Glial cells form boundaries for developing insect olfactory glomeruli

Morphology and physiology of antennal lobe projection neurons in the hawkmoth Agrius convolvuli

The neuronal pathways involved in the processing of sex pheromone information were investigated in the hawkmoth Agrius convolvuli (Lepidoptera: Sphingidae), which uses (E,E)-11,13-hexadecadienal (E11,E13-16:Ald) as the single sex pheromone component. We first clarified the anatomical organization of the antennal lobe of A. convolvuli. Subsequently, central neurons in the antennal lobe that responded to E11,E13-16:Ald were identified. The dendritic processes of these neurons were confined within a specific glomerulus (cumulus) in the antennal lobe. The axons of these neurons projected to the inferior lateral protocerebrum and mushroom body calyx. Although the anatomical organization and morphology of individual neurons in A. convolvuli were similar to other species in the superfamily Bombycoidea, the use of cumulus as the single pathway for sex pheromone information processing was characteristic to this species.

Review : Glial Boundaries and Scars: Programs for Normal Development and Wound Healing in the Brain

Early studies of glial boundaries, which are composed of immature astrocytes and extracellular matrix mol ecules (which they express), initially offered insight into the partitioning that occurs in the developing nervous system. More recently, however, it has been suggested that similar "boundaries" may have important roles in other processes occurring in the brain, including repair after traumatic brain injury. As more is understood about the expression and function of boundary molecules and glia, their potential importance is becoming apparent in numerous neuropathological conditions, including neurodegeneration and neuroregeneration in Alzheimer's and Huntington's diseases as well as in brain neoplasms. Furthermore, before we can hope to fully understand and facilitate regeneration in the compromised brain, our knowledge of the glial boundary, both during development and in the adult, must be more complete. The Neuroscientist 1:142-154, 1995

Nitric oxide affects short-term olfactory memory in the antennal lobe of Manduca sexta

Nitric oxide (NO) is thought to play an important neuromodulatory role in olfaction. We are using the hawkmoth Manduca sexta to investigate the function of NO signaling in the antennal lobe (AL; the primary olfactory network in invertebrates). We have found previously that NO is present at baseline levels, dramatically increases in response to odor stimulation, and alters the electrophysiology of AL neurons. It is unclear, however, how these effects contribute to common features of olfactory systems such as olfactory learning and memory, odor detection and odor discrimination. In this study, we used chemical detection and a behavioral approach to further examine the function of NO in the AL. We found that basal levels of NO fluctuate with the daily light cycle, being higher during the nocturnal active period. NO also appears to be necessary for short-term olfactory memory. NO does not appear to affect odor detection, odor discrimination between dissimilar odorants, or learning acquisition. These findings suggest a modulatory role for NO in the timing of olfactory-guided behaviors.

Development-based compartmentalization of the Drosophila central brain

The neuropile of the Drosophila brain is subdivided into anatomically discrete compartments. Compartments are rich in terminal neurite branching and synapses; they are the neuropile domains in which signal processing takes place. Compartment boundaries are defined by more or less dense layers of glial cells as well as long neurite fascicles. These fascicles are formed during the larval period, when the approximately 100 neuronal lineages that constitute the Drosophila central brain differentiate. Each lineage forms an axon tract with a characteristic trajectory in the neuropile; groups of spatially related tracts congregate into the brain fascicles that can be followed from the larva throughout metamorphosis into the adult stage. Here we provide a map of the adult brain compartments and the relevant fascicles defining compartmental boundaries. We have identified the neuronal lineages contributing to each fascicle, which allowed us to compare compartments of the larval and adult brain directly. Most adult compartments can be recognized already in the early larval brain, where they form a "protomap" of the later adult compartments. Our analysis highlights the morphogenetic changes shaping the Drosophila brain; the data will be important for studies that link early-acting genetic mechanisms to the adult neuronal structures and circuits controlled by these mechanisms.

