Defective myelination in the optic nerve of the Browman-Wyse (BW) mutant rat

Summary The Browman-Wyse (BW) rat displays a spectrum of ocular abnormalities which include myelination by Schwann cells of retinal ganglion cell (RGC) axons within the retina. Immunohistochemical and ultrastructural studies of the optic nerves of adult BW rats (30–60 days of age) with myelinated intraretinal axons were performed. Although individual nerves displayed considerable morphological variability, all were characterized by an initial dysmyelinated proximal segment which was separated from a normally myelinated distal segment by a transitional junctional zone. The proximal segment contained axons which were predominantly unmyelinated: where myelination occurred, almost all sheaths were P_o-positive, proteolipid protein-negative, and the myelinating cell was a Schwann cell. In the distal segment the distribution of myelinated axons appeared to be normal, sheaths were PLP⁺, and the myelinating cell was an oligodendrocyte. Within the proximal segment, axons that were myelinated by Schwann cells were isolated by a basal lamina and expanded extracellular spaces from the bulk of other RGC axons within the optic nerve. Few carbonic anhydrase (CAII)⁺ or GalC⁺ oligodendrocytes were seen in proximal segments that contained Schwann cells: anti-CAII antibody stained atypical cells within the proximal segments which did not resemble CAII⁺ oligodendrocytes in the distal segment, and which were probably GalC^–. Astrocytes appeared normal throughout the length of the nerve, and there was no morphological specialization at the junctional zone similar to that at the lamina cribrosa. The possible source (s) of the intraneural Schwann cells, and the pathogenetic mechanisms underlying the aberrant myelination of RGC axons within the BW optic nerve are discussed.     

Re-utilization of Schwann cells during ingrowth of ventral root afferents in perinatal kittens

Ventral roots in all mammalian species, including humans, contain significant numbers of unmyelinated axons, many of them afferents transmitting nociceptive signals from receptive fields in skin, viscera, muscles and joints. Observations in cats indicate that these afferents do not enter the spinal cord via the ventral root, but rather turn distally and enter the dorsal root. Some unmyelinated axons are postganglionic autonomic efferents that innervate blood vessels of the root and the pia mater. In the feline L7 segment, a substantial proportion of unmyelinated axons are not detectable until late in perinatal development. The mechanisms inducing this late ingrowth, and the recruitment of Schwann cells (indispensable, at this stage, for axonal survival and sustenance), are unknown. We have counted axons and Schwann cells in both ends of the L7 ventral root in young kittens and made the following observations. (1) The total number of axons detectable in the root increased throughout the range of investigated ages. (2) The number of myelinated axons was similar in the root's proximal and distal ends. The increased number of unmyelinated axons with age is thus due to increased numbers of small unmyelinated axons. (3) The number of separated large probably promyelin axons was about the same in the proximal and distal ends of the root. (4) Schwann cells appeared to undergo redistribution, from myelinated to unmyelinated axons. (5) During redistribution of Schwann cells they first appear as aberrant Schwann cells and then become endoneurial X-cells temporarily free of axonal contact. We hypothesize that unmyelinated axons invade the ventral root from its distal end, that this ingrowth is particularly intense during the first postnatal month and that disengaged Schwann cells, eliminated from myelinated motoneuron axons, provide the ingrowing axons with structural and trophic support.     

A light and electron microscopic study on pulpal nerve fibers in the lower incisor of the mouse.

