Neurogenesis and plasticity of the olfactory sensory neurons

Odorant receptor expression patterns are restored in lesion-recovered rat olfactory epithelium

Lesions of the olfactory periphery provide a means for examining the reconstitution of a diverse and highly regulated population of sensory neurons and the growth, en masse, of nascent axons to the bulb. The olfactory epithelium and its projection onto the bulb are reconstituted after ablation by methyl bromide gas, and some measure of olfactory function is restored. The extent to which the system regenerates the full repertoire of odorant receptor-expressing neurons, particularly their spatially restricted distribution across the epithelial sheet, is unknown, however, and altered odorant receptor expression might contribute to the persistent distortion of odorant quality that is observed in the lesioned-recovered animals. To address the question of receptor expression in the recovered epithelium, we performed in situ hybridization with digoxigenin-labeled riboprobes for eight odorant receptors on the olfactory epithelium from unilaterally methyl bromide-lesioned and control rats. The data demonstrate that the distribution of sensory neuron types, as identified and defined by odorant receptor expression, is restored to normal or nearly so by 3 months after lesion. Likewise, the numbers of probe-labeled neurons in the lesioned-recovered epithelium are nearly equivalent to the unlesioned side at this time. Finally, our evidence suggests that odorant receptors are distributed in multiple overlapping bands in the normal, unlesioned, and lesioned-recovered epithelium rather than in the conventionally accepted three or four zones. Thus, the primary sensory elements required for functional recovery of the olfactory system after damage are restored, and altered function implies the persistence of a more central failure in regeneration.

Neuronal replacement in adult brain

The discovery of spontaneous neuronal replacement in the adult vertebrate brain has changed the way in which we think about the biology of memory. This is because neuronal replacement is likely to have an impact on what a brain remembers and what it learns. Neuronal replacement has also changed the way in which we go about exploring new strategies for brain repair. Our new outlook on both these matters is all the more remarkable because of the pervasiveness of the earlier dogma, which for warm-blooded vertebrates relegated neurogenesis to embryonic development and, for a few neuronal classes, early postnatal life. The discovery of constant neuronal replacement in the adult brain was remarkable, too, in that it was not required by what we thought to be the logic of nervous system function. Moreover, no previous facts prepared us for it. Much of the modern theory of learning embraced the view of modifiable synapses as the key players in learning and as the repositories of memory. But if this were so, what would be the point of neuronal replacement in healthy brain tissue? In what follows, I will briefly review the work of Joseph Altman, because he was the first one to challenge the notion that new neurons were not produced in adulthood. I will then review what we know about neuronal replacement in the song system of birds, which my laboratory has studied for many years. In closing, I will offer a general theory of long-term memory that, if true, might explain why adult nervous systems constantly replace some of their neurons.

17beta-Estradiol increases, aging decreases, c-Fos expression in the rat accessory olfactory bulb

In the present paper we investigated the c-Fos immunoreactivity in the accessory olfactory bulb (AOB) of juvenile, adult and old rats of both sexes, as well as the effect of 17beta-estradiol on the expression of this immediate early gene. Basal c-Fos expression in the olfactory bulb decreased with age, and estrogen treatment caused an increase in the number of neurons expressing c-Fos in the AOB. The results indicate that both aging and estrogen have roles in the ability of neurons to co-ordinate genetic activity. Our observations may explain the decrease in age-related changes of brain plasticity, and provide data for the understanding of hormonally regulated neuronal plasticity.

