Distribution of P0 protein and the myelin-associated glycoprotein in peripheral nerves from Trembler mice.

The Trembler mouse has a dysymelination of peripheral nerves that includes hypomyelination, failure of myelin compaction, and demyelination/remyelination. We have localized the myelin proteins P0 and myelin associated glycoprotein in Trembler peripheral nerve and correlated their distributions with the ultrastructure of myelin internodes. Immunocytochemically, myelin-associated glycoprotein was localized in Schwann cell periaxonal membranes, Schmidt-Lanterman incisures, paranodal loops, and internal and external mesaxons. P0 staining was located over compact myelin and regions of Schwann cell cytoplasm rich in Golgi membranes. An unusual abundance of small, P0-stained, Golgi-related vesicles was found in some Schwann cells. P0 protein was also detected in multiple spiral wraps of myelin-associated glycoprotein-positive mesaxon membranes. At some sites the periodicity of the myelin membranes was intermediate to that found in mesaxon membranes and compact myelin. The distance between apposing extracellular leaflets was similar to that found in mesaxon membranes, while the cytoplasmic leaflets were fused but twice as thick as normal major dense lines. These intermediate membranes were stained by P0 and myelin-associated glycoprotein antiserum. These studies suggest that altered transport and/or translocation of P0 and myelin-associated glycoprotein results in defective myelin compaction in Trembler peripheral nerve.

Animals↗

PubMed1991

Mechanisms of synaptic plasticity. Changes in postsynaptic densities and glutamate receptors in chicken forebrain during maturation.

We have shown that the synapse maturation phase of synaptogenesis is a model for synaptic plasticity that can be particularly well-studied in chicken forebrain because for most forebrain synapses, the maturation changes occur slowly and are temporally well-separated from the synapse formation phase. We have used the synapse maturation phase of neuronal development in chicken forebrain to investigate the possible link between changes in the morphology and biochemical composition of the postsynaptic density (PSD) and the functional properties of glutamate receptors overlying the PSD. Morphometric studies of PSDs in forebrains and superior cervical ganglia of chickens and rats have shown that the morphological features of synapse maturation are characteristic of a synaptic type, but that the rate at which these changes occur can vary between types of synapses within one animal and between synapses of the same type in different species. We have investigated, during maturation in the chicken forebrain, the properties of the N-methyl-D-aspartate (NMDA) subtype of the glutamate receptors, which are concentrated in the junctional membranes overlying thick PSDs in the adult. There was no change in the number of NMDA receptors during maturation, but there was an increase in the rate of NMDA-stimulated uptake of 45Ca2+ into brain prisms. This functional change was not seen with the other ionotropic subtypes of the glutamate receptor and was NMDA receptor-mediated. The functional change also correlated with the increase in thickness of the PSD during maturation that has previously been shown to be due to an increase in the amount of PSD associated Ca(2+)-calmodulin stimulated protein kinase II (CaM-PK II). Our results provide strong circumstantial evidence for the regulation of NMDA receptors by the PSD and implicate changing local concentrations of CaM-PK II in this process. The results also indicate some of the ways in which properties of existing synapses can be modified by changes at the molecular level.

Animals↗

PubMed1991

A human colon cancer cell line established from collagen matrix cultures transplanted into nude mice.

A new human colon cancer cell line (020588) has been derived by means of a combined in vitro matrix-in vivo xenograft technique. The tumour cell line is carcino-embryonic antigen positive, displays a marker chromosome and proliferates in chemically-defined serum-free culture medium. The chemosensitivity pattern for the tumour cell line was similar to that observed for the parent tumour cells. The novel method used to establish this continuous human tumour cell line may have several advantages over standard techniques.

Animals↗

PubMed1991

Myelin sheath survival following axonal degeneration in doubly myelinated nerve fibers.