Light and electron microscopic studies were made on pulpal nerve fibers in mouse lower incisors, typical continuously growing teeth. Serial sections, from the apex of the odontogenic sheath to the incisal edge of the apical foramen, were examined by light microscopy to identify myelinated fibers passing through the apical foramen. The fine structure of the pulpal nerves was examined by electron microscopy at three sites: 1) the level at the incisal edge of the apical foramen; 2) a level 5 mm incisal from the apex of the odontogenic sheath; and 3) the level where the incisor comes out of the alveolar bone. No myelinated fibers were found passing through the apical foramen; they were also lacking at the three levels of the pulp. At level 2, unmyelinated axons were seen in close contact with smooth muscle fibers of arterioles. At level 3, nerve fibers were difficult to distinguish from processes of fibroblasts and odontoblasts. Degenerating axons were present in Schwann cells, and fine unmyelinated axons running through the odontoblast cell layer were seen. Various types of unmyelinated axons were observed in the apical region (level 1). These axons were classified into 6 types on the basis of their fine structures: Type I, bundles of unmyelinated axons completely or partly ensheathed by Schwann cell cytoplasm (mature type); Type II, bundles of unmyelinated axons in a space formed by a Schwann cell membrane (regenerating type); Type III, bundles of unmyelinated axons ensheathed not by a Schwann cell, but merely by a basal lamina (regenerating type); Type IV, single axons in direct contact with the basal lamina (regenerating or terminal type); Type V, naked, electron-dense axons with many vesicles and mitochondria (growth cone-like type); and Type VI, electron opaque axons, due to loss of axonal organellae (degenerating type). The significance of these structures is discussed in relation to the continuous growth of the rodent incisor.     

Regionally defective colonization of the terminal bowel by the precursors of enteric neurons in lethal spotted mutant mice

In order to gain insight into the process of colonization of the bowel by the neural crest-derived precursors of enteric neurons, the development of the enteric nervous system was examined in lethal spotted mutant mice, a strain in which a segment of bowel is congenitally aganglionic. In addition, nerve fibers within the ganglionic and aganglionic zones of the gut of adult mutant mice were investigated with respect to their content of acetylcholinesterase, immunoreactive substance P, vasoactive intestinal polypeptide and serotonin, and their ability to take up [³Hserotonin. In both the fetal gut of developing mutant mice and in the mature bowel of adult animals abnormalities were limited to the terminal 2 mm of colon. The enteric nervous system in the proximal alimentary tract was indistinguishable from that of control animals for all of the parameters examined. In the terminal bowel, the normal plexiform pattern of the innervation and ganglion cell bodies were replaced by a coarse reticulum of nerve fibers that stained for acetylcholineserase and were continuous with extrinsic nerves running between the colon and the pelvic plexus. These coarse nerve bundles contained greatly reduced numbers of fibers that displayed substance P- and vasoactive intestinal polypeptide-like immunoreactivity, but a serotonergic innervation was totally missing from the aganglionic bowel. During development, acetylcholineserase and uptake of [³Hserotonin appeared in neural elements in the foregut of mutant mice on the 12th day of embryonic life (E12), about the same time these markers appeared in the forgut in normal mice. By day E14, neurons expressing one or the other marker were recognizable as far distally as about 2 mm from the anus. The appearance of neurons in segments of gut grown for 2 weeks as expiants in culture was used as an assay for the presence of neuronal progenitor cells in the segments of fetal bowel at the time of explantation. Both acetyl- cholinesterase activity and uptake of [³Hserotonin developed in neuronsin vitro in expiants of proximal bowel between days E10 and E17. At all times, however, the terminal 2mm of mutant but not normal fetal gut gave rise to aneuronal cultures. In some mutant mice rare, small, ectopically-situated pelvic ganglia were found just outside aganglionic segments of fetal colon. Uptake of [³Hserotonin, normally a marker for intrinsic enteric neurites, was found in these ganglia.The experiments suppport the hypothesis that the terminal 2 mm of the gut in lethal spotted mutant mice is intrinsically abnormal and thus cannot be colonized by the precursors of enteric neurons. The defect seems to be specific in that both cells and processes of intrinsic enteric neurons, including all serotonergic and most peptidergic neurites, seem to be excluded from the abnormal region while extrinsic nerve fibers, including sympathetic and sensory axons, are able to enter the aganglionic zones. Since examination of neural progenitor cells has failed to reveal a significant proximo-distal displacement of these cells through the enteric tube during development of the murine bowel, a defect in the migration of precursor cells down the alimentary tract to the terminal gut seems unlikely to be substantially involved in the pathogenesis of aganglionosis. This conclusion is supported by the normal enteric nervous system in proximal regions of the mutant gut and the presence of enteric type neurons outside of, but at the same level as the aganglionic region.     