Neuropathology of mice carrying mutant APP(swe) and/or PS1(M146L) transgenes: alterations in the p75(NTR) cholinergic basal forebrain septohippocampal pathway

Cholinergic basal forebrain (CBF) projection systems are defective in late Alzheimer's disease (AD). We examined the brains of 12-month-old singly and doubly transgenic mice overexpressing mutant amyloid precursor protein (APP(swe)) and/or presenilin-1 (PS1(M146L)) to investigate the effects of these AD-related genes on plaque and tangle pathology, astrocytic expression, and the CBF projection system. Two types of beta-amyloid (Abeta)-immunoreactive (ir) plaques were observed: type 1 were darkly stained oval and elongated deposits of Abeta, and type 2 were diffuse plaques containing amyloid fibrils. APP(swe) and PS1(M146L) mouse brains contained some type 1 plaques, while the doubly transgenic (APP(swe)/PS1(M146L)) mice displayed a greater abundance of types 1 and 2 plaques. Sections immunostained for the p75 NGF receptor (p75(NTR)) revealed circular patches scattered throughout the cortex and hippocampus of the APP(swe)/PS1(M146L) mice that contained Abeta, were innervated by p75(NTR)-ir neurites, but displayed virtually no immunopositive neurons. Tau pathology was not seen in any transgenic genotype, although a massive glial response occurred in the APP(swe)/PS1(M146L) mice associated with amyloid plaques. Stereology revealed a significant increase in p75(NTR)-ir medial septal neurons in the APP(swe) and PS1(M146L) singly transgenic mice compared to the APP(swe)/PS1(M146L) mice. No differences in size or optical density of p75(NTR)-ir neurons were observed in these three mutants. p75(NTR)-ir fibers in hippocampus and cortex were more pronounced in the APP(swe) and PS1(M146L) mice, while the APP(swe)/PS1(M146L) mice showed the least p75(NTR)-ir fiber staining. These findings suggest a neurotrophic role for mutant APP and PS1 upon cholinergic hippocampal projection neurons at 12 months of age.

From stem cells towards neural layers: a lesson from re-aggregated embryonic retinal cells

Cells from dissociated embryonic avian retinae have the capacity to re-aggregate in rotation culture and form cellular spheres reconstituting a complete arrangement of all retinal layers. This exquisite phenomenon is based upon in vitro proliferation of multipotent precursor stem cells and spatial organization of their differentiating descendants. The addition of soluble factors from cultured retinal pigmented epithelial (RPE) or radial glial cells is essential to revert inside-out spheres (rosetted retinal spheres) into correctly laminated outside-out spheres (stratified spheres). Such complete restoration of a laminated brain tissue by cell re-aggregation has been achieved only for the embryonic avian retina, but not the mammalian retina, nor for other brain parts. This review summarises the history of the re-aggregation approach, presents avian retinal re-aggregate models, and analyses roles of the RPE and Müller cells for successful retinal tissue regeneration. It is predicted that these results will become biomedically relevant, as stem cell biology will soon open ways to produce large amounts of human retinal precursors.

Disruption of the olfactory placode and brain conditioned medium increase the number of LHRH immunostained neurons in explants

The olfactory placode gives rise to both olfactory receptor neurons, which remain as a component of the peripheral nervous system, and to luteinizing hormone-releasing hormone (LHRH) neurons, which migrate to the central nervous system. In this study, we used chick olfactory placode explants to ask several questions regarding LHRH neuronal differentiation. We found that explants of ectoderm from the fronto-nasal region of embryos as early as Hamilton & Hamburger (HH) stage 12 gave rise to LHRH neurons, that explants from all regions of the olfactory placode were able to generate LHRH neurons, that both brain conditioned medium and disruption of the olfactory placode increase the number of LHRH neurons observed in explants, and that the combination of these two manipulations results in the production of more LHRH neurons than either treatment alone. We conclude that LHRH neurons originate in the olfactory epithelium and that some of the same factors which influence olfactory receptor neuron development also affect LHRH neuronal development.