Morphogenesis and proliferation of the larval brain glia in Drosophila

Glial cells subserve a number of essential functions during development and function of the Drosophila brain, including the control of neuroblast proliferation, neuronal positioning and axonal pathfinding. Three major classes of glial cells have been identified. Surface glia surround the brain externally. Neuropile glia ensheath the neuropile and form septa within the neuropile that define distinct neuropile compartments. Cortex glia form a scaffold around neuronal cell bodies in the cortex. In this paper we have used global glial markers and GFP-labeled clones to describe the morphology, development and proliferation pattern of the three types of glial cells in the larval brain. We show that both surface glia and cortex glia contribute to the glial layer surrounding the brain. Cortex glia also form a significant part of the glial layer surrounding the neuropile. Glial cell numbers increase slowly during the first half of larval development but show a rapid incline in the third larval instar. This increase results from mitosis of differentiated glia, but, more significantly, from the proliferation of neuroblasts.

Organization of glomeruli in the main olfactory bulb of Xenopus laevis tadpoles

Structural and functional investigations were carried out to study olfactory glomeruli in the main olfactory bulb (OB) in tadpoles of the clawed frog, Xenopus laevis. Calcium imaging of odor response patterns of OB neurons revealed that the synapses within the glomeruli are functional. Tracing axons of individual olfactory receptor neurons (ORNs), dendrites of mitral/tufted (M/T) cells and processes of periglomerular interneurons indicate that the glomerular architecture is solely determined by terminal branches of ORN axons and tufts of M/T primary dendrites. The small population of periglomerular neurons forms wide-field arborizations that always extend over many glomeruli, enter the glomeruli, but lack any glomerular tufts. Antibodies to synaptophysin indicate a high density of synapses within glomeruli, which was further confirmed at the ultrastructural level and quantified to approximately 0.5 synaptic sites per microm(2). Combining immunocytochemistry and ultrastructural investigations, we show that glomeruli in Xenopus laevis tadpoles lack any cellular borders. Glomeruli are surrounded neither by periglomerular somata nor by glial processes. Taken together, our results demonstrate that olfactory glomeruli in Xenopus laevis tadpoles (1) are fully functional, (2) are spheroidal neuropil aggregations of terminal tufts of ORNs and tufts of primary dendrites of M/T cells, and (3) are not enwrapped by a border formed by juxtaglomerular cells.

Early development of the Drosophila brain: IV. Larval neuropile compartments defined by glial septa

In this study, we have analyzed the architecture of the brain neuropile of the Drosophila larva, which is formed by two main structural elements: long axon tracts and terminal axonal/dendritic arborizations carrying synapses. By using several molecular markers expressed in neurons and glial cells, we show that the early larval neuropile is subdivided by glial sheaths into numerous compartments. The three-dimensional layout of these compartments and their relationship to the pattern of long axon tracts described in the accompanying article (Nassif et al. [2003] J. Comp. Neurol 417-434) was modeled by using a three-dimensional illustration computer software. On the basis of their location relative to each other and to long axon tracts, larval brain compartments can be identified with compartments defined by structural and functional criteria for the adult fly brain. We find that small precursors of most of the compartments of the adult central brain can be identified in the early larva. Changes in brain compartmental organization occurring during larval growth are described. Neuropile compartments, representing easily identifiable landmark structures, will assist in future analyses of Drosophila brain development in which the exact location of neurons and their axonal trajectories is of importance.

Increasing the number of synapses modifies olfactory perception in Drosophila

The Drosophila mutant gigas produces an enlargement of postmitotic cells caused by additional rounds of DNA replication. In neurons, the mutant cell establishes more synapses than normal. We have taken advantage of this feature to study the effect of synapse number on odorant perception. Mosaic adults were generated in which one antenna was homozygous for gigas, whereas the contralateral side served as an internal control. Morphological analysis indicates that the number and type of sensory afferents forming the mutant antenna, as well as their projection to the olfactory glomeruli, are normal. In contrast, the volume of identified glomeruli increases to a variable extent, and mutant sensory neurons branch profusely. The number of synapses, estimated in the ventral (V) glomerulus that receives ipsilateral afferents only, is increased twofold to threefold. Large-dense-core vesicle-containing terminals that probably modulate olfactory centers are identified in the V glomerulus. Their number and size are not modified by the mutant input. Sensory transduction, measured by electroantennograms, is normal in amplitude and kinetics. In odorant tests, however, the profile of the behavioral response to ethyl acetate shows attractive responses to concentrations to which sibling controls remain indifferent (10(-)8 and 10(-)7 v/v). In addition, the intensity of the response is augmented both at attractive and repulsive odorant concentrations with respect to that of controls. These results demonstrate that increased synapse number in the sensory neurons can modify the behavior of the organism, allowing a higher sensitivity of perception.

