Periaxonal Space Research Articles

Spongy degeneration in the zitter rat: Ultrastructural and immunohistochemical studies

Pathological changes in the grey matter of the zitter rat were examined by electron microscopy and immunohistochemistry to investigate the pathogenesis of spongy degeneration. Vacuole formation was first detected in the pons and the outer thalamus at 2 weeks of age. The vacuoles arose from the periaxonal or inter-myelinic spaces as well as the cytoplasm of some oligodendrocytes or astrocytes. With increasing age, some dendrites and the cytoplasm of neurons developed an electron lucent area with sparse organelles and the vacuoles occasionally fused together. Although spongy degeneration gradually extended to the entire CNS, no inflammatory or phagocytotic cell infiltration and no viral particles were detected. Glial fibrillary acidic protein immunoreactivity increased transiently in the vacuolated areas from 2 to 15 weeks of age (maximal at 7 weeks of age). Although zitter rats older than 65 weeks showed some reactive astrocytes in vacuolated areas, their numbers and the intensity of immunostaining decreased with advanced vacuolation suggesting astrocytic hypofunction in response to tissue damage. Immunoreactivity for synaptophysin was weaker in the zitter rats than in the control rats throughout the observation period, which suggested that synapse formation was disturbed in the zitter rats, probably due to a combination of hypomyelination and vacuole formation in the grey matter. These findings suggest that an unknown genetic abnormality, probably related to cell membrane biosynthesis or cell-to-cell interactions, produces both hypomyelination and spongy degeneration in the zitter rat.

Myelination in the absence of myelin-associated glycoprotein.

The hypothesis that myelin-associated glycoprotein (MAG) initiates myelin formation is based in part on observations that MAG has an adhesive role in interactions between oligodendrocytes and neurons. Furthermore, the over- or underexpression of MAG in transfected Schwann cells in vitro leads to accelerated myelination or hypomyelination, respectively. Here we test this idea by creating a null mutation in the mag locus and deriving mice that are totally deficient in MAG expression at the RNA and protein level. In adult mutant animals the degree of myelination and its compaction are normal, whereas the organization of the periaxonal region is partially impaired. Mutant animals show a subtle intention tremor. Our findings do not support the widely held view that MAG is critical for myelin formation but rather indicate that MAG is necessary for maintenance of the cytoplasmic collar and periaxonal space of myelinated fibres.

Transient K + current and periaxonal potassium accumulation in the giant axon of the cockroach Periplaneta americana

Potassium currents were studied under voltage-clamp conditions in the cockroach isolated giant axon in the presence of 10 −7M tetrodotoxln. At room temperature, a transient K + current (inactivation time constant ∼ 5ms at + 30mV), having characteristics similar to the I A current, could be activated if the holding potential (HP) was maintained at values negative to −70mV. This current was not present at HP −50 mV or if the test pulse applied from a negative HP was preceded by a ∼ 15–20 ms conditioning predepolarization to −40 mV. Although this transient current could be attributed to the existence of inactivating K + channels, the current decrease during a voltage pulse could be likely explained by the external K + accumulation in the periaxonal space, revealed by the instantaneous K + tail current analysis. Even though the K + accumulation in the isolated cockroach axon is less than in many similar preparations, it can increase the external K + concentration by a factor of 10 under the experimental conditions and shift the equilibrium potential for K + ions by 30–50 mV towards positive values at the end of a 35 ms voltage pulse to +30 mV. Thus, the accumulation phenomena should always be taken into account in the studies of ionic conductances.

Prepubertal height velocity references over a wide age range.

