Myelinated dendritic segments in the monkey olfactory bulb

Expression of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase) in the developing olfactory bulb and subventricular zone rostral extension

The olfactory bulb (OB) presents a unique pattern of permanent acquisition of primary afferents and interneurons, but not much detail is known about the differentiation of its oligodendroglial cells. We studied the expression of 2',3'-cyclic nucleotide 3'-phosphodiesterase (CNPase), a protein related to axonal ensheathment by myelinating cells. Expression of CNPase in OB follows a general caudorostral gradient, with the exception of the glomerular layer (GL). At postnatal day 5-6 (P5-P6), the first CNPase(+) profiles appeared in the dorsal lateral olfactory tract adjacent to the accessory OB (AOB), followed by rare cell bodies and processes in AOB internal plexiform layer at P7. At P9, the main OB (MOB) granular cell layer (GrCL) already showed intensely stained CNPase(+) processes. From P5 to P12, small numbers of CNPase(+) cells were found in the subventricular zone (SVZ), throughout its rostral extension (SVZ-RE), and in the intrabulbar subependymal layer. The appearance of CNPase(+) profiles delimiting glomeruli started in the GL rostralmost region at P12, extending to all GL levels, but glomeruli remained open caudally at P15. At P18, oligodendroglial glomeruli were evident throughout OB, but the adult pattern was established only after P30. There was no age-related loss of CNPase immunoreactivity in glial cell bodies, possibly indicating de novo ensheathment of neurites. Our results show an earlier onset of oligodendroglial differentiation in OB than previously reported and a rostrocaudal gradient of formation of oligodendroglial glomeruli. They also raise the possibility that a minor fraction of OB oligodendrocytes might derive from the SVZ-RE.

Emergence of axons from distal dendrites of adult mammalian neurons following a permanent axotomy

The distinctive features of axons and dendrites divide most neurons into two compartments. This polarity is fundamental to the ability of most neurons to integrate synaptic signals and transmit action potentials. It is not known, however, if the polarity of neurons in the adult mammalian nervous system is fixed or plastic. Following axotomy, some distal dendrites of neck motoneurons in the adult cat give rise to unusual processes that, at a light microscopic level, resemble axons (Rose, P.K. & Odlozinski, M., J. Comp. Neurol., 1998, 390, 392). The goal of the present experiments was to characterize these unusual processes using well-established ultrastructural and molecular criteria that differentiate dendrites and axons. These processes were immunoreactive for growth-associated protein-43 (GAP-43), a protein that is normally confined to axons. In contrast, immunoreactivity for a protein that is widely used as a marker for dendrites, microtubule-associated protein (MAP)-2a/b, could not be detected in the unusual distal arborizations. At the electron microscopic level, unusual distal processes contained dense collections of neurofilaments and were frequently myelinated. These molecular and structural characteristics are typical of axons and suggest that the polarity of adult neurons in the mammalian nervous system can be disrupted by axotomy. If this transformation in neuronal polarity is common to other types of neurons, axon-like processes emerging from distal dendrites may represent a mechanism for replacing connections lost due to injury. Alternatively, the connections formed by these axons may be aberrant and therefore maladaptive.

Making brain connections: neuroanatomy and the work of TPS Powell, 1923-1996

Dr. Thomas PS Powell was one of the founders of modern neuroanatomy. His career spanned an era that saw techniques for analyzing connections in the central nervous system dramatically increase in number and resolving power. In tracing the history of his research, one can see how the introduction of each new technique provided an incremental step in analytical capacity although eventually revealing its own limitations. Also evident is the extent to which prejudices born in the days of applying earlier techniques could continue to influence the interpretation of results obtained with new ones. Powell's contributions to neuroscience were extremely wide-ranging, encompassing investigations of the circuitry of the basal ganglia, corticofugal connections, topographic maps in sensory systems, central olfactory pathways, corticocortical and commissural connections, and pathways for sensory convergence in the cerebral cortex. From these investigations, made with tract tracing techniques, much existing knowledge of forebrain organization is derived. He was also one of the earliest investigators to use electron microscopy in the investigation of the central nervous system, and his electron microscopic studies on the olfactory bulb, thalamus, cerebral cortex, and basal ganglia laid, to a large extent, the foundations for all modern research on the synaptic circuitry of these structures. He was given to synthesizing data across systems in order to arrive at common principles of brain organization. A number of these syntheses have been sources of great interest and, occasionally, controversy.