Müller cell differentiation in the zebrafish neural retina: evidence of distinct early and late stages in cell maturation

The vertebrate neural retina is mainly composed of cells of neuroectodermal origin. The primary cell types found in all vertebrate retinas are several categories of neurons and the archetypical retina glial cell the Müller cell. Although the neurons and the single glial cell type of the retina are specialized for very distinct functions, they all have a common developmental origin within the tissue. How the distinctions between cell types, in particular between neurons and glia, arise during embryonic development remains a central issue in neurobiology. In this report, we examine the genesis of Müller glial cells during zebrafish (Danio rerio) eye development. Particular emphasis is placed on the expression of the Müller cell maturation markers carbonic anhydrase and glutamine synthetase. In addition, we report that the HNK-1 monoclonal antibody, which identifies a particular glycoconjugate frequently found on cell surface recognition molecules, also identifies zebrafish retina Müller cells early in development. The expression patterns of these three markers clearly show that the Müller cells mature in stages: HNK-1 labeling and glutamine synthetase arise earlier than carbonic anhydrase expression. In addition, the embryonic zebrafish neural retina is characterized by the presence of amoeboid, carbonic anhydrase-positive microglial cells even before the genesis of retinal neuroectodermal glia. The stepwise maturation of the glia is likely to be indicative of an overall retinal maturational program in which cell differentiation and the expression of certain phenotype-defining gene products may be separately regulated.

Formation of antennal lobe and mushroom body neuropils during metamorphosis in the honeybee, apis mellifera

The projections to the mushroom bodies (mbs) have been clearly described in the brain of adult honeybees (Apis mellifera). Olfactory projection neurons arborize in the lip of the calyceal neuropil, whereas visual projection neurons project to the collar. To study the maturation of this pattern of innervation, as well as the development of uniglomerular projection neurons within the antennal lobes (als), we conducted the following three studies focused on the first four stages of pupal development: mass staining of olfactory projection neurons, single cell labeling of olfactory projection neurons, and simultaneous labeling of olfactory projection neurons and visual projection neurons. Examination of whole-mount preparations with the confocal laser scanning microscope revealed that the olfactory projection neurons achieved their adult arborization pattern within their main output region, the lip of the mb calyces, earlier during development (pupal stage 1) than their dendritic processes within their main input region, the al (pupal stage 2). Simultaneous labeling experiments showed further that the fiber terminals of olfactory projection neurons and visual projection neurons did not overlap but instead occupied their respective projection areas within the mb calyces as early as pupal stage 1. We conclude that selective innervation of different subregions of the calycal neuropil precedes the segregation of glomerular units within the antennal lobe neuropil, and that the Kenyon cells themselves provide a template for the innervation of olfactory and visual projection neurons.

Neuronal-glial interactions and behaviour

Both neurons and glia interact dynamically to enable information processing and behaviour. They have had increasingly intimate, numerous and differentiated associations during brain evolution. Radial glia form a scaffold for neuronal developmental migration and astrocytes enable later synapse elimination. Functionally syncytial glial cells are depolarised by elevated potassium to generate slow potential shifts that are quantitatively related to arousal, levels of motivation and accompany learning. Potassium stimulates astrocytic glycogenolysis and neuronal oxidative metabolism, the former of which is necessary for passive avoidance learning in chicks. Neurons oxidatively metabolise lactate/pyruvate derived from astrocytic glycolysis as their major energy source, stimulated by elevated glutamate. In astrocytes, noradrenaline activates both glycogenolysis and oxidative metabolism. Neuronal glutamate depends crucially on the supply of astrocytically derived glutamine. Released glutamate depolarises astrocytes and their handling of potassium and induces waves of elevated intracellular calcium. Serotonin causes astrocytic hyperpolarisation. Astrocytes alter their physical relationships with neurons to regulate neuronal communication in the hypothalamus during lactation, parturition and dehydration and in response to steroid hormones. There is also structural plasticity of astrocytes during learning in cortex and cerebellum.