<h3>ABSTRACT</h3> The block of voltage-dependent sodium channels by saxitoxin (STX) and tetrodotoxin (TTX) was investigated in voltage-clamped squid giant axons internally perfused with a variety of permeant monovalent cations. Substitution of internal Na⁺ by either NH₄⁺ or N₂H₅⁺ resulted in a reduction of outward current through sodium channels under control conditions. In contrast, anomalous increases in both inward and outward currents were seen for the same ions if some of the channels were blocked by STX or TTX, suggesting a relief of block by these internal cations. External NH₄⁺ was without effect on the apparent magnitude of toxin block. Likewise, internal inorganic monovalent cations were without effect, suggesting that proton donation by NH₄⁺ might be involved in reducing toxin block. Consistent with this hypothesis, decreases in internal pH mimicked internal perfusion with NH₄⁺ in reducing toxin block. The interaction between internally applied protons and externally applied toxin molecules appears to be competitive, as transient increases in sodium channel current were observed during step increases in intracellular pH in the presence of a fixed STX concentration. In addition to these effects on toxin block, low internal pH produced a voltage-dependent block of sodium channels and enhanced steady-state inactivation. Elevation of external buffer capacity only marginally diminished the modulation of STX block by internal NH₄⁺, suggesting that alkalinization of the periaxonal space and a resultant decrease in the cationic STX concentration during NH₄⁺ perfusion may play only a minor role in the effect. These observations indicate that internal monovalent cations can exert trans-channel influences on external toxin binding sites on sodium channels.

Ultrastructure and permeability of the Schwann cell layer surrounding the giant axon of the squid.

The ultrastructure of the Schwann cell layer surrounding the giant axon of the squid Alloteuthis subulata is described, and the permeability of extracellular compartments assessed by exposure to electron-dense tracers. Morphometric analysis is used to deduce the number, size and shape of the Schwann cells, and the routes for ion flux across the Schwann cell layer. Axons (mean diameter 233 microns) were surrounded by a 1-2 microns thick layer of Schwann cells which were approximately 1 micron thick, approximately 70 microns long and approximately 23 microns wide. There were around 62,000 Schwann cells per cm2 axon surface. The outer (abaxonal) surface of the Schwann cells was invaginated, with evidence for a covering of fine Schwann cells processes; the inner (adaxonal) surface of the Schwann cells was less folded. The percentage area occupied by mesaxonal cleft openings to the axon and to the basal lamina was 0.02% and 1.09% respectively. A system of tubules, the glial tubular system, occupied 3.9% of the Schwann cell volume, and opened to both axonal and basal lamina surfaces, with more elaborate lattice-like clusters towards the basal side of the cell. Tubule openings accounted for 0.26% of the surface area facing the axon and 0.37% of the area facing the basal lamina (where there was greater clustering of openings). The electron dense tracers horseradish peroxidase, ionic lanthanum and tannic acid filled mesaxon clefts, glial tubular system and periaxonal space. If ion flux occurred via the mesaxonal clefts, a theoretical series resistance (Rsth) of > 20 omega cm2, would be predicted, whereas if it occurred via the tubular system, the figure would be < 2 omega cm2, closer to physiological estimates. The results presented show that the glial tubular system is likely to be the major route for ion flux into and across the Schwann cell layer, and for clearance of K+ from the periaxonal space during periods of axonal stimulation. The implications for K+ homeostasis in the axonal microenvironment are discussed.

A model for axonal propagation incorporating both radial and axial ionic transport

We present an axonal model that explicitly includes ionic diffusion in the intracellular, periaxonal, and extracellular spaces and that incorporates a Hodgkin-Huxley membrane, extended with potassium channel inactivation and active ion transport. Although ionic concentration changes may not be significant in the time course of one action potential, they are important when considering the long-term behavior (seconds to minutes) of an axon. We demonstrate this point with simulations of transected axons where ions are moving between the intra- and extracellular spaces through an opening that is sealing with time. The model predicts that sealing must occur within a critical time interval after the initial injury to prevent the entire axon from becoming permanently depolarized. This critical time interval becomes considerably shorter when active ion transport is disabled. Furthermore, the model can be used to study the effects of sodium and potassium channel inactivation; e.g., sodium inactivation must be almost complete (within 0.02%) to obtain simulation results that are realistic.