Reinnervation of the rat olfactory bulb after methyl bromide-induced lesion: timing and extent of reinnervation

We used the inhalation of methyl bromide gas to produce a near-complete destruction of the rat olfactory epithelium and analyzed the reinnervation of the bulb during reconstitution of the epithelium. The degeneration of olfactory axons elicits a transient up-regulation of glial cell proliferation and glial fibrillary acidic protein expression in the olfactory nerve and olfactory nerve layer of the bulb. Anterograde transport after intranasal infusion of wheat germ agglutinin conjugated horseradish peroxidase demonstrates that the first nascent axons reach the bulb within the first week after lesion. Subsequently, a massive wave of fibers arrives at the bulb between 1 and 2 weeks postlesion, and enters the glomeruli between 2 and 3 weeks postlesion. However, the olfactory projection does not stabilize until 8 weeks after lesion judging from the return in growth associated protein-43 expression to control levels. The extent of reinnervation after lesion is correlated with the completeness with which the epithelium reconstitutes itself. In rats that are lesioned while fed ad libitum, there is near-complete reconstitution of the neuronal population, and the projection onto the bulb fills the glomerular layer in its entirety. However, in rats that are lesioned while food restricted, a significant fraction of olfactory epithelium becomes respiratory during its reconstitution, and the population of reinnervating fibers is less. As a consequence, the posterior half of the bulb remains hypoinnervated overall and denervated at its caudal margin. The preferential reinnervation of the anterior bulb in the food-restricted, methyl bromide gas-lesioned animals indicates that the mechanisms that guide the growth of the olfactory axons and restore receptotopy do not operate with the same precision in this setting as they do during development or during the lower level of turnover associated with the "normal" laboratory existence. Accordingly, we hypothesize that the persistence of a significant population of pre-existing neurons is needed to preserve receptotopy during reinnervation. In addition, the results suggest that in the face of massive turnover and a reduced afferent population, there is a tendency for reinnervating axons to fill available synaptic space.

Nasal route for direct delivery of solutes to the central nervous system: fact or fiction?

During this century, several investigators reported that certain viruses, metals, drugs, and other solutes could bypass systemic circulation and enter the brain and/or cerebrospinal fluid directly following nasal administration. Although evidence clearly suggests that the olfactory epithelium and its olfactory cells play a major role, little is known about the mechanisms of direct transport of solutes into the brain. An overview of what is known about these mechanisms may aid in further research in this field, including studies of direct drug delivery to the central nervous system. This review, in addition to summarizing the literature to date, clearly describes the intricate association of the anatomical features involved in direct entry of solutes into the brain following nasal administration. To aid in the understanding of the possible routes a solute can take after nasal administration, the anatomy of the olfactory epithelium and surrounding tissues is described, and a detailed scheme delineating the emerging pathways is presented. Techniques used in delineating these pathways and studies supporting a particular pathway are discussed in greater detail. Finally, some factors influencing the direct transport of solutes to the cerebrospinal fluid and brain are summarized.

Neuron addition during growth of the postmetamorphic bullfrog: sensory neuron and axon number

Neuron addition is one means whereby the nervous system can compensate for increased body size. Neurons can be added either by mitosis of stem cells or by late differentiation of committed precursors. Previously, the doubling of hind limb dorsal root ganglion (DRG) neurons in postmetamorphic bullfrogs (Rana catesbeiana) was found to occur in the absence of neuron proliferation (St. Wecker and Farel [1994] J. Comp. Neurol. 342:430-438). In the present study, we identify a population of cells in the DRGs of juvenile frogs that lack the appearance typical of sensory neurons yet are immunoreactive to a neuron-specific probe for neurofilament protein. These less differentiated (type-L neurons) could not be labeled retrogradely with horseradish peroxidase from the periphery or dorsal root. Despite their apparent immaturity, type-L neurons appear to have extended axons both centrally and toward the periphery, because axon number in dorsal roots and peripheral nerves was similar in juvenile and adult frogs. These findings are consistent with the existence in juvenile frogs of a population of incompletely differentiated DRG neurons that lack the physiological properties and appearance typical of mature neurons.