Oligodendrocyte/myelin-immunoreactivity in the developing olfactory system

Immunocytochemistry was used to characterize oligodendrocyte maturation in the developing mammalian olfactory system. Postnatal day 10-16, 20, 30 and adult rats were examined, as well as postnatal day 20, 30, 40 and adult Monodelphis domestica (the grey, short-tailed opossum). In rats, oligodendrocyte/myelin-immunoreactivity first appears in the accessory olfactory bulb by day 11, with labeling rapidly increasing throughout the entire bulb over the next five days. An adult pattern of immunoreactivity, characterized by dense labeling in the granule cell layer, sparse immunoreactivity in the external plexiform layer, and staining along the periphery of glomeruli, is attained by day 30. Staining is apparent in both the lateral olfactory tract and anterior commissure by day 11, and becomes heavy by day 20. While patterns of oligodendrocyte/myelin-immunoreactivity in the adult Monodelphis and rat bulb are similar, staining first appears much later in the opossum (around day 30), and maturation occurs more slowly. For example, rostral-caudal gradients in the development of staining in the anterior commissure were noted which were not seen in the rat. These differences emerge because Monodelphis' slower growth allows more resolution into developmental sequences. Finally, in rats, unilateral naris closure on the day after birth, which significantly alters normal patterns of bulb development, has no effect on the pattern and level of immunoreactivity even after long (30 day) survival periods. In both normal and naris occluded rats, oligodendrocyte/myelin-immunoreactivity is found in caudal aspects of the rat bulb on day 11 and subsequently progresses throughout the entire bulb over the next five days. Patterns in the Monodelphis bulb mirror those observed in the rat, however, staining appears later and progresses more slowly, suggesting Monodelphis is a useful animal for examining early myelin formation.

Neuroglial arrangements in the olfactory glomeruli of the hedgehog

The olfactory glomeruli represent morphological and functional units in which olfactory information is processed in specialized synaptic arrangements established between the central processes of sensory neurons, whose cell bodies are located in the olfactory epithelium, and the terminal (intraglomerular) portions of the dendrites of periglomerular, tufted, and mitral cells. The olfactory glomeruli are surrounded by distinctive glial formations in which the peripheral glia interacts with the central glia. We have studied the morphology and organization of neuroglial cells in the layer of olfactory nerves and the glomerular layer of the olfactory bulb in the insectivorous hedgehog (Erinaceus europaeus) with the electron microscope, Golgi method, and immunohistochemistry by using antibodies to glial fibrillary acidic protein (GFAP) and "rip," a monoclonal antibody that stains oligodendrocytes and their processes in the rat (Friedman et al.: Glia 2:380-390, '89). The peripheral glia is represented by a special category of cells that are closely related to astrocytes and known as sheathing cells. They accompany olfactory axons to their entrance in the glomeruli where they interact with the central glia, represented by astrocytes and oligodendrocytes. The sheathing cells typically display indented nuclei and protoplasmic expansions forming laminar processes wrapping several axons together. Astrocytes surrounding the glomerular neuropil belong to the velate type. They display numerous sheet-like processes enveloping dendritic segments and periglomerular cell bodies. Oligodendrocytes were found surrounding the glomeruli and at the interstices separating different glomeruli. Myelinated dendritic segments and cell bodies were found surrounding the olfactory glomeruli. These myelin coverings probably derive from oligodendrocytes. Together with the astrocytic lamellar expansions, they provide a rigid structural support that contributes to the segregation of group of different cells while remaining relatively isolated from other influences at the periphery of the glomeruli.