Regrowth of axons in lesioned adult rat spinal cord: promotion by implants of cultured Schwann cells

Summary Highly purified populations of Schwann cells were grafted into lesioned adult rat spinal cord to determine if they promote axonal regeneration. Dorsal spinal cord lesions were created by a photochemical lesioning technique. Schwann cells derived from E16 rat dorsal root ganglia, either elongated and associated with their extracellular matrix or dissociated and without matrix, were rolled in polymerized collagen to form an implant 4–6 mm long which was grafted at 5 or 28 days after lesioning. No immunosuppression was used. Acellular collagen rolls served as controls. At 14, 28 and 90 days and 4 and 6 months after grafting, animals were analysed histologically with silver and Toluidine Blue stains and EM. The grafts often filled the lesion and the host borders they apposed exhibited only limited astrogliosis. By 14 days, bundles of unmyelinated and occasional thinly myelinated axons populated the periphery of Schwann cell implants. By 28 days and thereafter, numerous unmyelinated and myelinated axons were present in most grafts. Silver staining revealed sprouted axons at the implant border at 28 days and long bundles of axons within the implant at 90 days. Photographs of entire 1 m plastic cross-sections of nine grafted areas were assembled into montages to count the number of myelinated axons at the graft midpoint; the number of myelinated axons ranged from 517–3214. Electron microscopy of implants showed typical Schwann cell ensheathment and myelination, increased myelin thickness by 90 days, and a preponderance of unmyelinated over myelinated axons. Random EM sampling of five Schwann cell grafts snowed that the ratio of unmyelinated to myelinated axons was highest (201) at 28 days. These ratios implied that axons numbered in the thousands at the graft midpoint. Dissociated Schwann cells without matrix promoted axonal ingrowth and longitudinal orientation as effectively as did elongated Schwann cells accompanied by matrix. There was a suggestion that axonal ingrowth was at least as successful, if not more so, when the delay between lesioning and grafting was 28 rather than 5 days. Acellular collagen grafts did not contain axons at 28 days, the only interval assessed. In sum, grafts of Schwann cells in a rolled collagen layer filled the lesion and were well tolerated by the host. The Schwann cells stimulated rapid and abundant growth of axons into grafts and they ensheathed and myelinated these axons in the normal manner.     

Electron microscopic immunocytochemistry of GAP-43 within proximal and chronically denervated distal stumps of transected peripheral nerve

Summary Growth-associated protein, GAP-43 was initially described as a neuron-specific molecule thought to play a critical role in axonal growth and regeneration. However, it is also expressedin vitro in certain CNS glia, Schwann cell precursors and non-myelinating Schwann cells. In this paper, we report the subcellular localization of GAP-43in vivo in chronically-denervated Schwann cells in the distal stumps of previously transected rat sciatic nerve. We have used a progressive lowering of temperature method combined with the non-polar acrylic resin Lowicryl HM20 and a post-embedding labelling regime to visualize the distribution of GAP-43, S-100 (marker for Schwann cells), RT97 and NF68 (markers for different subunits of the neurofilament molecule). We report that (1) the smallest calibre regrowing axons were GAP-43-positive, sometimes NF68-positive but always RT97-negative; (2) regenerating myelinated axons and larger unmyelinated axons (> 0.7 m diameter) were NF68-positive, RT97-positive but GAP-43-negative; (3) cytoplasmic processes within Schwann cell basal lamina tubes in the distal stumps were S-100-positive, GAP-43-positive but RT97- and NF68-negative. The similar localization of GAP-43 within regrowing axons and denervated Schwann cells suggests that GAP-43 may function similarly in both situations, and may thus be involved in motility and/or elongation of axons and Schwann cells during regeneration.     