The influence of nodal constriction on conduction velocity in myelinated nerve fibers

Myelinated nerve fibers exhibit a complex anatomy in the nodal region which includes a marked nodal-paranodal constriction and an intricate paranodal structure where the myelin sheath is separated from the axon by a narrow periaxonal space. In this study, a recently developed computational model of the mammalian myelinated nerve fiber based on electron microscopic data was employed to examine the effect of the nodal-paranodal axonal radius and periaxonal space width on the conduction of action potentials. These findings indicate that the nodal-paranodal constriction promotes higher conduction velocities by minimizing the component of the nodal capacity contributed by the paranodal axolemma. Model prediction of optimal nodal-paranodal radii is correlated with radii determined in experimental anatomical studies.

Neuropathology in Spontaneously Epileptic Rats(ESR). Comparative Study with Original Mutants Zitter and Tremor Rats.

The pathological alterations in the central nervous system (CNS) of spontaneously epileptic rats (SER) were reviewed in comparison with those of zitter (Zi) and tremor (Tm) rats which are original neurological mutants for SER. The main pathological findings were vacuolation and hypomyelination in the CNS. Vacuolation was prominent in SER and Tm, especially in the brain stem and thalamus. Ultrastructurally, vacuoles consisted mainly of swollen astrocytic processes and enlargement of extracellular space, as well as occasional enlargement of periaxonal spaces. Throughout the observation period, there was no inflammatory cell infiltrations nor phagocytotic cells. Vacuolation seemed to be mainly derived from tremor mutation.On the other hand, hypomyelination seemed to be related to zitter mutation, based on the chronological observation of early myelination in Zi from 1st to 28th days of age. The characteristic findings accompanied with hypomyelination were the presence of aberrant myelin sheaths in addition to a decreased number of myelinated fibers and increased number of radial components whose real significance has been still unknown. Zi revealed another characteristic alteration in oligodendrocytes, which was membranous abnormality associated with nuclear membrane or other intracellular organelles, such as mitochondria or endoplasmic reticulum. The structures of myelin sheath in these three kinds of mutants seemed to be quite normal and the expression of main structural proteins, such as myelin basic protein (MBP), proteolipid protein (PLP) and myelin associated glycoprotein (MAG) were also seemed to be normal.According to the pathological findings obtained to date, the two different genetic abnormalities (zitter and tremor mutations) interact and synergistically reinforce each other both in clinically and neuropathologically in this double mutant (SER) . To elucidate the etiology and pathogenesis of clinical symptoms and pathological alterations, further neuropathological investigations cooperated with other research fileds, such as genetics, biochemistry, pharmacology, electrophysiology and so on will be required.

Periaxonal ensheathment of lobster giant nerve fibres as revealed by freeze-fracture and lanthanum penetration

Sheath structure and permeability have been studied in the nerve fibres of lobster (Panulirus argus) walking limbs, in particular the individually ensheathed larger giant fibres, 100-150 microns in diameter, of which there are five or six in a peripheral bundle. They are easily distinguished and can be separated from neighbouring fibre bundles in which smaller giant axons (65-80 microns diameter) and many axons of much smaller diameter (5-15 microns) are ensheathed together. Each of the larger giant axons is enveloped by a Schwann cell layer outside of which is a multilayered sheath consisting of one-cell thick belts of flattened cells and interleaved zones of collagen fibrils and extracellular matrix. The cells in each belt lack basal lamina and, after freeze-fracture, as well as in thin sections, exhibit intercellular gap junctions and incomplete, fascia type, tight junctions; their most striking aspect is an exceedingly large number of exo-endocytic profiles. Permeability to lanthanum chloride in the bathing medium studied before or during fixation both in intact nerves and in nerves with surgically breached (slit) epineurium showed penetration of lanthanum tracer between the cells around the giant fibres, but the electron-dense tracer was excluded from the Schwann cell layer and the periaxonal space unless the epineurium had been slit. The extent of lanthanum diffusion was evaluated by transmission electron microscopy of thin sections and confirmed by X-ray microanalysis (EDAX) of comparable selected areas in such sections. The results indicate structural similarities but distinct permeability differences between the multilayered sheath surrounding the lobster giant axons and the vertebrate nerve perineurium. Other ultrastructural details provided by the freeze-fracture replicas concern the distribution of intramembrane particles in the axolemma and the Schwann and sheath cell membranes.

