Migrational analysis of the constitutively proliferating subependyma population in adult mouse forebrain

Initial experiments to evaluate the in vivo fate(s) of constitutively proliferating subependymal cells determined that, following in vivo labeling of this population by infection with a retrovirus containing a β-galactosidase reporter gene, there was a progressive and eventually complete loss of histochemically β-galactosidase-positive cells within the lateral ventricle subependyma with increasing survival times of up to 28 days after retroviral infection. Subsequent experiments were designed to ascertain the potential contributions of: (i) the migration of subependymal cells away from the forebrain lateral ventricles; and (ii) the down-regulation of the retroviral reporter gene expression. Retroviral lineage tracing experiments demonstrate that a major in vivo fate for constitutively proliferating subependymal cells is their rostral migration away from the walls of the lateral ventricle to the olfactory bulb. Although down-regulation of retroviral reporter gene expression does not contribute to the loss of detection of β-galactosidase-labeled cells from the lateral ventricle subependyma, it does result in an underestimation of the absolute number of retrovirally labeled cells in the olfactory bulb at longer survival times. Furthermore, a temporal decrease in the double labeling of β-galactosidase-labeled cells with [³H]thymidine was observed, indicating that only a subpopulation of the migratory subependymal-derived cells continue to actively proliferate en route to the olfactory bulb. These two events may contribute to the lack of a significant increase in the total number of retrovirally labeled subependymal cells during rostral migration. Evidence from separately published studies suggests that cell death is also an important regulator of the size of the constitutively proliferating subependymal population.In summary, in vivo studies utilizing retroviral reporter gene labeling demonstrate that constitutively proliferating subependymal cells born in the lateral ventricle migrate rostrally to the olfactory bulb. Loss of proliferation potential and retroviral reporter gene down-regulation contribute to the lack of any significant increase in the total number of labeled cells recovered in the olfactory bulb.     

Migration of bipolar subependymal cells, precursors of the granule cells of the rat olfactory bulb, with reference to the arrangement of the radial glial fibers

In order to examine the relationship between radial glial fibers and the migrating bipolar subependymal cells which are considered to be post-mitotic precursors of granule cells in the rat olfactory bulb, the arrangement of radial glial fibers along the anterior lateral and olfactory ventricles was analysed by Golgi techniques, immunohistochemical demonstration of glial fibrillary acidic protein, and electron microscopy. In rats during their first 3 weeks of life, the bipolar subependymal cells migrate along the anterior lateral and olfactory ventricles into the center of the olfactory bulb, whereas the radial glial fibers radiating from the ventricular surface are arranged rather perpendicularly to the direction of migration of bipolar cells. Hence radial glial fibers in this region are not considered to act as guides for the rostralwards migration of subependymal cells.     

GABAergic basal forebrain afferents innervate selectively GABAergic targets in the main olfactory bulb

In this work we have analyzed the targets of the GABAergic afferents to the main olfactory bulb originating in the basal forebrain of the rat. We combined anterograde tracing of 10 kD biotinylated dextran amine (BDA) injected in the region of the horizontal limb of the diagonal band of Broca that projects to the main olfactory bulb, with immunocytochemical detection of GABA under electron microscopy or vesicular GABA transporter (vGABAt) under confocal fluorescent microscopy. GABAergic afferents were identified as double labeled BDA-GABA boutons. Their targets were identified by their ultrastructure and GABA content. We found that GABAergic afferents from the basal forebrain were distributed all over the bulbar lamination, but were more abundant in the glomerular and inframitral layers (i.e. internal plexiform layer and granule cell layer). The fibers had thick varicosities with abundant mitochondria and large perforated synaptic specializations. They contacted exclusively GABAergic cells, corresponding to type 1 periglomerular cells in the glomerular layer, and to granule cells in inframitral layers. This innervation will synchronize the bulbar inhibition and consequently the response of the principal cells to the olfactory input. The effect of the activation of this pathway will produce a disinhibition of the bulbar principal cells. This facilitation might occur at two separate levels: first in the terminal tufts of mitral and tufted cells via inhibition of type 1 periglomerular cells; second at the level of the firing of the principal cells via inhibition of granule cells. The GABAergic projection from the basal forebrain ends selectively on interneurons, specifically on type 1 periglomerular cells and granule cells, and is likely to control the activity of the olfactory bulb via disinhibition of principal cells. Possible similarities of this pathway with the septo-hippocampal loop are discussed.     

