Fine structure of regenerated ependyma and spinal cord in Sternarchus albifrons

The caudal-most regenerated spinal cord in Sternarchus albifrons consists solely of an ependymal tube. Ependymal cells are enlarged radially and are more numerous than in unregenerated cord. Projections of ependymal cell cytoplasm and Reissner's fiber fill most of the central canal. Small groups of neurites and cell processes filled with dense-cored vesicles lie between abluminal processes of ependymal cells. Rostral to this, additional cells appear dorsal and lateral to the inner ependymal layer. Some cell bodies contain numerous dense-cored vesicles. Larger bundles of neurites, some with synapses, are present. Invaginations of the peripheral edge of the cord create enclosed spaces lined with basal lamina. In the peripheral region, longitudinally oriented neurites extend through extracellular spaces or channels. The ventral portion at some levels of regenerated cord is completely filled with neurites, processes containing dense-cord vesicles, and capillaries. Similar masses of neurites and processes containing dense-cored vesicles lie outside the cord proper, in or near the meningeal layer. In rostral-most sections, the organization of regenerated spinal cord approaches that of normal cord, with the regenerated cord exhibiting groups of myelinated axons, differentiated fibrous astrocytes and oligodendroglia, cell bodies containing dense-cored vesicles, and differentiated electromotor neurons. These observations indicate a degree of pluripotency in some of the ependymal cells in adult Sternarchus. Moreover, they are consistent with a role of ependymal cells in the guidance of regenerating neurites.     

Locomotor recovery in spinal-transected lamprey: role of functional regeneration of descending axons from brainstem locomotor command neurons

In spinal-transected lampreys, locomotor function is restored within a few weeks, and a number of mechanisms could potentially contribute to behavioral recovery. The present study examines the contribution of functional regeneration of descending axons from brainstem locomotor "command" centers to behavioral recovery using both whole animal and in vitro preparations. Under in vitro conditions activation of brainstem locomotor centers could elicit locomotor patterns below a healed transection of the rostral spinal cord. Additional experiments indicated that spinal locomotor networks below a spinal transection could be directly activated by descending axons arising from brainstem neurons. In contrast, mechanosensory inputs and regenerated spinal neurons did not contribute significantly to the initiation of locomotor activity below a spinal lesion. Regenerated descending axons from large reticulospinal Muller neurons did not contribute significantly to the recovery of locomotor function. These results suggest that functional regeneration of descending axons from neurons in brainstem locomotor command centers contribute significantly to the recovery of locomotion following spinal cord transection. This is the first demonstration in a vertebrate of functional regeneration of descending command axons which can initiate locomotion.     

Caudal spinal cord of the teleost Sternarchus albifrons resembles regenerating cord

The morphology of spinal cord in the caudal-most spinal segments of normal adult Sternarchus albifrons is different from that of more rostral adult cord. The caudal segments are strikingly similar to the regenerating spinal cord observed after amputation of the tail in Sternarchus. In the caudal-most vertebral segment of normal spinal cord, ependymal cells are radially enlarged and are more numerous than in more rostral adult cord. Large processes of the ependymal cells extend into the central canal, which also contains a prominent Reissner's fiber. Invaginations of the outer surface of the spinal cord, with the associated basal lamina, are common. Lateral to the immediate ependymal layer, extracellular spaces contain longitudinally oriented neurites. Cell bodies and cell processes filled with dense-cored vesicles occur throughout the caudal-most segment of spinal cord, and are especially concentrated in the ventral half, interspersed with numerous capillaries. In all these respects the caudal-most segments of normal adult spinal cord in Sternarchus closely resemble regenerating spinal cord of Sternarchus. In both regions, at least some of the ependymal cells retain the ability to divide and generate new neurons and glial cells.     