Axonal contact plays a critical role in initiating myelin formation by Schwann cells. However, recent studies of "double myelination" have indicated that myelin maintenance continues in Schwann cells completely displaced from physical contact with the axon. This raises the possibility either that diffusible trophic factors are produced by the axon, or that the axon is not required for myelin maintenance by these displaced Schwann cells. To test these hypotheses, the axons involved in double myelination in the mouse superior cervical ganglion (SCG) were transected surgically by a transganglionic lesion. The inferior pole of the SCG was resected to limit axonal regeneration. This method produced a typical Wallerian pattern of degeneration in the superior pole, without compromising the blood supply or introducing nonspecific trauma. EM analysis at 1 and 5 d postoperatively showed that initially the axon degenerated, followed by breakdown of the inner myelin sheath. In those configurations where the outer Schwann cell was only partly displaced from the axon, the outer myelin sheath degenerated simultaneously. However, in completely displaced internodes the outer sheath survived degeneration of the axon and inner sheath. Outer internodes remained intact for at least 5 weeks after transection (the longest time point in this study), at which time they enclosed reorganized processes of the inner Schwann cells, their basal lamina, and numerous collagen fibrils. Axonal regeneration within surviving outer internodes was rare and was characterized by the development of typical Remak ensheathment by the inner Schwann cells. We conclude that in the mouse SCG, myelin maintenance does not depend on the continued presence of the axon. These data suggest further that myelin breakdown in Wallerian degeneration may be initiated by mechanisms other than absence of a viable axon.

Animals↗

PubMed1988

A rapid Percoll gradient procedure for isolation of synaptosomes directly from an S1 fraction: homogeneity and morphology of subcellular fractions.

A method for preparation of synaptosomes from rat cerebral cortex, on a discontinuous Percoll gradient, was previously developed for use with a P2 pellet (Brain Research, 372 (1986) 115-129). Here the Percoll method has been adapted for use with an S1-supernatant which eliminates a potentially damaging resuspension step and saves over 30 min, representing a third of the total preparation time. The homogeneity of the synaptosomes in each of the 5 subcellular fractions obtained with the S1-Percoll method was determined biochemically by analysis of the distribution of total protein, myelin basic protein, synapsin I and pyruvate dehydrogenase across the gradient. Electron microscopy was also used to determine the homogeneity of the synaptosomes, as well as to determine their morphological characteristics. Fraction 4 was the most enriched in synaptosomes and contained the lowest level of contamination by myelin, extrasynaptosomal mitochondria and plasma membranes. The yield of synaptosomes in fraction 4 with the S1-Percoll method was 1.4-fold greater than with the P2-Percoll method. While all other fractions contained some synaptosomes the major additional content in fractions 1-3 and 5 was, respectively, unidentified small membranes, myelin, synaptic plasma membranes and extrasynaptosomal mitochondria. Fraction 1 was enriched for very small synaptosomes (0.34 micron mean diameter) only 8% of which contained mitochondria, while fractions 2-4 progressively included larger synaptosomes containing more mitochondria. Fraction 5 synaptosomes were approximately the same size as those in fraction 4 (0.63 micron mean diameter), but 83% contained mitochondria, significantly more than in fraction 4. The synaptosomes in fraction 5 were found to be relatively resistant to hypotonic lysis, explaining a previously observed lack of phosphorylation of synapsin I in this fraction. The differences in homogeneity and morphological characteristics of the synaptosomes in fractions 1-5 suggest that the basis for their fractionation on Percoll gradients is different from that achieved with the more traditional procedures for isolating synaptosomes and that unique synaptosomal fractions are obtained with the S1-Percoll procedure.

Animals↗

PubMed1988

Analysis of myelin proteins in sympathetic peripheral nerve of adult rats.

Biochemical analyses of myelin proteins in rat sympathetic peripheral nerve were correlated with morphological observations. Myelin proteins in superior cervical ganglia (SCG) and the paravertebral (thoraco-lumbar) chain of ganglia were quantitated by immunoassays and examined qualitatively by Western blotting. The results were compared to those obtained on sciatic nerves from the same animals. In rats aged one year, the concentrations of PO glycoprotein and myelin basic protein (MBP) in SCG were about 1% of those in sciatic nerve, consistent with the relatively low numbers of myelinated fibers in sympathetic nerve. The relative concentration of myelin-associated glycoprotein (MAG) was higher, being 6.7% of that in sciatic nerve. The latter finding is probably due to the greater proportion of MAG-containing membranes (periaxonal, paranodal, and Schmitt-Lanterman incisures) in myelinated fibers of the SCG, in which the internodes are both short and thinly myelinated. The proportion of the 21 kDa, 18 kDa and 17 kDa forms of MBP relative to the 14 kDa form was much higher in SCG than in sciatic nerve, probably reflecting the fact that myelin formation continues actively during adult life in the ganglia, whereas the deposition of myelin is complete at a much earlier age in somatic nerves. The levels of myelin proteins were 2- to 3-fold higher in the paravertebral chain ganglia. These studies indicate that quantitation of myelin proteins in sparsely myelinated sympathetic nerve tissue is feasible and provide a baseline for further studies on the control of myelination in sympathetic nerve during adult life.