Structure of the olfactory bulb of the hedgehog (Erinaceus europaeus): a Golgi study of the intrinsic organization of the superficial layers

The intrinsic organization of the mitral cell, external plexiform, and glomerular layers of the main olfactory bulb of the insectivore hedgehog were studied with the Golgi method. This study completes our previous description of the cell types in the granule cell layer in the same subject (López-Mascaraque et al., J. Comp. Neurol. 253:135-152, '86). In the present contribution the morphology of mitral, internal, and middle tufted cells is described with particular interest in the formation and arrangement of glomerular tufts by the primary dendrites, and the disposition of the secondary dendrites. Three types of intrinsic cells were found in the external plexiform layer: Van Gehuchten cells, satellite cells, and horizontal cells. All these cells display unusual patterns of branching processes that were difficult to classify as dendritic or axonal. The close relationship between some processes of the satellite cells with the mitral or tufted cell dendrites is noted, suggesting that these cells may be inhibitory in nature. The external tufted cells are described, and several aspects pertaining to their morphology were considered in order to distinguish them from periglomerular cells and from the remaining tufted cells. External tufted and periglomerular cells appear to be intrinsic neurons, having axons distributed in the periglomerular region, most probably devoted to relating different olfactory glomeruli. The arrangement of glomeruli in the glomerular layer and distinctive characteristics with respect to other mammals were considered from a comparative point of view.

Changes in granule cells of the ferret olfactory bulb associated with imprinting on prey odours

The maturation of the granule cells of the ferret olfactory bulb around the time of odour imprinting has been examined. Rapid Golgi impregnation studies revealed a temporal overshoot in the development of the spines on the external and internal dendrites of the granule cells. In contrast, the number of somatic spines decreased continuously. Electron microscopical examinations of the synaptic contacts in the external plexiform layer revealed that the time course of synapse and reciprocal synapse formation was similar to that of the formation of the spines on the external dendrites. The results were taken as evidence that both the Golgi and the electron microscopical investigations described the same developmental process of postnatal synaptic rearrangement.

Functional maturation of the oligodendrocytes and myelin basic protein expression in the olfactory bulb of the mouse

The timing of myelin basic protein (MBP) expression and myelin component synthesis by the oligodendrocytes of the olfactory bulb was investigated in the mouse. Immunostaining with an anti-MBP immunoserum and a radioimmunoassay determination of MBP allowed to study the timing of MBP deposition during the development in this structure. Immunostaining of dissociated cells with anti-MBP and anti-galactosylceramide (anti-GC) was used to determine the state of development when these markers become expressed by olfactory bulb oligodendrocytes. Investigations using dissociated cells showed that GC-positive oligodendrocytes are already detected 3 days after birth in the olfactory bulb of the mouse and MBP is expressed 4 days later. Myelinated fibers were not visible on cryostat sections of olfactory bulb before 8 days postnatal. This work has been initiated by observations on the timing of myelination of olfactory bulb oligodendrocytes in transplantation experiments.

Myelinated non-axonal neuronal elements in the feline olfactory bulb lack sites with a nodal structural differentiation

Myelinated dendrites in the external plexiform layer (EPL) of the feline olfactory bulb and myelinated axons in the lateral olfactory tract (LOT), were examined by transmission electron microscopy. The results show that the non-axonal myelin sheaths are extremely thin and short and that the number of myelin lamellae does not increase with increasing dendritic diameter. In myelinated LOT axons the sheaths tend to be thicker and the myelin lamellar number increases with axon diameter. Domains with node-like structural characteristics are not encountered along myelinated dendrites, neither between successive myelin sheaths nor where single sheaths terminate. The partly myelinated neuronal perikarya, which occur in the EPL, also lack node-like domains. In contrast, typical nodes are easily found in myelinated LOT axons. In the periglomerular region dendrites and neuronal perikarya are surrounded by non-compacted glial sheets. It is concluded that myelination and node formation are relatively independent events and that morphogenetic glial-neuronal interactions may give different results in different parts of the same neuron.