Regeneration of axons in the optic nerve of the adult Browman-Wyse (BW) mutant rat

Summary We have studied the regeneration of axons in the optic nerves of the BW rat in which both oligodendrocytes and CNS myelin are absent from a variable length of the proximal (retinal) end of the nerve. In the optic nerves of some of these animals, Schwann cells are present. Axons failed to regenerate in the exclusively astrocytic environment of the unmyelinated segment of BW optic nerves but readily regrew in the presence of Schwann cells even across the junctional zone and into the myelin debris filled distal segment. In the latter animals, the essential condition for regeneration was that the lesion was sited in a region of the nerve in which Schwann cells were resident. Regenerating fibres appeared to be sequestered within Schwann cell tubes although fibres traversed the neuropil intervening between the ends of discontinuous bundles of Schwann cell tubes, in both the proximal unmyelinated and myelin debris laden distal segments of the BW optic nerve. Regenerating axons never grew beyond the distal point of termination of the tubes. These observations demonstrate that central myelin is not an absolute requirement for regenerative failure, and that important contributing factors might include inhibition of astrocytes and/or absence of trophic factors. Regeneration presumably occurs in the BW optic nerve because trophic molecules are provided by resident Schwann cells, even in the presence of central myelin, oligodendrocytes and astrocytes. All the above experimental BW animals also have Schwann cells in their retinae which myelinate retinal ganglion cell axons in the fibre layer. Control animals comprised normal Long Evans Hooded rats, BW rats in which both retina and optic nerve were normal, and BW rats with Schwann cells in the retina but with normal, i.e. CNS myelinated, optic nerves. Regeneration was not observed in any of the control groups, demonstrating that, although the presence of Schwann cells in the retina may enhance the survival of retinal ganglion cells after crush, concomitant regrowth of axons cut in the optic nerve does not take place.     

Segregation of NGF receptor in sensory receptors, nerves and local cells of teeth and periodontium demonstrated by EM immunocytochemistry

Summary Nerve growth factor receptor immunoreactivity (NGFR-IR) in sensory nerves and somatosensory receptors of adult rat dental and periodontal tissue was analysed using a monoclonal antibody (192-IgG) and electron microscopy. In dental and periodontal nerves, the unmyelinated axons and their Schwann cells had occasional labelling of their cell membranes, and myelinated axons had none. Dental free nerve endings in predentin had varied NGFR-IR: 15% were unlabelled, 25% had some axonal membrane NGFR-IR, and 60% had intense membrane label and cytoplasmic staining. In periodontal ligament there were two types of NGFR-IR somatosensory receptors: Ruffini mechanoreceptors had extensive NGFR-IR on apposed membranes of the terminal Schwann cell and nerve endings, but no labelling of the neural fingers which extended out into the ligament tissue; and thin fibres had intense membrane NGFR-IR and cytoplasmic stain.Non-neuronal NGFR-IR had cell specific patterns: perineurial and endoneurial cells and Ruffini terminal Schwann cells had NGFR-IR on cell membranes and inside numerous pinocytotic vesicles; Schwann cells along unmyelinated axons had NGFR-IR cell membrane intensities which varied depending on the NGFR-IR intensity of the enclosed axons; odontoblasts were unlabelled except at sites of contact with the NGFR-IR pulpal or neural cells; pulp fibroblasts in the subodontoblast zone had intense NGFR-IR all along their cell membrane; and ligament fibroblasts were unlabelled.The diverse NGFR-IR patterns described here suggest that there are specific categories of cellular expression and localization which correlate with somatosensory receptor type, and that specific patterns also characterize various non-neuronal cells in dental and periodontal tissue. Only the endoneurial cells, perineurial cells, and Ruffini terminal Schwann cells had NGFR-IR endocytotic vesicles, suggesting NGF internalization by high-affinity receptors.     