Morphology and electrical properties of Schwann cells around the giant axon of the squids Loligo forbesi and Loligo Vulgaris

The first successful dye-fills of Schwann cells around the split giant axon of Loligo show them to be spindle-shaped cells ca. 600 microns long and 20 microns wide lying parallel to the axonal axis. There are some 50,000 Schwann cells per cm2 of axonal membrane. Only a small part (ca. 6% of each Schwann cell membrane) is in contact with the periaxonal space, the remainder is overlain by adjacent Schwann cells, or applied to the basal lamina. The mean membrane potential of the Schwann cells in artificial seawater (ASW) varies from around -40 mV in fresh split-axon preparations to around -60 to -70 mV after 1-2 h; this hyperpolarization is not seen in preparations dissected and maintained in Ca2(+)-free ASW. Electrical- and dye-coupling (abolished by prior octanol treatment) is present between Schwann cells, but is weaker in cells with lower (less negative) membrane potentials. The implications for potassium homeostasis around the axon are briefly discussed.

A distributed-parameter model of the myelinated nerve fiber

This paper presents a new model for the characterization of electrical activity in the nodal, paranodal and internodal regions of isolated amphibian and mammalian myelinated nerve fibers. It differs from previous models in the following ways: (1) in its ability to incorporate detailed anatomical and electrophysiological data; (2) in its approach to the myelinated nerve fiber as a multi-axial cable; and (3) in the numerical algorithm used to obtain distributed model equation solutions for potential and current. The morphometric properties are taken from detailed electron microscopic anatomical studies (Berthold & Rydmark, 1983a, Experientia 39, 964-976). The internodal axolemma is characterized as an excitable membrane and model-generated nodal and internodal membrane action potentials are presented. A system of describing equations for the equivalent network model is derived, based on the application of Kirchoff's Current Law, which take the form of multiple cross-coupled parabolic partial differential equations. An implicit numerical integration method is developed and the numerical solution implemented on a parallel processor. Non-uniform spatial step sizes are used, enabling detailed representation of the nodal region while minimizing the number of total segments necessary to represent the overall fiber. Conduction velocities of 20.2 m sec-1 at 20 degrees C for a 15 microns diameter amphibian fiber and 57.6 m sec-1 at 37 degrees C for a 17.5 microns diameter mammalian fiber are achieved, which agrees qualitatively with published experimental data at similar temperatures (Huxley & Stämpfli, 1949, J. Physiol., Lond. 108, 315-339; Rasminsky, 1973, Arch, Neurol. 28, 287-292). The simulation results demonstrate the ability of this model to produce detailed representations of the transaxonal, transmyelin and transfiber potentials and currents, as well as the longitudinal extra-axonal, periaxonal and intra-axonal currents. Also indicated is the potential contribution of the paranodal axolemma to nodal activity as well as the presence of significant longitudinal currents in the periaxonal space adjacent to the node of Ranvier.

CNS PATHOLOGY IN THE NEUROLOGICAL MUTANT RATS ZITTER, TREMOR AND ZITTER-TREMOR DOUBLE MUTANT (SPONTANEOUSLY EPILEPTIC RAT, SER)