The ultrastructure of the subnucleus gelatinosus of the nucleus of the tractus solitarius in the cat

The dorsomedial region of the nucleus of the tractus solitarius termed the subnucleus gelatinosus (SNG) was studied at the light and electron microscopic level in the cat. In cresyl violet and luxol fast blue stained sections the SNG contained small neuronal somata that were scattered throughout a pale-staining neuropil containing few myelinated fibers. These neurons were difficult to impregnate with Golgi staining techniques, but in successful impregnations the somata were observed to be 10--19 micrometers in diameter and bore few sparsely branching primary dendrites. Spines were present on the dendrites of some neurons and were more numerous on distal portions of the dendritic tree. Ultrastructural examination of the SNG revealed that the neuronal complement consisted of round, oval, or spindle shaped neurons with little or no organized Nissl substance. Rare myelin-like ensheathments of neuronal perikarya were also observed. Bundles of fine unmyelinated axons that coursed mainly longitudinally were a prominent feature of the area. The most common type of axon terminal observed contained mainly round clear vesicles, approximately 31 nm in diameter, and made asymmetrical synaptic contact with a dendritic profile. Pleomorphic vesicle-containing terminals involved in symmetrical synaptic contact were also commonly seen. Axodendritic and axosomatic synapses were associated with terminals containing either round clear vesicles or pleomorphic vesicles. Less commonly, dendrodendritic and dendrosomatic synapses were seen, the presynaptic elements of which contained pleomorphic vesicles. Following removal of a nodose ganglion, degenerating terminals of vagal afferent fibers were observed throughout the neuropil. Such terminals contained round, clear vesicles with an occasional large, dense-cored vesicle, and made axodendritic and axosomatic synaptic contacts.

Myelinated perikarya and dendrites in lateral geniculate nucleus of adult cat following chronic cortical deafferentation

Chronic (one year) cortical deafferentation resulted in the appearance of myelinated neuronal perikarya and dendritic processes in the lateral geniculate nucleus (LGN) of the cat. Four per cent of all nerve cell bodies, both large and small, were observed to be covered partially or totally by compact myelin sheath. The myelination of the dendrites was confined to the most proximal portion of the processes. A comparison of the fine structural features of the myelinated cell bodies and dendrites with those from intact LGN indicate that both surviving projection neurons and local interneurons may become myelinated. It is suggested that the unorthodox myelination of nerve cell somata and dendrites is caused primarily by the massive deafferentation of the nucleus. This indicates a relationship between synaptic input and the process of myelination of the postsynaptic nerve cells.

Distribution and morphology of myelinated perikarya and dendrites in the olfactory bulb of primates

Numerous myelinated perikarya occur in different layers of the olfactory bulbs of a chimpanzee and two species of New World primates, that is, the squirrel and the Cebus monkey. It appears that somata of all established neuron categories, except for the mitral cells, can become ensheathed in myelin. Myelinated dendritic segments are found in the periglomerular region and in the external plexiform layer; tufted and periglomerular cells most likely to give rise to these myelinated dendrites. The myelin sheath is predominantly of the compact C.N.S. type. Perikaryal and dendritic myelin often ends in typical feet of glial cytoplasm. The termination site of dendritic myelin is a preferred site of synaptic contacts. Myelinated profiles are more numerous in the two monkey species than in the chimpanzee.

Myelinated dendrites in the spinal cord of frogs (Rana esculenta)

Myelinated dendrites--probably of motoneurons--were found in the spinal cord of the frog. It is assumed that the myelin sheath, by increasing the membrane resistance, improves the function of the dendrite as a cable.

Ruffed cell: a new type of neuron with a distinctive initial unmyelinated portion of the axon in the olfactory bulb of the goldfish (Carassius auratus) I. Golgi impregnation and serial thin sectioning studies