Redistribution of Schwann cells at the developing PNS–CNS borderline. An ultrastructural and autoradiographic study on the S1 dorsal root of the cat

In order to test our hypothesis that myelin-forming Schwann cells early during development, after having been eliminated from their parent axons, colonize neighbouring unmyelinated axons, we studied the distribution of Schwann cells at the PNS–CNS border in the feline S1 dorsal spinal root during pre- and postnatal development using electron microscopy and autoradiography. Myelination of axons peripheral to the PNS–CNS border began about 1.5 weeks before birth. The adult distribution of one-third myelinated and two-thirds unmyelinated axons was noted 3 weeks after birth. Analysis based on to-scale reconstructions of axon and Schwann cell samples from the first 6 postnatal weeks gave the following results. 1) CNS tissue appeared in the proximal part of the root around birth and expanded peripherally during the first three postnatal weeks. (2) The number of Schwann cells associated with myelinated axons decreased. (3) The number of Schwann cells associated with unmyelinated axons increased. (4) The mitotic activity of the Schwann cells was low at birth and nil after the first postnatal weak. (5) Apoptotic cell units were virtually absent. (6) Aberrant Schwann cells, i.e. short and very short Schwann cells with distorted and degenerating myelin sheaths, were common. (7) The endoneurial space contained numerous Schwannoid cells i.e. solitary cells surrounded by a basal lamina. (8) Cytoplasmic contacts between unmyelinated axons and aberrant Schwann cells or Schwannoid cells were observed. We take these results to support our hypothesis.     

Protein 4.1B localizes on unmyelinated axonal membranes in the mouse enteric nervous system

Recent molecular studies on anchoring structures between myelin sheaths by glial cells (oligodendrocytes and Schwann cells (Sc) in the central (CNS) and peripheral nervous system (PNS), respectively) and axons indicated protein-protein interaction for the polarization of paranodes in the axons. The protein 4.1 (4.1) family was originally found in erythrocytes as a component of membrane skeletons, and genetic approaches revealed the precise family members. One of them, 4.1B, has been reported to be localized in paranodes and juxtaparanodes of myelinated axons. In this study, in addition to the myelinated axons, we also present the localization of 4.1B in nerve fibers in the adult mouse enteric nervous system, a subpopulation of mature unmyelinated nerve fibers in PNS. Ultrastructurally, 4.1B localized along the membranes of unmyelinated axons. Such unmyelinated axons were surrounded only by Sc, suggesting that the 4.1B may also have a role in direct Sc-axon interactions and maturation of the axons, as well as myelinating glial cell-axon interactions.     

Peripheral nerve grafts lacking viable Schwann cells fail to support central nervous system axonal regeneration

Summary Peripheral nerve grafts were implanted bilaterally into the diencephalon of adult hamsters. One graft segment contained both viable Schwann cells and their basal lamina tubes. The Schwann cell population in the second graft segment was killed by freezing prior to implantation. Seven weeks after graft implantations, the extracranial end of each graft segment was exposed, transected and labelled with a fluorescent tracer substance. One week after the labelling procedure each animal was perfused and the diencephalon and midbrain were examined. Ultrastructural analyses of both types of graft demonstrated the persistence of the Schwann cell-derived basal lamina tubes. Retrogradely labelled neurons were found in all cases in which an intact graft remained in place for two months, but were seen in only one case with a frozen graft. Large numbers of myelinated and unmyelinated axons were seen within the intact grafts, but no axons were found in the previously frozen grafts. These results indicate that lesioned CNS axons are able to regenerate vigorously when provided with an environment which includes viable Schwann cells. But, CNS axons regenerate less well, if at all, when Schwann cells are absent. Further, it appears that Schwann cell-derived basal lamina tubes, when isolated from their parent cells, are insufficient to initiate or sustain CNS axonal regeneration.This material is based upon work supported by the National Science Foundation under grant BNS-8416911     