Vasoactive intestinal polypeptide-immunoreactive neurons in the main olfactory bulb of the hedgehog (Erinaceus europaeus)

Vasoactive intestinal polypeptide (VIP) immunoreactivity was localized by the indirect antibody enzyme method (PAP technique) in the main olfactory bulb of the hedgehog. Most VIP-immunoreactive cells were located in the glomerular layer and throughout the external plexiform layer. Fewer cells were observed in the granule cell layer. At the morphological level they exhibit the characteristics of periglomerular, external tufted, superficial short axon, horizontal and Van Gehuchten cells. It should be mentioned that another specific neuronal type was found in the inner third of the external plexiform layer, which is not described in other animals. These results revealed that a high number of intrinsic neuronal types of the olfactory bulb of the hedgehog display a strong VIP immunoreactivity.     

Organization of the main olfactory bulb of lesser hedgehog tenrecs

Using a confocal laser scanning microscope (CLSM) and an electron microscope, we investigated the organization of the main olfactory bulb (MOB) of tenrecs, which were previously included into insectivores but now considered to be in a new order "Afrosoricida" in the superclade 'Afrotheria'. We confirmed that the overall structural organization of the tenrec MOB was similar to that of rodents: (1) the compartmental organization of glomeruli and two types of periglomerular cells we proposed as the common organizational principles were present; (2) there were characteristic dendrodendritic and axo-dendritic synapses in the glomerulus and external plexiform layer (EPL) and gap junctions in glomeruli; and (3) no nidi, particular synaptic regions reported only in laboratory musk shrew and mole MOBs, were encountered. However, instead of nidi, we often observed a few tangled olfactory nerves (ONs) with large irregular boutons in the glomerular-external plexiform layer border zone, with which dendrites of various displaced periglomerular cells were usually found to be intermingled. Electron microscopic (EM) examinations confirmed characteristic large mossy terminal-like ON terminals making asymmetrical synapses to presumed mitral/tufted cell and displaced periglomerular cell dendrites. In addition, gap junctions were also encountered between dendritic processes in these tiny particular regions, further showing their resemblance to glomeruli.     

Tyrosine hydroxylase immunoreactive structures in the aged human olfactory bulb and olfactory peduncle

We studied the anatomical distribution of dopaminergic structures in the normal, aged, human olfactory bulb and olfactory peduncle with a monoclonal antibody against tyrosine hydroxylase. Three different tyrosine hydroxylase containing cell groups are present in the olfactory bulbs: (1) a group of round, medium-sized cells within and around the glomeruli; (2) cells in the external plexiform layer; and (3) cells that are scattered in the stratum album. Occasionally, a few labeled neurons can be observed in the granule cell layer. In the olfactory peduncle a few labeled cells are present in the superficial layers just underneath the pia. Tyrosine hydroxylase containing terminal-like structures are present in the glomerular layer and the external plexiform layer. In a few cases dense terminal labeling is also observed in the cell groups that constitute the anterior olfactory nucleus. In the olfactory peduncle scattered labeled fibers are present. In addition, the present study makes clear that quantitative differences exist between the individual cases for which no explanation could be found.     

Analysis of synaptic events in the mouse accessory olfactory bulb with current source-density techniques.