A quantitative analysis of ultrastructural changes induced by electrical stimulation of identified spinal cord axons in the larval lamprey

Summary Microelectrodes filled with horseradish peroxidase (HRP) were used to label single identified giant axons in the isolated lamprey spinal cord. Subsequent to the iontophoretic injection of HRP, the spinal cord was stimulated at repetition rates of 20–30/s and the activity in labelled axons monitored. Immediately following failure of the action potential, the spinal cord was fixed by immersion and processed for light and electron microscopy. Electron micrographs were taken of synaptic contacts made by the labelled axons. Several quantitative measures were made from each synapse using a digitizing tablet interfaced with a digital computer. These measures included vesicle number (VN), vesicle area (VA), length of differentiated membrane (DM), vesicle density (VD=VN/VA), vesicle frequency (VF = VN/DM), and a relative measure of the amount of vesicle membrane added to the axolemma during the stimulation period, the curvature ratio (CR). Measures from 106 stimulated synapses were compared with 134 synapses from injected but unstimulated giant axons. The results from these experiments suggest that measurable ultrastructural changes occur during transmitter release at identified C.N.S. synapses, which are consistent with the hypothesis of synaptic vesicle recycling.     

Phenylethanolamine N-methyltransferase-immunoreactive terminals synapse on adrenal preganglionic neurons in the rat spinal cord

Adrenergic neurons in the C1 region in the ventrolateral medulla oblongata send descending axons into spinal cord which terminate in thoracic and upper lumbar segments, overlapping the distribution of sympathetic preganglionic neurons. The present study was undertaken to determine whether adrenergic fibers synapse directly on preganglionic neurons which innervate the adrenal medulla and to examine the ultrastructure of these fibers during development. The ultrastructure and synaptology of adrenergic axons in the intermediolateral nucleus of mid-thoracic spinal cord were studied in 7-, 9-, 24-, 30-, 60-, and 90-day-old rats using immunocytochemical staining for phenylethanolamine N-methyltransferase, the epinephrine-synthesizing enzyme. Phenylethanolamine N-methyltransferase-immunoreactivity was observed in the cytoplasm of unmyelinated axonal varicosities and intervaricose segments in the neuropil of intermediolateral nucleus. Phenylethanolamine N-methyltransferase-immunoreactive synaptic boutons were filled with spherical electron-lucent vesicles and occasional larger dense-core vesicles. These boutons were observed to form symmetrical synaptic contacts with dendritic processes at all ages examined. Asymmetrical synapses on dendrites were also observed in adult rats. Axosomatic synaptic contacts were frequently observed in immature rats, but were never observed in adult rats. To determine whether adrenergic axons synapse on preganglionic neurons which project to the adrenal medulla, adrenal preganglionic neurons were retrogradely labeled with horseradish peroxidase and adrenergic axons were stained for phenylethanolamine N-methyltransferase-immunoreactivity. In young rats, phenylethanolamine N-methyltransferase-immunoreactive boutons were observed to form symmetrical axosomatic and axodendritic synaptic contacts with adrenal preganglionic neurons in intermediolateral nucleus. These contacts had already formed by postnatal day 7, the youngest age studied. In contrast, it was not possible to verify that adrenal preganglionic neurons receive adrenergic innervation in adult rats, since phenylethanolamine N-methyltransferase-immunoreactive boutons were only observed in contact with small diameter dendrites that were not retrogradely labeled by horseradish peroxidase. These studies demonstrate that adrenal preganglionic neurons receive adrenergic synapses prior to the first postnatal week. The initial synapses which form on preganglionic somata and proximal dendrites appear to reorganize late in development. It is suggested that these become more distally located as the dendritic tree matures. More generally, these observations suggest that adrenergic bulbospinal neurons are involved in central regulation of adrenal development and function.     