Evidence for a Notch1‐mediated transition during olfactory ensheathing cell development

Olfactory ensheathing cells (OECs) are a unique glial population found in both the peripheral and central nervous system: they ensheath bundles of unmyelinated olfactory axons from their peripheral origin in the olfactory epithelium to their central synaptic targets in the glomerular layer of the olfactory bulb. Like all other peripheral glia (Schwann cells, satellite glia, enteric glia), OECs are derived from the embryonic neural crest. However, in contrast to Schwann cells, whose development has been extensively characterised, relatively little is known about their normal development in vivo. In the Schwann cell lineage, the transition from multipotent Schwann cell precursor to immature Schwann cell is promoted by canonical Notch signalling. Here, in situ hybridisation and immunohistochemistry data from chicken, mouse and human embryos are presented that suggest a canonical Notch‐mediated transition also occurs during OEC development.     

Interaction of regrowing PNS axons with transplanted aggregates of cultured CNS gliain vivo

Summary Aggregates of cultured neonatal mouse cerebellar astrocytes were implanted into adult mouse sciatic nerves. Two different experimental models were used: aggregates were either placed between proximal and distal stumps of totally transected nerves, or were placed in gaps in partially transected nerves in direct apposition with the cut surface of the proximal stumps. In the model where aggregates were not placed in contact with the proximal stump, regrowing axons rarely entered the aggregates. Where aggregates were placed in contact with the proximal stumps, axons entered the astrocyte-rich environment. Experimental depression of the supply of Schwann cells available to comigrate with regenerating axons proved to be unnecessary: astrocytes provided an alternative substrate for axons. Some axons became myelinated by oligodendrocytes which differentiated within the aggregates; however, few axons remained, unmyelinated, in long-term association with the transplanted astrocytes.     

De- and remyelination in spinal roots during normal perinatal development in the cat: a brief summary of structural observations and a conceptual hypothesis

We have studied the perinatal development of large myelinated axons (adult D > 10 microm) in cat ventral and dorsal lumbosacral spinal roots using autoradiography and electron microscopy (serial section analysis). These axons acquire their first myelin sheaths 2-3 weeks before birth and show nearly mature functional properties first at a diameter of 4-5 microm, i.e. 3-4 weeks after birth. The most conspicuous event during this development takes place around birth, when a transient primary myelin sheath degeneration strikes already well myelinated although short 'aberrant' Schwann cells. The aberrant Schwann cells become completely demyelinated, then measuring about 10 microm in length, and are subsequently eliminated from their parent axons. Morphometry indicates that on average 50% of the Schwann cells originally present along a prospective large spinal root axon suffer elimination. Here it should be noted that in cat lumbo-sacral spinal roots, the longitudinal growth of myelinated Schwann cells that belong to the group containing what will be the largest fibers is on average twice that of their parent axons. The elimination phenomenon is particularly striking in the dorsal roots close to the spinal cord where CNS tissue invades the root for several hundred micrometres. Our observations suggest that, once demyelinated and then eliminated, Schwann cells (i.e. aberrant Schwann cells) colonize neighbouring axons, future myelinated as well as future unmyelinated ones. In the former case the immigrant Schwann cells appear to start myelin production, possibly risking a second demyelination and elimination. We take our observations to indicate that Schwann cells in the cat, during normal development, may switch iteratively between a 'myelin-producing' and a 'non-myelin-producing' phenotype. From a functional point of view the transient presence along a myelinated axon of intercalated unmyelinated segments approximately 10 microm long, due to aberrant Schwann cells, would mean a slowing down of the action potential. The rapid disappearance of aberrant Schwann cells during the two first postnatal weeks could then explain the progressing normalization of the leg-length conduction time.     