The accessory olfactory bulb of the mouse was studied by current source-density analysis of field potentials to determine the laminar and temporal distribution of synaptic currents evoked by electrical stimulation of the vomeronasal organ. The one-dimensional current source-density analysis revealed two major spatially and temporally distinct inward membrane currents (sinks): one in the glomerular layer and the other in the external plexiform layer. The glomerular layer sink preceded the external plexiform layer sink by a mean of 5.5 ms. Local infusions of the broad-spectrum excitatory amino acid antagonist, kynurenate, into the accessory olfactory bulb blocked the external plexiform layer sink without an obvious effect on the glomerular layer sink. The selective non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione produced a dose-dependent blockade of the external plexiform layer sink, whereas the selective N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovalerate was without effect. These results, taken together with the cytoarchitecture of the accessory olfactory bulb, suggest that the glomerular layer sink results mainly from synaptic excitation evoked in the glomerular dendritic branches of mitral cells by the vomeronasal afferent fibres and the external plexiform layer sink mainly from non-N-methyl-D-aspartate receptor-mediated synaptic excitation in the peripheral processes of granule cells via the mitral to granule cell dendrodendritic synapse.     

Tyrosine hydroxylase-like immunoreactive neurons in the olfactory bulb of the snake,Elaphe quadrivirgata,with special reference to the colocalization of tyrosine hydroxylase- and GABA-like immunoreactivities

Summary The distribution and structural features of tyrosine hydroxylase-like immunoreactive (TH-LI) neurons were studied in the olfactory bulb of a snake, Elaphe quadrivirgata, by using pre-and post-embedding immunocytochemistry at the light microscopic level. In contrast to rodent olfactory bulbs previously reported, many TH-LI neurons were seen not only in the main olfactory bulb (MOB) but also in the accessory olfactory bulb (AOB). With regard to the TH-like immunoreactivity, there appeared no appreciable differences between MOB and AOB. As in mammalian MOB, the majority of TH-LI neurons were clustered in the periglomerular region and appeared to send their dendritic branches into glomeruli, which as a whole make an intense TH-LI band in the glomerular layer (GML). In the external plexiform/mitral cell layer (EPL/ML) of MOB and AOB as well as in the outer sublamina of the internal plexiform layer (OSL) of AOB, an appreciable number of TH-LI neurons were scattered, extending dendritic processes which appeared to make a loose meshwork. TH-LI neurons in EPL/ML (including OSL) appeared to consist of at least two morphologically different types. The first had a small perikaryon and one or two smooth dendrites which usually extended to GML and were frequently confirmed to enter into glomeruli. The second had a larger perikaryon and 2–3 dendrites which branched into several varicose processes extending in EPL/ML/OSL but appeared not to enter into glomeruli. The TH-like immunoreactivity was rarely seen in the internal plexiform layer and internal granule cell layer. The colocalization of GABA-like and TH-like immunoreactivities was further studied. Almost all TH-LI neurons in both EPL/ ML/OSL and GML contained GABA-like immunoreactivity irrespectively of the type of TH-LI cells.Abbreviations in Figures AOB accessory olfactory bulb - MOB main olfactory bulb - Hem hemisphere - ON olfactory nerve layer - VN vomeronasal nerve layer - GM glomerular layer - EP/M external plexiform layer/Mitral cell layer - IP internal plexiform layer - IG internal granular layer - OS outer sublamina of the IPL of AOB - MS middle sublamina of the IPL of AOB - IS inner sublamina of the IPL of AOB     

Autoradiographic analysis of [3H]dopamine and [3H]dopa uptake in the turtle olfactory bulb

Uptake and retention of exogenous tritiated dopamine and L-dopa was observed within turtle olfactory bulb slices. In the more superficial layers, periglomerular and superficial tufted cells, as well as their processes, and intraglomerular dendrites were recognized as labeled. Within the deeper part of the bulb, some labeled cells between the tanycytes, as well as nerve fibers and terminals within the granule cell layer, are reported. The results confirm the presence of specific intrinsic dopaminergic cells within the reptilian olfactory bulb.     