The pathological alterations in the central nervous system (CNS) were examined in three kinds of mutant rat; the zitter (zi/zi; Zi), the tremor rat (tm/tm; Tm) and the spontaneously epileptic rat (SER) which is a double mutant carrying both zitter and tremor genes. Two major alterations demonstrated in these mutants were hypomyelination and vacuolation or spongy degeneration. Hypomyelination was observed predominantly in SER and to a lesser extent in Zi, and was accompanied by a redundant or aberrant myelin sheath formation in addition to a decreased number of myelinated fibres. This appeared to be related to the occurrence of tremor. There was no abnormality in the structure of the myelin lamellae and oligodendrocytes or any destruction of myelin sheaths by phagocytic cells. The number of radial components in CNS myelin was increased almost equally in Zi, Tm and SER. Vacuolation was prominent in SER and Tm, especially in the brainstem and thalamus. Zi also developed mild vacuolation with advancing age. Vacuolation seemed to be related to the epileptic phenomena in SER and Tm. Vacuoles consisted mainly of swollen astrocytic processes and enlargement of the extracellular space, as well as occasional enlargement of periaxonal spaces. Thus both pathological findings--the hypomyelination derived from the zitter mutation with tremor, and the vacuolation from the tremor mutation with epileptic symptoms--were mutually exaggerated in SER. It is postulated that the two different genetic loci with zi and tm mutations interact and synergistically reinforce each other both clinically and pathologically in SER.

Functions and distribution of voltage‐gated sodium and potassium channels in mammalian schwann cells

Recent patch-clamp studies on freshly isolated mammalian Schwann cells suggest that voltage-gated sodium and potassium channels, first demonstrated in cells under culture conditions, are present in vivo. The expression of these channels, at least at the cell body region, appears to be dependent on the myelinogenic and proliferative states of the Schwann cell. Specifically, myelin elaboration is accompanied by a down regulation of functional potassium channel density at the cell body. One possibility to account for this is a progressive regionalization of ion channels on a Schwann cell during myelin formation. In adult myelinating Schwann cells, voltage-gated potassium channels appear to be localized at the paranodal region. Theoretical calculations have been made of activity-dependent potassium accumulations in various compartments of a mature myelinated nerve fibre; the largest potassium accumulation occurs not at the nodal gap but rather at the adjacent 2-4 microns length of periaxonal space at the paranodal junction. Schwann cell potassium channels at the paranode may contribute to ionic regulation during nerve activities.

Ultrastructural Studies on the Barrier Properties of the Paraganglia in the Rat Recurrent Laryngeal Nerve

The permeability of blood capillaries in the paraganglia of the rat recurrent laryngeal nerve (RLN) was investigated by employing the ionic lanthanum tracer at ultrastructural level. Two types of blood capillaries, namely, fenestrated and nonfenestrated types, were observed in the rat RLN and its associated paraganglia (RLN paraganglia). A preferential distribution of fenestrated capillaries in the RLN paraganglia was noted. Nonfenestrated capillaries were distributed in the area of RLN devoid of paraganglia. Minute aberrant ganglia consisting of 4-8 neurons were frequently encountered in the rat RLN near the paraganglia. The capillaries in these neuronal areas were also nonfenestrated. The lanthanum tracer was limited within the vascular lumen, but not in the extravascular space, in the RLN proper and in the area of RLN paraganglia where the neurons were identified. In the RLN paraganglia, the tracer was located in the vascular lumen, extravascular space, periaxonal space of nerve fibers, and the intercellular space of the RLN paraganglionic cells. We concluded that (1) a blood-nerve barrier and a blood-ganglion (or blood-neuron) barrier exist in the area of RLN devoid of paraganglia, and (2) blood-paraganglion barrier and blood-nerve barrier were lacking in the rat RLN paraganglia.

Voltage‐gated potassium currents in myelinating Schwann cells in the mouse.