Lesion-induced proliferation of neuronal progenitors in the dentate gyrus of the adult rat

In order to determine whether granule cell death stimulates the proliferation of granule cell precursors in the dentate gyrus of the adult rat, we performed both excitotoxic and mechanical lesions of the granule cell layer and examined the numbers of proliferating cells at different survival times. Using [3H]thymidine autoradiography, bromodeoxyuridine labelling and proliferating cell nuclear antigen immunohistochemistry, we observed an increase in proliferating cells on the lesioned side compared to the unlesioned side 24 h after surgery. A significant positive correlation between the extent of granule cell damage and the number of proliferating cells was observed. Combined [3H]thymidine autoradiography and immunohistochemistry for cell-specific markers revealed that the vast majority of proliferating cells had the morphological characteristics of granule cell precursors and were not immunoreactive for vimentin, a marker of immature glia. Combined [3H]thymidine autoradiography and terminal deoxynucleotidyl transferase-mediated dUTP nick end-labelling for degenerating cells showed that the proliferating cells did not rapidly degenerate. Three weeks after the lesion, most cells produced in response to the lesion had the morphological characteristics of mature granule neurons, were located in the granule cell layer and expressed markers of mature granule neurons, including neuron-specific enolase, the N-methyl-D-aspartate receptor subunit NRI and calbindin. These findings suggest that granule cell death stimulates the proliferation of precursor cells, many of which survive and differentiate into mature granule neurons.

Non-conventional role of lysosomal acid phosphatase in olfactory receptor axons: co-localization with growth-associated phosphoprotein-43

Olfactory receptor neurons undergo a continuous turnover in adult mammals. It is largely unknown how their axons invade the olfactory bulb and induce synaptic re-organization in glomeruli. Here, the cytochemical localization of lysosomal acid phosphatase has been studied in olfactory bulbs of adult rats and mice. The enzyme has been identified by specific substrate, inhibitors and absence in lysosomal acid phosphatase-knockout mice. Lysosomal acid phosphatase is located in primary and secondary lysosomes, which are unevenly distributed in the olfactory nerve layer and among olfactory glomeruli. In consecutive sections of glomeruli, the intensity of lysosomal acid phosphatase immunoreactivity co-varied with that of growth-associated phosphoprotein. Electron microscopically, differential lysosomal acid phosphatase staining in glomeruli corresponded to different proportions of labelled and unlabelled axons. Quantification revealed that lysosomal acid phosphatase labelling was strongest in non-synaptic profiles of terminal axons, while it was weak in or even missing from most synaptic profiles. Hence, growing olfactory axons apparently carry more lysosomal acid phosphatase than those which have established synaptic contacts. Following olfactory deafferentation both lysosomal acid phosphatase activity and growth-associated phosphoprotein-43 are lost from glomeruli, suggesting that both proteins are expressed in olfactory sensory axons during growth, while lysosomal acid phosphatase is apparently not a marker of anterograde terminal degeneration.

Early olfactory fiber projections and cell migration into the rat telencephalon

The formation and development of primary olfactory axons was studied in the rat embryo using acetylcholinesterase histochemistry, immunocytochemistry for neuron-specific beta-tubulin (TuJ1) and growth associated protein 43 (GAP43), and a fluorescent tracer DiI. Olfactory axons extend from the olfactory receptor neurons localized in the olfactory epithelium. These fibers grow to reach and enter the olfactory bulbs, where they form the first relay and integrative synaptic station in the olfactory system: the olfactory glomerulus. In this communication we address the development of primary olfactory fibers: first from the olfactory placode and later from the olfactory epithelium. Olfactory fibers enter the olfactory bulbs apparently in a disordered manner but soon arrange themselves in hook shaped aggregates of fibers, with many boutons (immature synaptic terminals), to form the glomeruli. We detected this kind of structure for the first time at embryonic day 16. The olfactory receptor cells are usually anchored in the basal lamina of the olfactory epithelium but some of them, after reaching their targets, lose their epithelial attachment, leave the olfactory epithelium and migrate to and enter the olfactory bulbs. The traffic of cells between the olfactory epithelium and the brain lasts late into embryonic development. We describe four types of migratory mechanism used by different populations of cells to reach their targets in the telencephalic vesicle and propose the existence of migrating cells that enter the telencephalon. These data were corroborated by injections into the olfactory epithelium a of murine retrovirus carrying the Escherichia coli lac-Z gene.