Immunolocalization of membrane skeletal protein, 4.1G, in enteric glial cells in the mouse large intestine

4.1 family proteins are membrane skeletal proteins that interact with spectrin-actin networks and intramembraneous proteins. We reported that one of them, 4.1G, was immunolocalized in myelinated nerve fibers of the mouse peripheral nervous system, especially along cell membranes of paranodes and Schmidt-Lanterman incisures in Schwann cells. In this study, to examine 4.1G's appearance in unmyelinated peripheral nerve fibers, we focused on the enteric nervous system in mouse large intestines. In intestinal tissues prepared by an "in vivo cryotechnique" followed by freeze-substitution fixation, 4.1G was immunolocalized in Auerbach's myenteric plexus and connecting nerve fiber networks. Its immunostaining was mostly colocalized with glial fibrillar acidic protein, a marker of enteric glial cells, but not with c-Kit, a marker of interstitial cells of Cajal. Using whole-mount preparation after splitting inner and outer muscle layers, the nerve fiber networks including the plexus were clearly detected by the 4.1G immunostaining. By conventional pre-embedding immunoelectron microscopy, 4.1G was detected along cell membranes of enteric glial cells and their processes surrounding axons. These indicate that 4.1G may have some roles in adhesion and/or signal transduction in unmylinated PNS nerve fibers.     

Structure of rat aortic baroreceptors and their relationship to connective tissue

Summary The ultrastructure of fibres and sensory terminals of the aortic nerve innervating the aorta between the left common carotid and left subclavian arteries was investigated in the rat. This is the region from which most baroreceptor responses are recorded electrophysiologically. The fibres of the aortic nerve enter the adventitia and separate into bundles generally containing one myelinated fibre and four or five unmyelinated fibres of various sizes. The bundles pursue a roughly helical course through the adventitia; when they are close to the aortic media, the myelinated fibre loses its myelin sheath. A complex sensory terminal region is formed, as both the unmyelinated and premyelinated axons become irregularly varicose. The concentration of mitochondria becomes very dense and cytoplasmic deposits of glycogen are observed. Both unmyelinated and premyelinated axons branch, and the unmyelinated axons wind irregularly around the premyelinated axon. The latter may have several loops and small holes. The terminal regions of both types of axon contain clusters of clear 40 nm vesicles. Part of the surface of each terminal region is ensheathed by Schwann cells, but the rest of the axolemma is directly exposed to extracellular connective tissue. There are often several layers of basal lamina around the sensory terminals and parts of the axolemma and Schwann cell membranes are attached to it by fine fibrillar material. The basal laminae are also attached to fibroblasts, fibroblast-like perineurial cells and elastic laminae, and the whole cellular and extracellular system appears to be tightly bound together. No differences between baroreceptors of spontaneously hypertensive and normal rats were found.     

Afferent and efferent axons in the medial and posterior articular nerves of the cat

The goal of this study is to determine the average numbers of afferent axons and postganglionic autonomic (sympathetic) efferent axons supplying the cat knee joint through the medial and posterior articular nerves. Interestingly, both nerves are composed primarily of unmyelinated axons. Only 20% of the axons in the medial articular nerve are myelinated, with the overwhelming majority, 80%, being unmyelinated. The posterior articular nerve has 78% unmyelinated and 22% myelinated axons. Neither nerve contains ventral root efferent axons. The sympathetic chain, in both nerves, contributes no myelinated and only 50% of the unmyelinated axons. The medial and posterior articular nerves are therefore predominantly afferent, since all myelinated and the remaining 50% of the unmyelinated axons arise from the dorsal root ganglion cell. The ratio of afferent unmyelinated to myelinated axons is 2:1. The roles of these afferent unmyelinated axons must now be considered in regard to joint kinesthetics and pain.     