Vasoactive intestinal polypeptide-containing elements in the olfactory bulb of the hedgehog (Erinaceus europaeus)

The distribution of vasoactive intestinal polypeptide (VIP)-immunopositive elements was analyzed in the olfactory bulb (OB) of the Western European hedgehog (Erinaceus europaeus) under light and electron microscopy. The immunoreactivity appeared in an abundant population of periglomerular cells of the glomerular layer, in interneurons of the external plexiform layer, and in a restricted group of deep short-axon cells of the internal plexiform layer, the granule cell layer and the white matter. In the glomerular layer, VIP-containing periglomerular cells constituted a population of non-GABAergic neurons and did not receive synapses from olfactory axons. In the EPL, VIP-immunoreactivity appeared in a morphologically heterogeneous population of GABAergic interneurons, most of them identified as satellite cells and Van Gehuchten cells. These interneurons exerted an abundant and selective innervation of the somata, primary and secondary dendrites of the principal mitral and tufted cells, but did not contact granule cells. Perisomatic innervation of the principal cells followed two different patterns. The first included 'normal' basket-like arrangements of VIP-containing varicosities surrounding the somata of mitral and tufted cells. In the second, a set of satellite cells gave rise to short dendritic shafts that embraced the somata of principal cells in an 'exuberant' basket-like arrangement. These two morphological patterns of perisomatic innervation of principal cells were correlated with a neurochemical specificity of the target. In this sense, the 'exuberant' basket-like structures were always found surrounding a subpopulation of principal cells that did not contain the calcium-binding protein parvalbumin (PV). By contrast, they were never found surrounding the subpopulation of PV-containing principal cells, which only showed 'normal' basket-like structures. This study provides new data on the connectivity and neurochemical features of the hedgehog olfactory bulb and suggests that the olfactory circuits in this species are more complex than those described in other mammals.     

An experimental study of the projection of the amygdala to the accessory olfactory bulb and its relationship to the concept of a dual olfactory system

Summary The present anatomical findings point to the existence of a separate subdivision of the olfactory system whose connections are quite different from the principal part. The main olfactory bulb has olfactory afferents from the receptors of the general olfactory mucosa, while the accessory bulb has afferents from receptors in the vomeronasal organ. The main bulb projects to the olfactory tubercle and pyriform cortex, while the accessory bulb projects to the amygdala. In turn these areas are further related with the medial forebrain bundle in the case of the pyriform cortex and olfactory tubercle, and with the medial preoptic area and medial hypothalamus in the case of the amygdala. The main and accessory olfactory bulbs are further distinguished by their centrifugal connections, the main bulb receiving fibres from the olfactory tubercle passing through the lateral olfactory tract, and the accessory olfactory bulb receiving fibres from the amygdala through the stria terminalis. The centrifugals to the accessory olfactory bulb resemble those to the main bulb in that both appear to terminate upon granule cells, although further projections to the external plexiform layer or to the periglomerular region have not been demonstrated for the accessory bulb. By virtue of its neural connections the accessory olfactory system is ideally placed to mediate the effects of olfactory stimuli on reproduction.     

Microglia in the olfactory bulb of rats during postnatal development and olfactory nerve injury with zinc sulfate: a lectin labeling and ultrastrucutural study