Lineage specification of olfactory neural precursor cells depends on continuous cell interactions

We transplanted, as a single cell suspension, cells dissociated from the mature and immature olfactory epithelium of rats or TgR(ROSA26)26Sor mice expressing constitutively the LacZ gene into the developing brain (cerebellum, striatum, inferior colliculus, lateral ventricles) of E15 rat fetuses. Grafted cells or their descendants were still present in the central nervous system more than a month after transplantation. Transplanted cells either integrated as isolated cells or, during the first day after transplantation, reaggregated into clusters. Scattered cells, despite their placodal origin, differentiated into neuron or glial cells with a central phenotype. This was demonstrated by anatomical methods and selective amplification of cDNA encoding for neuronal specific transcripts (microtubule-associated protein 2 and middle-molecular-mass neurofilament protein) expressed by the engrafted cells. Cells in large clusters generated an epithelium containing mature olfactory neurons. Some of them were immunoreactive for the olfactory marker protein. Our findings show that cells dissociated from the developing and adult olfactory organs when transplanted into the rat fetal brain can either completely change their fate and differentiate according to their final position or generate an olfactory epithelium if they reaggregate into large clusters.

Time course of reinnervation of the olfactory bulb after transection of the primary olfactory nerve in the pigeon

Horseradish peroxidase (HRP) histochemistry was used to study the time course of reinnervation of the pigeon olfactory bulb after simple transection of the primary olfactory nerve. At selected intervals (9, 13, 29, 61 and 93 days) after transection of the right olfactory nerve, a concentrated solution of HRP was instilled in both nasal cavities. Intracarotid perfusion was performed 3 days after nasal instillation of HRP and 40-microns sections of olfactory bulb processed with the tetramethylbenzidine (TMB)-HRP histochemical protocol to visualize olfactory receptor axon terminals reinnervating the glomerular layer of the bulb. The total area of reinnervation of four representative regions in the bulb of the transected side were compared with that on the control bulb. The area of innervation by newly reconstituted olfactory axons was approximately 17% of control values at the 12-day posttransection time interval. A progressive increase in the area of reinnervation was observed over time. Reinnervation of the right bulb was approximately 70% complete at the 32-day posttransection time interval and indistinguishable from the left control bulb at the 64- and 96-day posttransection time intervals. A uniform pattern of reinnervation of different bulb regions was observed at all time intervals. These results indicate that the peripheral olfactory system of the pigeon is capable of complete reconstitution after nerve transection. Our findings should be useful in guiding functional comparisons of normal and newly reconstituted peripheral olfactory systems in the pigeon.

Cell migration from the transplanted olfactory placode in Xenopus

The eye vesicle of Xenopus borealis has been replaced with the transplanted olfactory primordium from Xenopus laevis in an attempt to determine whether cells from the transplant could migrate along the regrowing olfactory nerve and become incorporated into the CNS of the host. The use of X. laevis and X. borealis pairs allowed us to distinguish the cells of the host from those of the donor at the cellular level by means of the characteristic fluorescent nuclear spots (Q bands) of X. borealis. Transplantation was performed on pairs of animals at stages 23/24. The olfactory anlage was readily incorporated into the host, often fusing with the host homolateral organ and inhibiting the regrowth of the eye vesicle. An olfactory nerve developed from the transplanted organ. In the majority of cases, the nerve reached the diencephalon at the level of entrance of the optic nerve. Along the nerve originating from the transplanted organ we observed a stream of cells with the characteristics of the donor. These cells penetrated the host's CNS and became incorporated into it. The nature of these cells has not been ascertained by specific neuronal markers. However, on the basis of their morphology and disposition, the hypothesis suggested is that some of the migrating cells are neurons.