1. The whole-cell variation of the patch-clamp technique was used to record ionic currents in Schwann cells obtained from enzyme-treated mouse sciatic nerves before and after the onset of myelination. 2. Only outward currents were evoked in embryonic Schwann cells, which had no myelin, at membrane potentials more positive than -40 mV. Neonatal myelinating cells developed depolarization-activated outward currents and hyperpolarization-activated inward currents. For large hyperpolarizations below -160 mV, inward currents exhibited a sag following a peak which appeared to be mainly due to Na+ blockade. 3. Membrane potentials of neonatal myelinating cells were more negative than those of embryonic cells. The depolarization of the membrane potentials per 10-fold increase in external K+ concentrations in neonatal myelinating cells was 57 mV which fits the Nernst equation for a K+ electrode. 4. Quinine (0.5-2 mM) blocked the outward currents in embryonic cells and Ba2+ (2 mM) blocked both outward and inward currents in neonatal myelinating cells leaving quinine-sensitive outward currents of the embryonic type. External Cs+ (5 mM) blocked mainly inward currents and internal Cs+ blocked outward currents. 5. Developmental changes of these voltage-gated K+ currents in myelinating cells showed that Ba2(+)-sensitive K+ currents disappeared rapidly during the first week of life in association with the membrane potential becoming more positive. In contrast, quinine-sensitive outward K+ currents of the embryonic type disappeared slowly during the first 3-4 weeks after birth. 6. It is concluded that neonatal myelinating Schwann cells developed new voltage-gated K+ channels, which are Ba2(+)-sensitive and set a new membrane potential, in addition to the voltage-gated K+ channels of embryonic type. The Ba2(+)-sensitive K+ channels in myelinating cells were suggested to play an important role in siphoning K+ ions accumulated in periaxonal space during nerve activities.

Edema formation and cellular alterations following spinal cord injury in the rat and their modification with p-chlorophenylalanine

The possibility that serotonin can modify the early pathological sequences occurring in spinal cord trauma was investigated in a rat model. To that end we took advantage of the possibility of influencing serotonin pharmacologically by treating animals with a serotonin synthesis inhibitor, p-chlorophenylalanine (p-CPA) before the production of the injury and compared the results with injured, untreated controls. A unilateral incision was made into the dorsal horn of the lower thoracic cord (about 2.5 mm deep, 4.5 mm long) and the rats were allowed to survive up to 5 h after the trauma. The injured region from untreated animals showed macroscopically at that time a pronounced swelling and the water content had increased by 3.5% as compared to intact controls. The segments rostral and caudal to the lesion also exhibited a profound increase in water content. Light microscopy revealed a significant expansion of the spinal cord as compared to controls. The swelling was most pronounced in the gray matter on the injured side. Electron microscopy showed distorted neurons, swollen astrocytes and extracellular edema in the gray matter in and around the primary lesion. There was also a sponginess in the surrounding white matter with disruption of myelin, collapsed axons and widened periaxonal spaces. Pretreatment of the rats with p-CPA significantly reduced the swelling of the injured spinal cord and there was no visible expansion. The ipsilateral edema in the central gray matter was considerable less pronounced as compared to that in untreated animals. The increase in water content was less than 1% in these animals.(ABSTRACT TRUNCATED AT 250 WORDS)

ULTRASTRUCTURAL EXAMINATION OF BRAIN EDEMA IN RATS CAUSED BY HIGH-DOSE 2, 5-HEXANEDIONE TREATMENT

High dose (700 or 500mg/kg/day) of 2, 5-hexanedione was orally administered to female Sprague-Dawley rats for 2 to 6 weeks. and the cerebellar and vestibular nuclei were examined ultrastructurally. Abnormal gait was observed at the 2nd week (700mg/kg) or the 3rd week (500mg/kg) of treatment, and one of six animals dosed 700mg/kg died at the 4th week of experiment. In the histopathological examination of the dead rat, edema and petechial hemorrhage were noted in the thalamus and vacuolation was observed in the cerebellar nuclei. Ultrastructural examination of the cerebellar and vestibular nuclei of the rats, which were dosed 700mg/kg for 2 weeks and fixed with an isotonic neutral fixative by gentle perfusion method, revealed dilatation of the periaxonal space, axonal degeneration, and extracellular edema. Nerve endings in these nuclei were occasionally detached from the nerve cells, and some of the endings were degenerated or destroyed. In the animals which survived and administered for a longer period, however, there were accumulation of neurofilaments and swelling of the nerve endings.