Using isolectin (GSA I-B4) as a marker, this study examined the possible alterations of lectin-labeled membranous glycoproteins in microglial cells in the olfactory bulb of normal development and under experimentally induced degeneration. In light microscopy, several morphological types of microglial cells representing different degrees of cell differentiation were distributed in the bulb laminae. A gradient of microglial differentiation extending from the intermediate to superficial and intermediate to deep occurs in the bulb layers. The differentiation gradient and lectin labeling pattern of microglial cells in the developing bulb resembled those in other areas of the brain tissues. Differentiating microglia showed a gradual diminution of lectin staining when the nascent round cells transformed into the mature ramified cells. Microglia in the external plexiform layer of the olfactory bulb were the first to mature and the cells expressed very weak lectin reactivity. In mature or adult rats, some microglial cells showing intense lectin labeling were observed in the olfactory nerve layer, granule cell layer and subependymal layer. Ultrastructurally, lectin labeling was localized at the trans saccules of the Golgi apparatus. Microglial cells in other bulb laminae, however, exhibited a negative reaction for the isolectin at the Golgi apparatus. Following intranasal irrigation of zinc sulfate, some microglial cells in the olfactory nerve layer and glomerular layer were activated to become phagocytic cells with increased lectin labeling at their ramified processes. GSA I-B4 staining was also localized at their trans saccules of the Golgi apparatus. The lectin labeling pattern of these phagocytic cells resembled that of differentiating microglia in postnatal bulbs, suggesting that bulb microglia in the lesioned sites were activated through cell dedifferentiation into macrophages.     

Distribution of enkephalin-like immunoreactivity in the rat main olfactory bulb

Enkephalin-like immunoreactivity was localized within the main olfactory bulb of the rat using immunohistochemical techniques. These studies utilized well characterized antisera directed to either leu⁵- or met⁵-enkephalin. Specificity was established by absorption of the antisera with either 10 μM synthetic leu⁵- or met⁵-enkephalin.Specific enkephalin-like immunoreactivity was observed within several different cell populations including (1) periglomerular cells, (2) granule cells and their processes within the external plexiform layer and (3) occasional short-axon (horizontal) cells within the granule and external plaxiform layers. The granule cell layer contained the greatest number of immunoreactive cells. Only a limited number of immunoreactive cells were found in both the periglomerular and granule cell layers, suggesting the enkephalin-containing neurons represent a sub-population within each layer.The absence of immunoreactive processes in the periventribular white matter, as well as the morphologies of immunoreactive bulbar neurons, indicates that enkephalin is found exclusively within intrinsic olfactory bulb neurons.     

Morphological characteristics of dopaminergic immunoreactive neurons in the olfactory bulb of the common marmoset monkey (Callithrix jacchus)

The present study describes the distribution of tyrosine hydroxylase (TH)-immunoreactive (IR) elements in the olfactory bulb of the common marmoset monkey (Callithrix jacchus), a primate species by immunohistochemistry. We identified six layers of the olfactory bulb of the common marmoset monkey in sections stained with cresyl violet. The majority of TH-IR cells were found in the glomerular layer. A few TH-IR cells were present in the external plexiform and granule cell layers. TH-IR fibers were identified in all layers of the olfactory bulb. The density of these nerve fibers was high in the internal plexiform and granule cell layers. The results in the olfactory bulb of the common marmoset monkey are generally similar to previous reports in some mammals. These data suggest that TH in the olfactory bulb of the common marmoset monkey may play a role in olfactory transmission via the glomeruli like in other mammals.     

3H-thymidine-radiographic studies of neurogenesis in the rat olfactory bulb

Neurogenesis in the rat olfactory bulb was examined with 3H-thymidine-radiography. For the animals in the prenatal groups, the initial 3H-thymidine exposures were separated by 24 h; they were the offspring of pregnant females given two injections on consecutive embryonic (E) days (E12-E13, E13-E14, . . . E21-E22). For the animals in the postnatal (P) groups, the initial 3H-thymidine injections were separated by 48 h, each group receiving either four (PO-P3, P2-P4, . . . P6-P9) or two (P8-P9, P10-P11, . . . P20-P21) consecutive daily injections. On P60, the percentage of labeled cells and the proportion of cells added during either 24 h or 48 h periods were quantified at several anatomical levels for each neuronal population in the main olfactory bulb (mitral cells, tufted cells, granule cells, interneurons in the external plexiform layer, periglomerular granule cells) and accessory olfactory bulb (output neurons, granule cells, periglomerular granule cells). The total time span of neurogenesis extends from E12 to beyond P20. Output neurons are prenatally generated over 5-9 day periods (with most neurogenesis occurring over 2-4 days) in a strict sequential order beginning with the accessory bulb output neurons (E13-E14) and ending with the interstitial tufted cells lying between the glomeruli in the main bulb (E20-E22). These data are correlated with the main and accessory bulb projection fields in the amygdala and with the chronology of amygdala neurogenesis. With the exception of the granule cells in the accessory bulb (88% generated between E15-E22), the rest of the interneuronal populations are generated postnatally and nearly simultaneously. While most neurons (75-80%) originate during the first three weeks of life, all interneuronal populations, including accessory bulb granule cells, show some neurogenesis beyond P20. Injections of 3H-thymidine in juvenile and adult rats indicates neurogenesis up to P60 in the accessory bulb and up to P180 in the main bulb, especially in the main bulb granule cell population. There is circumstantial evidence for turnover of main bulb granule cells during adult life.     