Morphology and synaptic relationships of physiologically identified low-threshold dorsal root axons stained with intra-axonal horseradish peroxidase in the cat and monkey

The arborizations and synaptic relationships of intra-axonally stained horseradish peroxidase- (HRP) labeled primary afferent fibers to the dorsal horn of the cat and monkey spinal cord have been studied by light and electron microscopic methods. The light microscopic arborizations of the afferent fiber types (hair follicle afferents, pacinian corpuscle afferents, type I and type II slowly adapting afferents) are similar to those described by Brown and his colleagues (1) in the cat. The synaptic profiles formed by labeled afferents contain rounded synaptic vesicles. In serial thin sections, it was found that single dorsal root axons may make hundreds or thousands of synapses with neuronal structures of the dorsal horn. The vast majority of synaptic contacts are on the dendritic trees of dorsal horn neurons. The synapses made by these low-threshold afferent axons are almost all in the deeper laminae (III-VI) of the dorsal horn. The hair follicle afferent axons and the pacinian corpuscle afferents have numerous vesicle-containing structures that synapse on them to form either axoaxonal synapses or dendroaxonal synapses. The slowly adapting afferent axons are less often found to be postsynaptic to axons or dendrites. It is concluded that different physiological classes of primary afferent axons have different morphological characteristics, both at the light and electron microscopic level.     

Development of spinal cord in the isolated CNS of a neonatal mammal (the opossumMonodelphis domestica) maintained in longterm culture

Summary The CNS of the newly born opossum removed in its entirety survives and maintains its electrical excitability in suitable culture media for up to ten days at 25 ° C. The structure of the developing neonatal spinal cord has been studied in the intact animal and in the cultured CNS. The differentiation and survival of individual cells and subcellular structures were followed at the light and electron microscopic level. The expression of cell markers in neuronal and glial cells was studied immunocytochemically using commercially available antibodies. Both mono- and polyclonal antibodies raised against antigens from several other species cross-reacted with Monodelphis antigens. The spinal cord of preparations removed from three-day-old-animals showed many neuron specific enolase-positive large neurons in the ventral horn as well as vimentin- and glial fibrillary acidic protein-positive radial glial cells and numerous small diameter unmyelinated axons, abundant dendrites and synaptic structures. From post natal day 5 to post natal day 8 continued differentiation of neurons and differentiation of radial glial cells into astrocytes were apparent. Radial glial fibres and astrocytes reacted positively to antibodies against glial fibrillary acidic protein. Myelin had not appeared at 8 days. A comparison of material obtained from postnatal day 3-postnatal day 4 preparations fixed immediately after dissection and from postnatal day 3-postnatal day 4 preparations fixed after 5 days in culture showed growth with continued mitotic activity of the neuroepithelial cells and further neuronal and glial maturation in the spinal cord especially in the more rostral end. In successful experimentsin vitro, the preservation of individual cells, organelles, membranes and synapses was similar in the freshly dissected and cultured preparations apart from a distinct loss of the youngest and some of the oldest neurons in the spinal cord. Also the main fibre tracts (dorsal, lateral and ventromedial funiculus) survived. Virtually all preparations that had not been damaged or injured showed these results. Possible reasons for the death or survival of individual neuronal or glial cell populations in these preparations are discussed.     

Unique in vivo properties of olfactory ensheathing cells that may contribute to neural repair and protection following spinal cord injury

Olfactory ensheathing cells (OECs) are specialized glial cells that guide olfactory receptor axons from the nasal mucosa into the brain where they make synaptic contacts in the olfactory bulb. While a number of studies have demonstrated that in vivo transplantation of OECs into injured spinal cord results in improved functional outcome, precise cellular mechanisms underlying this improvement are not fully understood. Current thinking is that OECs can encourage axonal regeneration, provide trophic support for injured neurons and for angiogenesis, and remyelinate axons. However, Schwann cell (SC) transplantation also results in significant functional improvement in animal models of spinal cord injury. In culture SCs and OECs share a number of phenotypic properties such as expression of the low affinity NGF receptor (p75). An important area of research has been to distinguish potential differences in the in vivo behavior of OECs and SCs to determine if one cell type may offer greater advantage as a cellular therapeutic candidate. In this review we focus on several unique features of OECs when they are transplanted into the spinal cord.     