The central pathway of primary olfactory axons is abnormal in mice lacking the N-CAM-180 isoform

Although N-CAM has previously been implicated in the growth and fasciculation of axons, the development of axon tracts in transgenic mice with a targeted deletion of the 180-kD isoform of the neural cell adhesion molecule (N-CAM-180) appears grossly normal in comparison to wild-type mice. We examined the organization of the olfactory nerve projection from the olfactory neuroepithelium to glomeruli in the olfactory bulb of postnatal N-CAM-180 null mutant mice. Immunostaining for olfactory marker protein revealed the normal presence of fully mature primary olfactory neurons within the olfactory neuroepithelium of mutant mice. The axons of these neurons form an olfactory nerve, enter the nerve fiber layer of the olfactory bulb, and terminate in olfactory glomeruli as in wild-type control animals. The olfactory bulb is smaller and the nerve fiber layer is relatively thicker in mutants than in wild-type mice. Previous studies have revealed that the plant lectin Dolichos biflorus agglutinin (DBA) clearly stains the perikarya and axons of a subpopulation of primary olfactory neurons. Thus, DBA staining enabled the morphology of the olfactory nerve pathway to be examined at higher resolution in both control and mutant animals. Despite a normal spatial pattern of DBA-stained neurons within the nasal cavity, there was a distorted axonal projection of these neurons onto the surface of the olfactory bulb in N-CAM-180 null mutants. In particular, DBA-stained axons formed fewer and smaller glomeruli in the olfactory bulbs of mutants in comparison to wild-type mice. Many primary olfactory axons failed to exit the nerve fiber layer and contribute to glomerular formation. These results indicate that N-CAM-180 plays an important role in the growth and fasciculation of primary olfactory axons and is essential for normal development of olfactory glomeruli.

Activity blockade does not prevent the construction of olfactory glomeruli in the moth Manduca sexta

During metamorphic development, the arrival at the olfactory (antennal) lobe of olfactory receptor axons initiates the process of glomerulus formation. The glomeruli are discrete spheroidal regions of neuropil that are the sites of synaptic interactions among receptor neurons and their target antennal-lobe neurons. The process of glomerulus formation begins as groups of receptor axons form protoglomeruli. These dense clusters of terminal branches mostly are discrete entities from the time they can be recognized, although a few branches from neighboring protoglomeruli overlap laterally. A previous study by Schweitzer et al. [Schweitzer E. S., Sanes J. R. and Hildebrand J. G. (1976) Ontogeny of electroantennogram responses in the moth, Manduca sexta. J. Insect Physiol. 22, 955-960] has shown that odor-induced activity in the receptor neurons can be detected first in recordings from the axons in the antennal nerve only in the last few days of metamorphic development and thus could not influence the process of glomerulus formation. In this study, we have tested directly the possibility that an earlier presence of spontaneous activity in either the receptor axons or the antennal-lobe neurons could affect the process. Tetrodotoxin, a Na(+)-channel blocker, was injected into the hemolymph prior to the onset of glomerulus formation to block any spontaneous Na(+)-dependent activity. Subsequent intracellular recordings from antennal-lobe neurons revealed no spike activity. Comparison with vehicle-injected control animals at stages during and after glomerulus formation revealed no differences in the localization of receptor-axon terminal branches in the glomeruli, in the border of glial cells that forms around each glomerulus, or in the morphology of the tufted glomerular arbors of one of the antennal-lobe neurons. We conclude that: (1) the process of glomerulus formation is largely independent of activity; and (2) glomeruli as modular units of the CNS more closely resemble cortical barrels than cortical columns, both in their ontogeny and in the lack of an obvious effect of activity on the morphology of the neurons arborizing within them.

Preparation of primary cultures and acute slices of the nervous system of the moth Manduca sexta

Interactions among receptor neurons, glial cells and neurons intrinsic to the antennal lobe of the moth underlie the formation of olfactory glomeruli. To isolate these interactions, as well as to understand the effect of a variety of humoral agents on differentiation of the neurons and glia, we generate primary cultures of neurons or glia. These methods are described. In addition. we describe a protocol for producing slice preparations of the developing moth brain that we are using to study the biophysical and morphological development of glial cells. This technique allows us to examine a class of glial cells associated with the glomeruli that otherwise are nearly inaccessible using standard intracellular recording techniques. It also preserves the 3-dimensional arrangement of glia that may strongly influence the development of glomeruli.