The age-dependent change in Olfactory periglomerular neuronal populations is not affected by interrupting subventricular neuroblast migration in adult rats

The olfactory bulb (OB) is rich in the number and variety of neurotransmitter and neuropeptide containing cells, in particular in the glomerular layer. Several reports suggest that numbers of some periglomerular phenotypes could change depending on age. However, it is unclear whether the different classes of periglomerular interneurons are modified or are maintained stable throughout life. Thus, our first objective was to obtain the absolute number of cells belonging to the different periglomerular phenotypes at adulthood. On the other hand, the olfactory bulb is continously supplied with newly generated periglomerular neurons produced by stem cells located in the subventricular zone (SVZ) and rostral migratory stream. Previously, we demonstrated that the implantation of a physical barrier completely prevents SVZ neuroblast migration towards the OB. Then, another objective of this study was to evaluate whether stopping the continuous supply of SVZ neuroblasts modified the different periglomerular populations throughout time. In summary, we estimated the total number of TH-IR, CalB-IR, CalR-IR and GAD-IR cells in the OB glomerular layer at several time points in control and barrier implanted adult rats. In addition, we estimated the volume of glomerular, granular and complete OB. Our main finding was that the number of the four main periglomerular populations is age-dependent, even after impairment of subventricular neuroblast migration. Furthermore, we established that these changes do not correlate with changes in the volume of glomerular layer.     

A Golgi study on the main olfactory bulb in the snake Elaphe quadrivirgata

The intrinsic organization of the main olfactory bulb in the snake was studied using the rapid Golgi method. A distinct laminar structure was recognized. From the periphery inward, the following layers were distinguished: the layer of the olfactory fibers, the olfactory glomeruli, the mitral cells, the deep fiber plexus, the granule cells and the ependymal cells. Olfactory fibers derived from the nasal cavity reached the entire surface of the bulb, forming a dense fiber plexus, then swung deeply and terminated in the olfactory glomeruli which were arranged in 2-4 rows. The mitral cell layer occupied a wide zone and was composed of scattered mitral cells. The mitral cells had 2-9 primary dendrites proceeding externally to terminate in the olfactory glomeruli and 2-4 secondary dendrites extending tangentially in the mitral cell layer to be distributed therein. The axons of the mitral cells travelled deeply and entered the layer of the deep fiber plexus. The deep fiber plexus was the path for the bulbar efferent and afferent fibers and could be traced caudally as the main olfactory tract, up to the anterior olfactory nucleus and vicinity. The granule cell layer was composed of small cells, the granule cells, packed closely with no special arrangement. The granule cells had long processes which extended superficially to be distributed mainly in the mitral cell layer. The ependymal cells were located at the deepest layer forming the wall of the olfactory ventricle and generated a long process which extended towards the surface to terminate in the peripheral portion of the bulb. In the snake bulb, the well-documented external and internal plexiform layers were considered to be included in the wide mitral cell layer. Thus, while several specific structures were observed, the fundamental organization of the main olfactory bulb in the snake seemed to be identical to that of the main olfactory bulb in various other vertebrate species.     