Reconstruction of the glial environment of a photochemically induced lesion in the rat spinal cord by transplantation of mixed glial cells

Summary It is becoming increasingly apparent that the astrocytic environment is critical to the normal development and functioning of the CNS, and that acute injury to the spinal cord causes destruction of glial cells in addition to neurones and axons. The aims of this study were to assess the viability of reconstructing the astrocytic environment of a cystic spinal cord lesion by transplantation of glial cells and to examine the effect of the transplanted cells on meningeal cell invasion and revascularisation of the lesion and on axonal regeneration. Neonatal rat and kitten mixed glial cells and the CG-4 rat O-2A progenitor cell line were transplanted into a lesion produced in the dorsal funiculus of the rat spinal cord by photochemical infarction. The animals were killed 4 weeks after injury, their cords examined with light and electron microscopy and compared with control animals that were injected with medium alone. Transplantation of all three preparations resulted in increased numbers of astrocytes in the area of Wallerian degeneration cranial to the lesion and within the cyst. Mixed glial cell cultures prepared from neonatal rat forebrain contained cells within vitro characteristics of type-1 astrocytes, and produced dense clusters of astrocytes that were surrounded by meningeal cells, resulting in a fragmented environment in the cyst. In contrast, glial cell cultures prepared from kitten forebrain and the CG-4 cell line produced cells that filled the cyst with a loose network of fine processes and reduced meningeal cell infiltration of the lesion. The CG-4 cell line significantly increased the density of blood vessels in the centre of the lesion and the number of spared axons present dorsal to the lesion, but none of the preparations significantly increased the number of axons regenerating at the caudal end of the lesion. We conclude that O-2A progenitorderived astrocytes are more suitable for reconstruction of the glial environment of a cystic lesion in the rat spinal cord than type-1 like astrocytes and would therefore be the cell of choice to engineer to produce factors that promote axonal regeneration.     

Architecture and synaptic relationships in the intermediolateral nucleus of the thoracic spinal cord of the rat: HRP labelling, catecholamine histochemistry and electron microscopic studies

Summary The organization of the intermediolateral nucleus (IML) of the thoracic spinal cord was examined using glyoxylic acid-induced fluorescence histochemistry, retrograde horseradish peroxidase (HRP) labelling and electron microscopy. In serial sections of T2, it was found that the distribution of catecholamine nerve terminals was intimately related to the neuronal perikarya of IML. Potassium permanganate fixation and 5-hydroxydopamine treatment revealed small dense-cored vesicles in axon varicosities with or without synaptic specializations. A gelatinous region, composed of small diameter dendrites and unmyelinated axons, formed a narrow longitudinal bundle in the centre of the nucleus. The population of the axon varicosities in the IML was 0.17 ± 0.02/m² in 75 nm sections. The average size of the axon varicosities with flat synaptic vesicles was 1.44 ± 0.05 m² and that of varicosities with spherical vesicles was 0.97 ± 0.02 m². After HRP injection into the superior cervical ganglion, ipsilateral IML neurons were labelled in T1–T3 segments of the spinal cord. Axon varicosities with flat and others with spherical synaptic vesicles synapsed on the dendrites labelled by HRP. Among axon varicosities synapsing on the preganglionic sympathetic neurons, 74.8 ± 7.1% at axo-somatic synapses and 46.0 ± 6.7% at synapses on proximal dendrites contained flat synaptic vesicles.     

Matrix inclusion within synthetic hydrogel guidance channels improves specific supraspinal and local axonal regeneration after complete spinal cord transection