A new type of neuron was recognized in the olfactory bulb of the goldfish (Carassius auratus) by means of light and high voltage electron microscopy of Golgi-impregnated material, combined Golgi-electron microscopy, and electron microscopy of serial thin sections. The neuron is located in the layer between the olfactory nerve layer and the anterior olfactory nucleus. It has a spherical cell body, about 10--20 microns in diameter, and several dendrites which form a spherical dendritic field, about 70--100 microns in diameter, in the vicinity of the cell body. The most remarkable structural feature of this neuron is that its initial unmyelinated portion of the axon (IP) has elaborate protrusions with many synapses. The IP can be divided into three parts, parts 1, 2 and 3, based on its structural features. Part 1 is the initial part of the IP, about 20--40 microns in length. Many elaborate protrusions arise from the shaft and intermingle with one another to constitute a spherical field, about 20--40 microns in diameter, around the shaft. Part 2 is the middle part of the IP, about 10--20 microns in length. There are several collateral-like protrusions, which are scattered along the shaft and extended laterally about 5--15 microns. Part 3 is the last part of the IP, and is cylindrical without protrusions. The length of part 3 varies from 20 to more than 100 microns. The axon acquires a myelin sheath at distance of 70--250 microns from its origin. Protrusions make synaptic contacts mainly with granule cell dendrites. Some of them are of the reciprocal type. Protrusion are presynaptic in asymmetrical synapses, and postsynaptic in symmetrical synapses with granule cell dendrites. The shaft of the IP also has synapses similar to those on protrusions. The neuron described is a new type of neuron in the vertebrate central nervous system. We propose for it the name "ruffed cell."

The fine structure of myelinated nerve cell bodies in the bulbus olfactorius of man

In the bulbus olfactorius of man numerous myelinated nerve cell bodies occur in the stratum plexiforme internum and stratum granulosum internum. In many respects they resemble the neighbouring granule cells: small chromatin clumps border on more than half of the circumference of the nucleus, the thin cytoplasmic rim contains abundant polysomes and sometimes pigment complexes with numerous light vacuoles, the cells often show a process which extends up to the stratum glomerulosum, the perikarya are devoid of synaptic contacts whereas the proximal segment of the peripheral processes display rare contacts. The myelin sheath varies in thickness consisting of 2 to 24 lamellae with distances between the major dense lines ranging from 9.3 to 11.3 nm. The myelin sheath may enclose the cell body completely or partially and accompany the proximal segment of the process arising from the perikaryon. On partially enveloped perikarya, the myelin lamellae end in formations like those of the node of Ranvier, though often less regularly. Within the compact myelin sheath all of its lamellae may be distended for a short distance by glial cytoplasm as in the Schmidt-Lanterman incisures of peripheral nerve fibres. Adjacent to the outermost myelin lamella myelinated axons and cell bodies, tentatively identified as oligodendrocytes, as well as granule cells may be closely joined.

Myelinated granule cell bodies in the cerebellum of the monkey (Saimiri sciureus)

Electronmicrographs of both the vermis and hemisphere of the cerebellum of the squirrel monkey (Saimiri sciureus) show numerous granule cell bodies partially or completely surrounded by myelin. The myelin is of the compact type and consists of 1 to 13 lamellae. In several cases of partially ensheathed cells the myelin is clearly derived from extensions of myelin sheaths that surround small caliber axons. Either all or only the outer lamellae surrounding the axon contribute to the extensions. In the first instance the myelin buckles at its mesaxon pole and the resulting doubled flap extends for a variable distance along the cell surface.

Myelinated nerve cell bodies in the dorsal horn of the monkey (Saimiri sciureus)

While observing electronmicroscopic preparations of laminae I-III of Rexed ('52), taken from the lumbosacral region of squirrel monkey spinal cord, several small neuronal cell bodies were found which were partially or completely encircled by compact myelin sheaths of varying thickness. Though found in all three laminae, the occurrence of these perikaryal sheaths was less frequent in the "inner zone" of lamina II where there were few myelinated fibers. Perikaryal profiles which were completely surrounded by myelin exhibited meither internal mesaxons nor external tongue processes and the origin of their myelin is obscure. In cases of partially enveloped cells the myelin was often clearly derived from extensions of myelin sheaths surrounding small-diameter axons. These overgrowths of myelin extended away from their axons at a pole near their internal mesaxon and spread out across the surface of neighboring nerve cells. In some cases the extensions were derived from the entire axonal sheath while in others only the external lamellae were included. The external tongue process, when observed, was located at the distal end of the axonal myelin extension. Overgrowths of axonal myelin which were unrelated to neuronal cell bodies were also found but these formations were less extensive.