In vivo proliferation,migration and phenotypic changes of Schwann cells in the presence of myelinated fibers

Following injury to a peripheral nerve, changes in the behavior of Schwann cells help to define the subsequent microenvironment for regeneration. Such changes, however, have almost exclusively been considered in the context of Wallerian degeneration distal to an injury, where loss of axonal contact or input is thought to be critical to the changes that occur. This supposition, however, may be incorrect in the proximal stumps where axons are still in contact with their cell bodies. In this work, we studied aspects of in vivo Schwann cell behavior after injury within the microenvironment of proximal stumps of transected rat sciatic nerves, where axons are preserved. In particular we studied this microenvironment proximal to the outgrowth zone, in an area containing intact myelinated fibers and a perineurial layer, by using double immunolabelling of Schwann cell markers and 5-bromo-2'-deoxyuridine (BrdU) labeling of proliferating cells.In normal sciatic nerve, Schwann cells were differentiated, in an orderly fashion, into those associated with unmyelinated fibers that labeled with glial fibrillary acidic protein (GFAP) and those associated with myelinated fibers that could be identified by individual axons and myelin sheaths. After sciatic nerve transection, there was rapid and early expansion in the population of GFAP-labeled cells in proximal stumps that was generated in part, by de novo expression of GFAP in Schwann cells of myelinated fibers. Schwann cells from this population also underwent proliferation, indicated by progressive rises in BrdU and GFAP double labeling. Finally, this Schwann cell pool also developed the property of migration, traveling to the distal outgrowth zone, but also with lateral penetration into the perineurium and epineurium, while in intimate contact with new axons.The findings suggest that other signals, in the injured proximal nerve stumps, beyond actual loss of axons, induce 'mature' Schwann cells of myelinated axons to dedifferentiate into those that up-regulated their GFAP expression, proliferate and migrate with axons.     

Immunoelectron microscopic localization of E-cadherin in dorsal root ganglia, dorsal root and dorsal horn of postnatal mice

Summary Sensory neurons and associated glial cells are known to express the cell-cell adhesion molecule E-cadherin. The cellular and subcellular localization of this molecule in the dorsal root ganglion, dorsal root, and spinal cord of postnatal mice was studied by the pre-embedding immunoelectron microscopic labelling technique. In the dorsal root and the superficial layer of the dorsal horn, a subset of fasciculating unmyelinated axons expressed E-cadherin at their axon-axon contacts at all ages studied, and these axons were clustered together and segregated from E-cadherin-negative axons. In contrast, pre-myelinating large-diameter axons in P2 mice as well as myelinated axons in mice from P14 to adulthood were E-cadherin-negative. Glial cells also expressed E-cadherin: In the dorsal root ganglia, all of the satellite cells expressed E-cadherin at contact sites with neurons, other satellite cells, and basal lamina, at all ages studied. In dorsal roots from P14 to adulthood, myelin-forming Schwann cells expressed E-cadherin at the outer mesaxons and the contact sites with basal lamina. Non-myelin-forming Schwann cells occasionally stained for this molecule at contact sites with the plasma membrane of E-cadherin-positive axons and at other sites. These results strongly suggest that E-cadherin plays an important role in the selective fasciculation of a particular subset of unmyelinated sensory fibres, and also in glial cell contacts.     

Schwann cell myelin ensheathing C.N.S. axons in the nerve fibre layer of the cat retina

Summary In the retina of the cat the axons of the nerve fibre layer are unmyelinated and are provided with a C.N.S. myelin sheath only in the extraocular part of the optic nerve. The present study demonstrates that in the apparently normal cat retina close to the optic disc, some axons of the nerve fibre layer run for a short distance in the perivascular space of the retinal arteries. While coursing in the perivascular space, these C.N.S. axons become transiently myelinated by Schwann cells, which form a typical P.N.S. myelin sheath. These P.N.S. myelin sheaths terminate at a heminode in the transitional zone in which the C.N.S. axons penetrate the perivascular glial sheath in order to leave or to re-enter the nerve fibre layer. It is suggested that the Schwann cells, which elaborate the P.N.S. myelin around C.N.S. axons, are descendants of the Schwann cells of the perivascular autonomie nerves. The present study shows that Schwann cells are able to provide previously unmyelinated C.N.S. axons with a P.N.S. myelin sheath.