Differential expression of tenascin by astrocytes associated with the supraoptic nucleus (SON) of hydrated and dehydrated adult rats

The present study evaluated the expression of tenascin by astrocytes in the supraoptic nucleus and associated ventral glial limitans (SON-VGL) under conditions that induce reversible changes in neuronal organization (dehydration and rehydration). Immunostaining of astroglia cultured from rat neonatal SON-VGL confirmed that these cells are capable of both expressing and secreting tenascin. Observations of immunostained tissue sections from adult rats revealed tenascin immunoreactivity primarily in the VGL and dendritic zone, subjacent to SON neuronal somata. Comparison of immunostained tissues from hydrated and dehydrated animals showed an apparent decrease in the intensity of immunostaining with dehydration. Subsequent Western blots of similar tissues confirmed the presence of the 210-220-kDa tenascin protein in the SON-VGL. SON-VGL tissues from control, dehydrated, and rehydrated rats were then studied by using SDS-PAGE and quantitative gel densitometry. A consistent decrease in tenascin concentration was observed by 6 days of dehydration that, with rehydration, reversed back toward or beyond control levels. Together, these observations indicate that SON-VGL astrocytes variably express tenascin and that this protein may play a role in adult SON plasticity.

Glial cells stabilize axonal protoglomeruli in the developing olfactory lobe of the moth Manduca sexta

Odor information is processed in spherical structures called glomeruli, which in all animals with differentiated olfactory systems are sites of densely spaced synaptic contacts between olfactory sensory axons and target central nervous system (CNS) neurons. Glomerulus development in the antennal (olfactory) lobe of the moth brain, which is initiated by the arrival of antennal receptor axons, requires interaction among three elements: glial cells, receptor axons, and their targets, the antennal-lobe neurons. Receptor axons form an array of protoglomeruli that become surrounded by glia and serve as a template for mature glomeruli. Previous experiments showed that when the number of glial cells is sharply reduced during development either by irradiation or by an anti-mitotic agent, receptor axons form protoglomeruli, but in the mature lobes, glomeruli are absent and central neurons lack the characteristic glomerular tufting of their arbors. The current investigation was conducted to determine which cellular events in the process of glomerulus formation are disrupted by severe reduction in glial-cell number. The branching patterns of receptor axons and antennal-lobe neurons were examined in animals that had been irradiated to produce glia-deficient antennal lobes at stages during which glomeruli normally would develop. We found that the receptor axons did form protoglomeruli, but that the protoglomeruli quickly disintegrated in glia-deficient antennal lobes; the receptor axons branched diffusely, except where several neighboring glia survived irradiation and together formed a wall of processes that appeared to block the passage of neuronal processes. Multi-glomerular antennal-lobe neurons never developed tufted arbors even at early stages. These results suggest that maintenance of protoglomeruli depends on the border of glia that forms around each protoglomerulus and that the subsequent tufting of antennal-lobe neurons depends on maintenance of the protoglomerular template during the period of dendritic growth.

Effects of NMDA receptor blockade in the developing rat somatosensory cortex on the expression of the glia-derived extracellular matrix glycoprotein tenascin-C

The patterning of synaptic connections during development is thought to be influenced by the correlation of neuronal impulse activity. N-methyl-D-aspartate (NMDA) receptors have been implicated in the reorganization of thalamocortical afferents in the visual system. The topographic mapping of the periphery of sensory systems onto the somatosensory cortex in the whisker-barrel field of rodents has served as another important paradigm in the study of extrinsic influences on synaptic rearrangements. In a search for the molecular cues that may contribute to synaptic plasticity, we have investigated the distribution of the glia-derived extracellular matrix glycoprotein tenascin-C, which is highly expressed during the formation of the barrel field map around birth and delineates the boundaries between barrel fields after segregation of afferent inputs. Here we show that systemic and local application of NMDA receptor antagonists at postnatal day 2 inhibited the down-regulation of tenascin mRNA and protein by postnatal day 6 and prevented the appearance of tenascin-positive barrel field boundaries. Furthermore, barrels were not distinguishable by Nissl staining, and segregation of thalamocortical afferents as monitored by anterograde Dil tracing and acetylcholinesterase histochemistry was not complete. These observations indicate that expression of tenascin-C and segregation of afferent inputs are modified by NMDA receptor-dependent neuronal activity.