Ultrastructural identification of substance P immunoreactive neurons in the main olfactory bulb of the hamster

The neurons containing substance P immunoreactivity in the main olfactory bulb of the hamster are located in the glomerular layer. Their cell bodies lie in the periglomerular region and contain spherical or ovoid nuclei which lack invaginations of the nuclear membrane and tend to be positioned eccentrically in the cell body. Dendrites of these neurons extend throughout the periglomerular region and project into the glomerular neuropil. Within the glomerular neuropil, processes with substance P immunoreactivity contain agranular, spherical synaptic vesicles. Primary olfactory axons, and processes of uncertain origin which contain pleomorphic synaptic vesicles, form synaptic contacts with substance P immunoreactive processes.These ultrastructural findings confirm that the substance P immunoreactive neurons are external tufted cells. Their likely physiological properties are considered in relation to the synaptic organization in the glomerular layer of the main olfactory bulb and to the other putative neurotransmitters or neuromodulators located in this layer.     

Retention of J1/tenascin and the polysialylated form of the neural cell adhesion molecule (N-CAM) in the adult olfactory bulb

Summary To gain insight into the cellular and molecular mechanisms underlying neurogenesis in adult mouse olfactory bulb, several adhesion molecules expressed by glial cells and neurons were investigated. In the germinal zone of the olfactory bulb, the subependymal layer of the rostral region of the lateral ventricles, two adhesion molecules are detectable that are characteristic of early morphogenetic events: J1/tenascin and the polysialylated form, the so-called embryonic form, of N-CAM. The polysialylated form of N-CAM is expressed by most cells in the subependymal layer, and by some astrocytes and neurons in the granular layer adjacent to the subependymal layer. This suggests that bipotential precursor cells retain expression of the embryonic form during their migration from the subependymal layer and during the first stages of differentiation into neurons and glia.Expression of the polysialylated form of N-CAM is also retained in monolayer cultures of six-day-old olfactory bulbs, 55 days after seedingin vitro. J1/tenascin was detectable in the subependymal layer using two monoclonal antibodies. The immunostaining pattern was different between the two antibodies and more restricted to the subependymal layer than when staining with polyclonal J1 antibodies was performed, indicating that J1/tenascin exists in distinct isoforms.Finally, our observations suggest that, in the adult olfactory bulb, L1 is not only a neuron-neuron adhesion molecule, but it may also be involved in neuron-glia interactions, since it is found at contact sites between these two cell types. L1, therefore, may be a neuron-glia adhesion molecule in some parts of the CNS, while it is not in others.     

A Golgi study on the olfactory bulb in the lamprey, Lampetra japonica

The intrinsic organization of the olfactory bulb in the lamprey was studied using the rapid Golgi method. Although not as discrete as in many vertebrates, a laminar organization was recognized. From the periphery inward, the following layers were discernible: the layer of the olfactory fibers, the olfactory glomeruli with the mitral cells, the granule cells, and the ependymal cells. Just beneath the surface of the olfactory bulb, the olfactory fibers extended over the entire bulb forming a dense fiber plexus terminating in the olfactory glomeruli which were arranged in one to two layers internally to the layer of the olfactory fibers. The mitral cells formed no discrete layer and were located mainly around the olfactory glomeruli. The mitral cells in the lamprey were lacking in secondary dendrites, but had two or more primary dendrites which terminated in the olfactory glomeruli. The axons of the mitral cells proceeded inwardly and accumulated diffusely in the granule cell layer which occupied a wide area internally to the layer of the olfactory glomeruli with the mitral cells. The granule cell layer was composed of densely packed small spindle or fusiform axonless cells, the processes of which extended superficially to be distributed in the olfactory glomeruli. At the deepest region of the bulb was a layer of the ependymal cells lining the surface of the olfactory ventricle. The external and internal plexiform layers were not evident. Thus, while the major constituents of the olfactory bulb of the vertebrate could be identified in that of the lamprey, the general laminar organization seemed indiscrete.