Animals↗

PubMed1988

Ultrastructural and immunohistochemical analysis of axonal regrowth and myelination in membranes which form over lesion sites in the rat visual system.

Glial-connective tissue membranes which form bridges over lesion cavities in the brachial and pretectal region of the rat visual system contain regenerated myelinated and unmyelinated axons. The lesions were made between 10 and 16 days postnatal--a time at which neonatal regeneration would not be expected. A detailed ultrastructural study of these membrane bridges has been undertaken in order to describe the cellular and extracellular conditions that are associated with the regeneration, myelination and continued survival of identified retinal and other axons. The lesion-induced membrane bridges possessed a limiting surface of fibroblasts and were composed of glial cells, macrophages, endothelial cells, pericytes and collagen. There was some variability in the ultrastructural appearance of the glial cells; the majority of criteria indicate that they were astrocytes. These astrocytes formed 'glia limitans'-like surfaces beneath the fibroblasts. They contained numerous filaments and extended fine, electron-dense cytoplasmic processes, often arranged into lamellated stacks. Basal lamina was present on the outer surfaces of the astrocytes. Astrocytic processes isolated clusters of myelinated and unmyelinated axons in lacunae which may have served as conduits for axonal elongation. This suggests a role for these astrocytes in the regeneration and maintenance process which appears to recapitulate events which occur during normal development. Interestingly, regrowing retinal axons were never found adjacent to astrocytic surfaces possessing a basal lamina. We did not detect evidence of Schwann cell invasion into the lesion. By ultrastructural criteria the myelin ensheathment which occurred on the larger axons in the membrane bridge was of central rather than peripheral type. The cytoplasmic domain external to the sheath was limited to a small tongue; no basal lamina invested the fibre; and the periodicity of the myelin was equivalent to that of other CNS structures. Similarly, the CNS character of the myelin was demonstrated by intense immunostaining of myelin sheaths for myelin basic protein and proteolipid [corrected] protein and lack of staining for the PNS component PO. The oligodendrocytes responsible for this myelination may either have extended cytoplasmic processes from the adjacent neuropil, or may have differentiated from precursor cells within the membrane bridge.

Animals↗

PubMed1988

Double myelination of axons in the sympathetic nervous system of the mouse. I. Ultrastructural features and distribution.

This study has examined the structural features and distribution of 'doubly myelinated' axons in normal adult and aged mice. Investigation focused on the superior cervical ganglion (SCG) and paravertebral sympathetic ganglia, which were extensively serial-sectioned for light and electron microscopy. In the SCG, the principal features of doubly myelinated regions were that an apparently normal myelinated axon was enclosed for part of its length by an additional (outer) myelinating Schwann cell. The separate nature of the inner and outer Schwann cells was emphasized by the consistent presence of individual nuclei in each, and by the presence of endoneurial space, often containing collagen fibrils, between the inner and outer cells. In some cases more than a single outer Schwann cell was present, arranged serially along the inner myelinated fibre. While double myelination forms through a mechanism involving displacement of an original myelinating Schwann cell by an interposed Schwann cell (see companion paper), we here provide evidence that in some instances the outer Schwann cell fails to retain any direct axonal contact, either with the axon centrally enclosed within the configuration or with any neighbouring axon. In contrast to the rat, delicate cytoplasmic processes often extended from the lateral extremes of outer Schwann cells. However, again no evidence for axonal contact was found, and similar processes also extended from the paranodal region of some singly myelinated non-displaced Schwann cells. Without exception the outer myelin sheath remained structurally intact, and characteristically underwent a series of conformational changes (progressive infolding of the paranodes and new areas of myelin compaction) which infer a continuing capacity of the outer Schwann cell to translocate myelin-specific components in a co-ordinated manner. A basal lamina was always present on the 'abaxonal' plasma membrane of the outer cell, but not on the 'adaxonal' surface except in areas involved in infolding, thus retaining the polarity which existed at the time of displacement from the axon. At single cross-sectional levels through the SCG, up to approximately 4% of myelinated axons were involved in double myelination. Double myelination was not detected in the sciatic nerve or in the paravertebral ganglia, thus indicating a predilection for the SCG as a site of development of these configurations. Though not challenging the role of the axon in initiating the formation of myelin, these data indicate that in this tissue myelin maintenance does not require direct contact between axonal and Schwann cell plasma membranes.

Animals↗