Immunocytochemical analyses of astrocyte development in the olfactory bulb

Several lines of evidence suggest an important role for glia in establishing boundaries during development of mammalian cortex and insect olfactory lobe. In the adult rat olfactory bulb distinct morphological categories of astroglial cells with clear laminar specificity are easily recognized following immunocytochemical staining of glial fibrillary acidic protein (GFAP). To explore the developmental distribution of olfactory bulb astrocytes and their possible role in establishing the segregation of neurons in specific olfactory bulb laminae, we used immunocytochemical localization of GFAP in rats at 0, 6, 9, 12, 15 and 21 days postnatal plus the adult. In the adult we confirmed prior observations and identified five morphological categories of astrocytes: linear, wedge, elongate, semicircular, and circular. Each category had a unique sublaminar distribution across the olfactory bulb, although categories could occur in more than one lamina. Between 0 and 21 days postnatal a 6th category was apparent, radial glial cells. The mature astrocyte morphologies did not emerge uniformly. Astrocytes found in the outermost glomerular layer developed first with the appearance of the linear, wedge and elongate morphologies. Deeper laminate of the olfactory bulb followed in a successive fashion until the adult pattern was evident around 15 days postnatal. As radial glia disappeared, the mature morphologies assumed their final position. The data suggest that the maturation of olfactory bulb astrocytes may be linked to the final migration and maturation of olfactory bulb neurons.

Hedgehog, transmitted along retinal axons, triggers neurogenesis in the developing visual centers of the Drosophila brain

The development of the visual centers of the Drosophila brain is tightly regulated by the ingrowth of retinal axons from the developing eye. In the first optic ganglion, the lamina, arriving retinal axons trigger the precursors of their synaptic partners to complete a final cell division and commence neural differentiation. The secreted product of the hedgehog gene regulates the temporal assembly of photoreceptor precursor cells into ommatidial clusters in the compound eye. Here, we show that Hedgehog is transmitted along the retinal axons to serve as the inductive signal in the brain. Hedgehog acts in the first of two retinal axon-mediated steps in the assembly of lamina synaptic cartridges. These observations provide a novel insight into the molecular interactions that orchestrate the assembly of neural precursor cells into precise synaptic circuits.

Influence of receptor axons on the formation of olfactory glomeruli in a hemimetabolous insect, the cockroach Periplaneta americana

The embryonic development of the hemimetabolous insect Periplaneta americana requires approximately 31 days. Deafferentation experiments were used to investigate the role of ingrowing receptor axons during embryogenesis, specifically their influence 1) on the subdivision of the antennal lobe neuropil into glomeruli, 2) on the morphology and number of glial cells, and 3) on the arborization pattern of central neurons. The flagellum of one antenna was removed from embryos at different developmental stages starting with day 10. Subsequently, they were raised in culture until a total age of 26 days. At day 10, the deutocerebrum has received only a very small number (ca. 0.4%) of antennal receptor axons; deafferentation at this stage allowed us to deprive the deutocerebrum of approximately 99% of its normal antennal input. Deafferentation has marked effects on the organization of the antennal lobe neuropil. The deafferented lobe is reduced in volume compared to the control side; the characteristic glomeruli are missing. During normal development glomeruli are formed between day 19 and 22, first in dorsal and then in ventral antennal lobe regions. By removing the antenna before day 20, their formation is disturbed in all parts of the antennal lobe. If deafferentation is performed after stage 20, glomeruli persist in dorsal regions, but are missing in ventral regions. On day 24 or later, glomeruli in both dorsal and ventral regions are unaffected by deafferentation. Glial cells continue to extend fine processes into the neuropil in the absence of ingrowing receptor axons. The number of glial cells is reduced compared to control lobes. Multiglomerular local interneurons and other gamma-amino butyric acid-immunoreactive neurons, as well as projection neurons, fail to develop glomerular arborization patterns in antennal lobes deprived of sensory axons.

Multiple factors shape development of olfactory glomeruli: insights from an insect model system

The antennal system of the moth Manduca sexta is a useful model for studies of the development of olfactory glomeruli, the complex synaptic structures that typically underlie the initial processing of olfactory input in vertebrates and invertebrates. In this review, we summarize cellular events in the construction of glomeruli in Manduca and highlight experiments that reveal factors that influence glomerulus development. By methodically manipulating each of various cell types, both neuronal and glial, that contribute to glomerular architecture, we have found that: olfactory receptor axons lay a template for developing glomeruli, stabilization of the template by glial cells is necessary to permit subsequent steps in development of the glomeruli, and the hormone that regulates adult development causes production of adequate numbers of glial cells. Neither electrical activity nor the presence of a serotonin-containing neuron that persists throughout development is required for a glomerular pattern to develop; these factors might, however, influence the synaptic organization of individual glomeruli.