Embryonic development of the antennal lobes of a hemimetabolous insect, the cockroach Periplaneta americana: light and electron microscopic observations

In the hemimetabolous insect Periplaneta americana, the adult-like organization of the primary olfactory centers, the antennal lobes, is established during the approximately 31 days of embryogenesis. This report describes the temporal sequence of developmental events as viewed in the light and electron microscope by means of histological stains and by DiI labeling of antennal receptor axons with subsequent photoconversion. Glomeruli, characteristic differentiations of the antennal lobe neuropil, are first observed on day 19; their development, which is not synchronous in the various parts of the antennal lobe, lasts until about day 22. From day 10 on, glial cells begin to form a narrow boundary layer between the soma cortex and the central neuropil. They exhibit a lengthening of their processes in parallel with the formation of glomeruli. Marked proliferation or migration of these glial cells into the neuropil between glomeruli has not been observed. Antennal receptor axons could be labeled from stage 15 on. They terminate in an elongated growth cone with numerous filopodia. From day 18 on, some of these become bent or show an initial bifurcation. From day 22 on, the first afferent axons develop an adult-like arborization pattern. Synaptic contacts between receptor axons and unidentified neurons were observed as early as stages 16 and 19, in which the axons still have a growth cone-like form. In stage 27, in which the fibers have adult-like arborizations, many output contacts and few input contacts were found.

Hair cell replacement in avian vestibular epithelium: supporting cell to type I hair cell

Previous investigations have demonstrated that the sensory epithelium of the avian vestibular system possesses the capacity to replace hair cells both on an ongoing basis and following severe damage. Supporting cells, within the sensory epithelium, are believed to be the progenitors of the regenerated hair cells. In the present study we describe the series of events leading to the formation of a regenerated vestibular hair cell in post-hatched birds. Young chickens received injections of streptomycin sulfate in order to damage the sensory epithelium of the vestibular system. These injections were followed by injections of the cell proliferation marker tritiated-thymidine. At predetermined intervals, the animals were killed, and the vestibular organs were processed for tissue autoradiography. Our results confirm that hair cells originate from supporting cells. The data also indicate that postmitotic cells migrate towards the lumen of the epithelium where they differentiate into Type II hair cells. At a later time, some of the new Type II hair cells further differentiate into Type I hair cells. These results suggest that both types of avian vestibular hair cells have a common ancestor. The data also provide evidence in support of the hypothesis that calyx enclosed Type I hair cells, only present in birds and mammals, are a more differentiated stage of Type II hair cells.

Cell cycle of transdifferentiating supporting cells in the basilar papilla

Mitosis of supporting cells has been shown to contribute to the cellular repopulation of the basilar papilla after acoustic trauma. In the present work we report data obtained with light and transmission electron microscopy after acoustic trauma in chicks. We report changes that occur in cell shape, surface morphology, intercellular junctions, nuclear shape and location, and cytoplasmic organization of supporting cells after trauma. The findings strongly suggest that supporting cells transdifferentiate and that the proliferative pattern is similar to interkinetic nuclear migration, as previously shown in the developing neural tube and basilar papilla. S-phase nuclei were positioned adjacent to the basement membrane, suggesting that interaction with the extracellular matrix may occur during the cell cycle. Supporting cells divided with the long axis of the spindle parallel to the reticular lamina and displayed no signs of intercellular communication during mitosis. This suggested to us that the fate of the progeny cells is determined prior to mitosis and that the progeny may be of identical phenotypic fate. Dividing cells had a smooth apical surface. The smooth surface may provide a marker to help identify dividing cells with scanning electron microscope analysis.

Microscopic structure of the olfactory organ of the clearnose skate, Raja eglanteria