Neuropathological Changes and Neurological Function After Spinal Cord Compression in the Rat

As part of a series of experimental investigations of the effects of various pharmacological agents on the outcome of compressive spinal cord trauma in the rat, the time course of the cell changes in the cord at the site of and distal to the compression was studied at the light microscopic level. The degree of compression used with the present model results in a transient paraparesis that recovers almost completely over a period of 3 weeks as judged by the inclined plane technique. The most significant morphological findings were as follows. Initially (1 and 24 h after the impact) there was pronounced swelling and hemorrhage at the compression site, chiefly in the gray matter of the cord. On day 4 there was severe necrosis in the same region, with numerous macrophages and leukocytes. Rats killed after 21 days showed either minor residual signs of necrosis or essentially normal tissue architecture. Surprisingly, necrosis with delayed onset also developed in the dorsal columns, involving the pyramidal tracts. This necrosis was detected in animals killed after 9 and 21 days but not in those observed after 4 days or earlier. The longitudinal tracts of the white matter showed reduced staining in paraffin sections of the compression site. Epon sections revealed splits in the myelin sheaths and enlarged periaxonal spaces as early as 1 h after the impact. The alterations in the longitudinal tracts persisted throughout the 21-day observation period and extended down to L2-L4. There was gradual functional recovery, documented by the inclined plane test. Preinjury values were almost reached on day 21, although the cord still showed some morphological damage. In individual animals, no relation was found between degree of function as tested by inclined plane and extent of morphologic injury. Additional functional and morphological methods obviously are needed in future investigations of the effects of treatments on the outcome of compressive spinal cord injury.

The myelin-associated glycoprotein is enriched in multivesicular bodies and periaxonal membranes of actively myelinating oligodendrocytes.

The myelin-associated glycoprotein (MAG) is a member of the immunoglobulin gene superfamily that is selectively expressed by myelin-forming cells. A developmentally regulated, alternative splicing of a single MAG transcript produces two MAG polypeptides (72 and 67 kD) in the central nervous system (CNS). MAG occurs predominantly as the 67-kD polypeptide in the peripheral nervous system (PNS). This study determined the subcellular localization of CNS MAG at different postnatal times when the 72-kD form (7-d) and 67-kD form (adult) are quantitatively abundant. These distributions were also compared to those of MAG in the PNS. In adult rat, MAG is selectively enriched in periaxonal membranes of CNS myelin internodes. This restricted distribution differs from that in PNS myelin internodes where MAG is also enriched in paranodal loops, Schmidt-Lanterman incisures, and mesaxon membranes. In 7-d-old rat CNS, MAG was associated with periaxonal membranes during axonal ensheathment and enriched in Golgi membranes and cytoplasmic organelles having the appearance of multivesicular bodies (MVBs). MAG-enriched MVBs were found in oligodendrocyte perinuclear regions, in processes extending to myelin internodes, and along the myelin internode in outer tongue processes and paranodal loops. MAG-enriched MVBs were not found in oligodendrocytes from adult animals or in myelinating Schwann cells. These findings raise the possibility that the 72-kD MAG polypeptide is associated with receptor-mediated endocytosis of components from the periaxonal space or axolemma during active stages of myelination.

Cytochemical demonstration of TPPase in myelinated fibers in the central and peripheral nervous system of the rat

Thiamine pyrophosphatase (TPPase) activity was demonstrated by means of cytochemistry and electron microscopy in association with myelinated fibers in the central and peripheral nervous system of the rat. The areas studied included corpus callosum, hippocampus, cerebral cortex, cervical spinal cord and sciatic nerves. In the myelin sheaths, the enzymatic activity was found in 3 locations: (1) within oligodendroglial and Schwann cytoplasmic clefts between myelin lamellae; (2) in the major dense line of myelin; and (3) within the periaxonal space. In addition to this myelin-associated TPPase, enzymatic activity was also observed in specific cytoplasmic localizations in myelinogenic cells. Oligodendrocytes displayed TPPase activity within the nuclear envelope and the endoplasmic reticulum cisternae, whereas Schwann cells displayed TPPase activity within the endoplasmic reticulum and Golgi saccules. The results are discussed in relation to the role that TPPase might play in myelinated fibers, including roles in the conduction of nerve impulses or roles in the maintenance of structural configuration of myelin sheaths.