Development of olfactory glomeruli: temporal and spatial interactions between olfactory receptor axons and mitral cells in opossums and rats

Mitral cells are the primary output neurons of the vertebrate olfactory bulb and are major recipients of sensory input from the periphery. The morphogenesis of mitral cell dendrites was followed to elucidate their early spatial and temporal interactions with olfactory receptor neurons and glia during the construction of olfactory glomeruli. Monodelphis domestica, a marsupial born at an extremely immature stage, and rats were examined. Mitral cells were retrogradely labeled by application of the lipophilic dye 1,1' dihexadecyl-3,3,3'3'-tetramethylin-docarbocyanine perchlorate (DiI) to the lateral olfactory tract. In double-labeling experiments, olfactory receptor neurons were stained with 3,3' dihexadecyloxacarbocyanine perchlorate (DiO), or olfactory nerve Schwann cells were visualized using S-100 protein immunohistochemistry. Tissue was examined with a confocal laser scanning microscope. Some preparations were subsequently investigated with an electron microscope. In Monodelphis, differentiation of mitral cells starts with an outgrowth of numerous, uniform, and widespread dendrites. As soon as terminals of olfactory receptor axons coalesce into glomerular knots within the presumptive glomerular layer, dendrites of individual mitral cells innervate several adjacent glomeruli where they receive sensory synaptic input. With maturation, supernumerary mitral cell dendrites retract, leaving one primary dendrite bearing a terminal glomerular tuft. Simultaneously, secondary dendrites begin to arise. The formation of glomeruli begins earlier and progresses faster in the rat compared to Monodelphis. Nevertheless, mitral cell differentiation in both species follows a common sequence: overproduction of dendrites, selection of usually one primary apical dendrite, and elimination of supernumerary processes. Since olfactory receptor neurons form synaptic contacts with the widespread mitral cell dendrites, considerable synaptic rearrangement must occur within the olfactory glomeruli during maturation.

Glomerular organization in developing olfactory and accessory lobes of American lobsters: stabilization of numbers and increase in size after metamorphosis

Olfactory glomeruli are columnar and radially arranged at the periphery of the primary chemosensory areas, the olfactory lobes (OLs), in the American lobster Homarus americanus. The number of olfactory glomeruli reaches nearly 100/lobe in midembryonic life, increases rapidly during larval life, and stabilizes at about 200 in juvenile and adult lobsters. The accessory lobes (ALs), higher order integration areas, are composed of cortical columns and of spherical glomeruli. Two populations of spherical glomeruli are defined, the cortical glomeruli located at the bases of the columns, and the medullary glomeruli scattered throughout the ALs. Both cortical columns and spherical glomeruli are seen for the first time in the second larval stage. There are about 1000 cortical columns and 1700 glomeruli/AL in the postlarva and these numbers remain constant during the life of the lobster. In both OLs and ALs, it is the size of the interglomerular spaces and of the glomeruli themselves that increases. Therefore, the data suggest that in both OLs and ALs the glomeruli were already generated when the lobster metamorphoses (stage III to IV) and switches from a planktonic to a benthic existence, and that the new sensory neurons that are formed at each molt in the antennulae grow into existing olfactory glomeruli. Stability of the glomerular population in the primary olfactory centers, once the full complement of glomeruli is acquired, has also been reported in insects, fish, and mammals.

Development of an identified serotonergic neuron in the antennal lobe of the moth and effects of reduction in serotonin during construction of olfactory glomeruli