The olfactory organ of juvenile clearnose skates (Raja eglanteria) was studied with the light and electron microscopes. The organ is ovoid in shape, and its free surface is complicated by the presence of some 20 lamellae. Each lamella has a folded surface lined by a typical neurosensory olfactory epithelium. Bipolar olfactory receptor neurons, ciliated sustentacular cells, and basal cells are the pre-eminent cellular components of the epithelium. Two types of receptor neurons, both bearing microvilli but not cilia, were identified. The type 1 neuron is similar to that previously described in other fishes. The type 2 neuron has a characteristic morphology justifying a separate description. Its dendritic knob is larger than that of type 1, and its microvilli, which are shorter and thicker, are straight and regularly arranged. Tight bundles of filaments provide a skeleton to each microvillus, and these filament bundles reach more than 5 microns down into the dendrite. Type 2 receptor neurons have a well-developed Golgi complex and sparse rough endoplasmic reticulum (rER), whereas type 1 receptor neurons have a less well-developed Golgi complex and a conspicuous system of rER lamellae. The mucous layer on the epithelial surface is provided by the secretion of goblet cells that are situated mostly in the peripheral regions of each lamella. Secretory granules in the sustentacular cells and glands in the lamina propria were not observed.

Histochemical and immunocytochemical study of the migration of neurons from the rat olfactory placode

Immunocytochemical and histochemical methods have been used to describe the neuronal population migrating from the rat olfactory placode and to analyze the spatio-temporal evolution of this neuronal migration during development. Several neuronal markers, such as binding to the lectin Ulex europaeus (UEA I) and the presence of neuron-specific enolase (NSE), olfactory marker protein (OMP), and luteinizing hormone-releasing hormone (LHRH), have been tested in order to determine whether migrating neurons originate from both the medial and the lateral parts of the placode and whether they all express LHRH. Our data show that a large population of differentiated migrating neurons can be identified with an antibody against NSE from the 14th day of gestation and with UEA I one day later. Migrating neurons are closely associated with both the vomeronasal axon fascicles emerging from the medial pit and the olfactory axons originating from the lateral pit. However, the neuron migration from the lateral pit appears to be more discrete than that from the medial pit. No LHRH immunoreactivity has been detected among neurons migrating from the lateral pit. Some neurons accompanying the olfactory axon fascicles exhibit a high level of maturation as shown by their OMP-positivity. Numerous neurons positive for both NSE and UEA I have also been observed within the presumptive olfactory nerve layer in early embryonic stages.

Reorganization of the chick basilar papilla after acoustic trauma

The auditory epithelium in birds and mammals consists of a postmitotic population of hair cells and supporting cells. Unlike mammals, birds can regenerate their auditory epithelia after trauma. Recent evidence indicates that supporting cells undergo mitosis after acoustic trauma, suggesting that supporting cells may transdifferentiate into hair cells. The goals of this study were to 1) characterize the responses of hair cells and supporting cells to acoustic trauma, and 2) determine whether hair cell loss is a prerequisite for generation of new hair cells. Chicks were exposed to an octave-band noise and their inner ears assayed with fluorescence or scanning electron microscopy. In one area of the basilar papilla, defined as the center of the lesion, extensive hair cell degeneration occurred. Expanded supporting cells obliterated degenerating hair cells and invaded spaces normally occupied by hair cells. Aggregates of DNA were found within the basilar papilla, suggesting that hair cell death and disintegration may occur within the epithelium. The epithelial sheet appeared structurally confluent at all times examined. Supporting cells exhibited altered apical contour in distal regions of the basilar papilla, where hair cell damage was mild or inconspicuous. Four days after noise exposure, newly generated hair cells were found in the center of the lesion and in the distal areas, where no hair cell loss could be detected. The results suggest that supporting cells may play an important role in maintenance and repair of the traumatized basilar papilla and raise the possibility that production of new hair cells is not dependent on hair cell loss in the immediate vicinity.

Size-related increase in motoneuron number: evidence for late differentiation

The number of motoneurons in the lumbar lateral motor column (LMC) was compared in bullfrogs (Rana catesbeiana) ranging in body length from 2.5 to 19 cm. Large frogs had 36% more motoneurons than small frogs; however, within the caudal third of the LMC, large frogs had over 70% more motoneurons than small frogs. Injection of small frogs with [3H]thymidine every third day for 20-22 weeks gave no evidence of motoneuron birth. Instead, a pool of small, incompletely differentiated (type L) motoneurons appears to be converted into mature (type M) motoneurons as the animal grows. This hypothesis is supported by several lines of evidence: (1) the number of type-M motoneurons varies directly with body size while the number of type-L cells varies inversely; (2) the increase in type-M motoneurons and the decrease in type-L cells are restricted to the same regions of the LMC; and (3) type-L cells exhibited both immunoreactivity to neurofilament antibodies and histochemical evidence of acetylcholinesterase activity, a marker for spinal motoneurons.