We have previously shown that a novel synthetic hydrogel channel composed of poly(2-hydroxyethyl methacrylate-co-methyl methacrylate) (pHEMA-MMA) is biocompatible and supports axonal regeneration after spinal cord injury. Our goal was to improve the number and type of regenerated axons within the spinal cord through the addition of different matrices and growth factors incorporated within the lumen of the channel. After complete spinal cord transection at T8, pHEMA-MMA channels, having an elastic modulus of 263+/-13 kPa were implanted into adult Sprague Dawley rats. The channels were then filled with one of the following matrices: collagen, fibrin, Matrigel, methylcellulose, or smaller pHEMA-MMA tubes placed within a larger pHEMA-MMA channel (called tubes within channels, TWC). We also supplemented selected matrices (collagen and fibrin) with neurotrophic factors, fibroblast growth factor-1 (FGF-1) and neurotrophin-3 (NT-3). After channel implantation, fibrin glue was applied to the cord-channel interface, and a duraplasty was performed with an expanded polytetrafluoroethylene (ePTFE) membrane. Controls included animals that had either complete spinal cord transection and implantation of unfilled pHEMA-MMA channels or complete spinal cord transection. Regeneration was assessed by retrograde axonal tracing with Fluoro-Gold, and immunohistochemistry with NF-200 (for total axon counts) and calcitonin gene related peptide (CGRP, for sensory axon counts) after 8 weeks survival. Fibrin, Matrigel, methylcellulose, collagen with FGF-1, collagen with NT-3, fibrin with FGF-1, and fibrin with NT-3 increased the total axon density within the channel (ANOVA, p<0.05) compared to unfilled channel controls. Only fibrin with FGF-1 decreased the sensory axon density compared to unfilled channel controls (ANOVA, p<0.05). Fibrin promoted the greatest axonal regeneration from reticular neurons, and methylcellulose promoted the greatest regeneration from vestibular and red nucleus neurons. With Matrigel, there was no axonal regeneration from brainstem motor neurons. The addition of FGF-1 increased the axonal regeneration of vestibular neurons, and the addition of NT-3 decreased the total number of axons regenerating from brainstem neurons. The fibrin and TWC showed a consistent improvement in locomotor function at both 7 and 8 weeks. Thus, the present study shows that the presence and type of matrix contained within synthetic hydrogel guidance channels affects the quantity and origin of axons that regenerate after complete spinal cord transection, and can improve functional recovery. Determining the optimum matrices and growth factors for insertion into these guidance channels will improve regeneration of the injured spinal cord.     

胚胎脊髓移植在成鼠损伤脊髓神经通路重建中的作用——电镜CB—HRP示踪

目的：探讨胎鼠脊髓在成鼠损伤脊髓神经通路修复中的作用。方法：将Ｅ１４大鼠胚胎脊髓植入成鼠损伤脊髓后３０、５０、７０天时，通过坐骨神经和红核引入ＣＢ－ＨＲＰ，取移植部位脊髓做冰冻切片，经ＴＭＢ组化显色后，再分步制成电镜标本，然后在电镜下观察移植物和宿主脊髓的纤维联系。结果：术后３０天时，来自背根的轴突再生进入移植物，附近运动神经元和中间神经元的突起此时向移植物内发送新支。术后５０天时，来自背根有的轴     

Nerve regeneration in the dorsal funiculus of the rat spinal cord: a light and electron microscopic study

In an attempt to identify regenerating axons in the central nervous system, a partial transection of the dorsal funiculus in the rat spinal cord was carried out with a pair of microdissection scissors, and a nylon thread loop was inserted into the lesion to demarcate the severed tissue. Nerve regeneration through the demarcated lesion was observed 4-20 days after the operation by light and electron microscopy. In the early stage, many naked axons appeared from the caudal part of the lesion, and some of these further extended into the demarcated space. They contained an accumulation of mitochondria, smooth-surfaced endoplasmic reticulum and vesicles in the axoplasm; this axoplasmic feature indicated that they were regenerating axons. They gradually increased in number, and took highly irregular courses exhibiting various fluctuations in diameter throughout their lengths. Immature Schwann cells as well as glial cells including oligodendrocytes and astrocytes appeared in close association with these regenerating axons. Oligodendrocytes eventually formed thin myelin sheaths. On the other hand, naked axons were present deflecting outside the thread loop; they showed no axoplasmic characteristics as described above. These axons could be regarded as uninjured ones merely undergoing demyelination due to the surgery. Thus, regenerating axons were clearly distinguished from merely demyelinated ones, and some of them were shown to grow through the traumatic lesion in the dorsal funiculus of the rat spinal cord.     