Are there structural and functional modules in the vertebrate olfactory bulb?

A number of different recording methods have shown that odorants elicit patterns of neuronal activity widely distributed across cells of the olfactory receptor epithelium, olfactory bulb, and piriform cortex in the vertebrate olfactory system. These findings suggest that the physicochemical properties of odorant molecules are processed by distributed coding mechanisms activated in parallel in olfactory circuits in order to characterize a single, "monomolecular" odorant. These findings also suggest that the response patterns seen at higher levels are set up by differential responses in peripheral receptor cells of the olfactory epithelium. One requirement for understanding the details of this proposed encoding scheme is correlation of odor-generated patterns with the components of these circuits. In this paper, results from 2-deoxyglucose and voltage-sensitive dye studies suggest that certain components of these responses may relate to patterns established in reproducibly identifiable aggregates of bulbar cells. These findings are consistent with previous observations suggesting that columnar groups of periglomerular, mitral/tufted and granule cells, oriented perpendicular to the laminae of the bulb, are functionally related to one another. Such cell groups or modules, when activated in parallel, could serve as building block components of the complete ensemble response. According to this hypothesis, different sets of such modules would be activated with different odorant stimuli and modules could be shared to the degree to which the physicochemical properties of the different stimuli overlap.

The persistent expression of a highly polysialylated NCAM in the dentate gyrus of the adult rat

The expression of a highly polysialylated form of the neural cell adhesion molecule (NCAM-H), often termed 'embryonic NCAM', has been investigated in the hippocampal formation of developing and adult rats. To determine the immunohistochemical localization of NCAM-H, a monoclonal antibody that reacts with the polysialic acid portion of NCAM-H was used. In the late embryonic and early postnatal periods, immunoreactivity for NCAM-H was found throughout the hippocampal formation, except for the ventricular layer. Thereafter, the immunoreactivity gradually decreased and almost vanished in most parts in the adult. However, a strong immunoreactivity remained on a number of cells in the dentate gyrus of adult rats, particularly in the deepest part of the granular layer. The immunoreactive arborized dendrites, mostly arising from the primary apical pole of the granule cells, were found to enter the molecular layer. The mossy fibers also were positive. Electron-microscopic examination of the hilus portion showed that the immunoreactivity was detected on the plasma membrane of some axons in the mossy fiber bundles. Since postnatal neurogenesis is known to continue into adulthood in the deepest part of the granule cell layer of the dentate gyrus, these results suggest that, in the adult dentate gyrus, NCAM-H is expressed by newly generated granule cells, and that the NCAM-H-expressing new cells may participate in the formation of new neural circuits.

Experimental studies on the olfactory marker protein. V. Olfactory marker protein in the olfactory neurons transplanted within the olfactory bulb

The olfactory mucosa of neonatal rats was transplanted within the olfactory bulb of littermates to investigate whether the olfactory bulb would have played a role in the differentiation of the olfactory neurons and whether the olfactory axons, growing out from the transplant, would have interacted with the olfactory glomeruli of the host. The observations were conducted on sections stained with Gill's hematoxylin, Loots' silver method, and the immunohistochemical technique for the demonstration of the olfactory marker protein (OMP). The olfactory neurons of the transplant (those localized in the neuroepithelium and those migrating from it into the bulbar parenchyma) could become fully differentiated but only few of them were OMP positive. Numerous sensory axons originated from the transplanted olfactory mucosa, however, they did not form ectopic glomeruli nor did they interact with the glomeruli of the host. These results indicate that the olfactory bulb, in vivo, does not affect the number of olfactory neurons expressing OMP and that the ectopically located neurons lack the cues to recognize the host glomeruli.