Long-term culture of human fetal spinal cord neurons: morphological, immunocytochemical and electrophysiological characteristics

Cultures were prepared from ventral spinal cord tissue from 8-11-week gestational human fetuses and grown for a period of up to 6 months. These cultures were studied by morphological, immunocytochemical and intracellular electrophysiological techniques. From 2 weeks in vitro and onward, small bipolar cells were found in outgrowths of spinal cord explants and were identified as neurons by positive immunoreactions with an antibody specific for neurofilament protein. In addition, a large population of glial fibrillary acidic protein-positive astrocytes and a smaller number of galactocerebroside-positive oligodendrocytes were recognized in these cultures. The development of synaptic terminals was also studied by electron microscopy. The first appearance of synaptic terminal was found in a 3-week culture and was an axo-dendritic synapse. During the next 2 months, there was a steady increase in number and structural maturation of synaptic profiles. In addition to axo-dendritic synapses, which were most common, axo-somatic and axo-axonic synapses were demonstrated. After 3 months in culture, the occurrence of large neurons possessing the characteristic features of mature neurons was also noted. Although the occurrence of oligodendrocytes in these cultures was confirmed, no myelination of axons was demonstrated by electron microscopy. Intracellular recordings were obtained from the cultured spinal cord cells, and these cells were identified clearly as neurons by their action potential responses to depolarizing current pulses. The average input resistance of these neurons was 31 M omega with resting membrane potential of -52 +/- 2.3 mV.     

Endomorphin-2 immunoreactivity in the cervical dorsal horn of the rat spinal cord at the electron microscopic level

Endomorphin-2 is a newly discovered endogenous opioid peptide with high affinity and selectivity for the micro-opioid receptor, and potent analgesic activity, particularly in the spinal cord. Using immunoelectron microscopy, we examined the ultrastructure of the endomorphin-2-like immunoreactive processes and their synaptic relationships in the spinal cord. Endomorphin-2-like immunopositive dense-cored vesicles were observed in many axon terminals, and, in a few cases, were observed together with immunonegative dense-cored vesicles. Immunopositive axons with or without myelination were also observed. The endomorphin-2-like immunoreactive axon terminals formed synapses with both immunopositive and immunonegative processes. Most synapses were asymmetrical, but symmetrical synapses were also found. Examples of axo-dendritic, axo-somatic and axo-axonic contacts were observed.This first demonstration of the ultrastructure and synaptic relationships of endomorphin-2-like immunoreactive axon terminals in the spinal cord dorsal horn provides morphological evidence that this peptide functions as a transmitter regulating pain processes.     

Locus coeruleus terminals in intraocularly transplanted spinal cords as compared with catecholamine terminals in normal spinal cords: Their synaptic densities and functional considerations

Locus coeruleus terminals in intraocularly transplanted spinal cords and catecholamine terminals in defined areas of normal spinal cords were investigated qualitatively and quantitatively by immunoelectron microscopy. Results showed that the morphological features of synapses formed in the grafts closely resembled those of normal spinal cords. The incidences of synapses per varicosities, as observed in single sections, were 30.1, 40.2 and 22.8% for the ventral horn, dorsal horn and grafted spinal cord, respectively. In all three groups, most of the postsynaptic targets were small dendrites, although high frequencies of large dendrites were found in the ventral horn. Spines and axons in the grafts were also postsynaptic targets. Several characteristics of relative immaturity were observed in the grafts. It is suggested that the inhibition of spinal neurons by locus coeruleus terminals may be mediated not only by volume transmission through nonsynaptic contacts, but also by direct contacts with catecholamine terminals, and that the excitation of facilitation observed at those terminals may be explained by the suppression of inhibitory neurons by axoaxonic contacts.     

Robust CNS regeneration after complete spinal cord transection using aligned poly-L-lactic acid microfibers

Following spinal cord injury, axons fail to regenerate without exogenous intervention. In this study we report that aligned microfiber-based grafts foster robust regeneration of vascularized CNS tissue. Film, random, and aligned microfiber-based conduits were grafted into a 3 mm thoracic rat spinal cord gap created by complete transection. Over the course of 4 weeks, microtopography presented by aligned or random poly-L-lactic acid microfibers facilitated infiltration of host tissue, and the initial 3 mm gap was closed by endogenous cell populations. This bulk tissue response was composed of regenerating axons accompanied by morphologically aligned astrocytes. Aligned fibers promoted long distance (2055 ± 150 μm), rostrocaudal axonal regeneration, significantly greater than random fiber (1162 ± 87 μm) and film (413 ± 199 μm) controls. Retrograde tracing indicated that regenerating axons originated from propriospinal neurons of the rostral spinal cord, and supraspinal neurons of the reticular formation, red nucleus, raphe and vestibular nuclei. Our findings outline a form of regeneration within the central nervous system that holds important implications for regeneration biology.     

Effect of embryonal nerve tissue transplantation on regeneration of dorsal roots of the rat spinal nerves

Using cobalt salts axonal ionophoresis posttraumatic regeneration of TXII dorsal roots nerve fibres in the zone of hemisection in conditions of 14 wks embryo spinal cord transplantation into the zone of trauma of spinal cord. Regro Invasion of dorsal roots nerve fibres into recipients posterior cords and Lissawers tract through the transitional zone "spinal cord--dorsal roots" was observed on posttransplantation d 14-120. It was show that afferent axons predominantly spread in substantia alba and substantia grisea caudal to the level of spinal cord transection with only individual fibres invading rostrad through the neuronal plate. In the transplants neurons were encountered up to d 120 of the observation although transplant neuropil was limited from recipient tissue brain by a glial and connective tissue scar. The influence of embryonal nervous tissue transplantation on intraspinal regeneration of dorsal roots afferents was discussed.     

The relationship between some measures of synaptic ultrastructure as a function of distance from the soma on lamprey reticulospinal neurons

Summary Synaptic junctions located on the dendrites of lamprey (Petromyzon marinus) reticulospinal neurons labelled with intracellularly-injected horseradish peroxidase were studied. The normal ultrastructure of the synaptic junctions was defined and several quantitative measures made from each junction in order to test the hypothesis that distally-located synapses are ultrastructurally different from those located at proximal dendritic sites. A total of 820 contacts from one neuron and 279 from a second neuron ranging from 20 to 340 m from the soma were quantified. The vast majority of the presynaptic endings contained round, clear-cored vesicles and formed an asymmetrical membrane differentiation with the postsynaptic dendrite. A small fraction of the population contained flattened or pleomorphic vesicles and these synapses were equally distributed with respect to distance from the soma. Many of the terminals contained a few large dark- and clear-cored vesicles. Four quantitative measures of each synaptic contact were made. These included vesicle number, length of differentiated membrane, vesicle area and terminal area. Four ratios relating the different quantitative measures were also calculated. Each ratio or measurement from the synaptic junctions was plotted as a function of distance from the soma to determine if differences existed at any distance. It was found that synaptic junctions are uniformly similar and that distal junctions did not differ significantly (P > 0.05) from those at proximal dendritic sites. It is concluded that if distal synapses do compensate for their remote location they do this in some other way, possibly by increasing the number of synaptic contacts made by each presynaptic axon.     

Contributions of identifiable neurons and neuron classes to lamprey vertebrate neurobiology

Among the advantages offered by the lamprey brainstem and spinal cord for studies of the structure and function of the nervous system is the unique identifiability of several pairs of reticulospinal neurons in the brainstem. These neurons have been exploited in investigations of the patterns of sensory input to these cells and the patterns of their outputs to spinal neurons, but no doubt these cells could be used much more effectively in exploring their roles in descending control of the spinal cord. The variability of cell positions of neurons in the spinal cord has precluded the recognition of unique spinal neurons. However, classes of nerve cells can be readily defined and characterized within the lamprey spinal cord and this has led to progress in understanding the cellular and synaptic mechanisms of locomotor activity. In addition, both the identifiable reticulospinal cells and the various spinal nerve cell classes and their known synaptic interactions have been used to demonstrate the degree and specificity of regeneration within the lamprey nervous system. The lack of uniquely identifiable cells within the lamprey spinal cord has hampered progress in these areas, especially in gaining a full understanding of the locomotor network and how neuromodulation of the network is accomplished.