Each olfactory (antennal) lobe of the moth Manduca sexta contains a single serotonin (5-HT) immunoreactive neuron whose processes form tufted arbors in the olfactory glomeruli. To extend our present understanding of the intercellular interactions involved in glomerulus development to the level of an individual, identified antennal lobe neuron, we first studied the morphological development of the 5-HT neuron in the presence and absence of receptor axons. Development of the neuron's glomerular tufts depends, as it does in the case of other multiglomerular neurons, on the presence of receptor axons. Processes of the 5-HT neuron are excluded from the region in which the initial steps of glomerulus construction occur and thus cannot provide a physical scaffolding on which the array of glomeruli is organized. Because the neuron's processes are present in the antennal lobe neuropil throughout postembryonic development, 5-HT could provide signals that influence the pattern of development in the lobe. By surgically producing 5-HT-depleted antennal lobes, we also tested the importance of 5-HT in the construction of olfactory glomeruli. Even in the apparent absence of 5-HT, the glomerular array initiated by the receptor axons was histologically normal, glial cells migrated to form glomerular borders, and receptor axons formed terminal branches in their normal region within each glomerulus. In some cases, 5-HT-immunoreactive processes from abnormal sources entered the lobe and formed the tufted intraglomerular branches typical of most antennal lobe neurons, suggesting that local cues strongly influence the branching patterns of developing antennal lobe neurons.

Migrating luteinizing hormone-releasing hormone (LHRH) neurons and processes are associated with a substrate that expresses S100

Luteinizing hormone-releasing hormone (LHRH) containing neurons arise in the region of the medial olfactory placode and migrate into the developing olfactory bulbs and basal forebrain along branches of the terminal and vomeronasal nerves. The neurons ultimately come to reside in olfactory and septo-preoptic areas and project extensively to several brain regions, including the preoptic area and median eminence. The present study examined the expression of a glial-associated guidance molecule, S100, as a possible substrate for this migration. Monodelphis domestica (the Brazilian grey, short-tailed opossum) was studied since this species gives birth to very immature, young, allowing access to early periods of mammalian forebrain development. Immunoreactivity for both S100 and LHRH-containing neurons and fibers were observed to be closely associated along the entire LHRH migratory route from the vomeronasal organ to the septo-preoptic areas as early as the day of birth (PO). By P10, S100-immunoreactivity was also seen in areas containing LHRH-immunoreactive fibers such as the preoptic area and median eminence. We suggest that S100, a protein with neurotrophic properties in vitro, acts as a guidance molecule for migrating LHRH-immunoreactive neurons and elongating processes.

Distribution, classification, and development ofDrosophila glial cells in the late embryonic and early larval ventral nerve cord

To facilitate the investigation of glial development inDrosophila, we present a detailed description of theDrosophila glial cells in the ventral nerve cord. A GAL4 enhancer-trap screen for glial-specific expression was performed. Using UAS-lacZ and UAS-kinesin-lacZ as reporter constructs, we describe the distribution and morphology of the identified glial cells in the fully differentiated ventral nerve cord of first-instar larvae just after hatching. The three-dimensional structure of the glial network was reconstructed using a computer. Using the strains with consistent GAL4 expression during late embryogenesis, we traced back the development of the identified cells to provide a glial map at embryonic stage 16. We identify typically 60 (54-64) glial cells per abdominal neuromere both in embryos and early larvae. They are divided into six subtypes under three categories: surface-associated glia (16-18 subperineurial glial cells and 6-8 channel glial cells), cortex-associated glia (6-8 cell body glial cells), and neuropile-associated glia (8-10 nerve root glial cells, 14-16 interface glial cells, and 3-4 midline glial cells). The proposed glial classification system is discussed in comparison with previous insect glial classifications.

Boundaries and inhibitory molecules in developing neural tissues

Numerous studies of the past decade have illuminated the importance of intercellular adhesion events for neural pattern formation. It has been documented that members of the Ig and cadherin gene superfamilies, that glycoproteins and, probably to some extent, proteoglycans of the extracellular matrix play a role in this context. Recent observations suggest that, in addition to adhesive interactions, repulsive and/or inhibitory phenoma are also of importance in regulating neural pattern formation. Several molecules are under study which are considered possible mediators of inhibitory interactions in the nervous system. The hypothesis has been advanced that some of these might be partially responsible for restrictive, boundary-like properties ascribed to glial cells in developing and regenerating tissues. The current review summarizes these studies and focusses on molecular aspects of boundary and compartmentation phenomena.

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.

Uniglomerular projection neurons participate in early development of olfactory glomeruli in the moth Manduca sexta

Glomerular organization of the antennal (olfactory) lobe is initiated by the arrival of sensory axons from the antenna. Bundles of axon terminals coalesce into spheroidal knots of neuropil called protoglomeruli. Previous studies have suggested that the protoglomeruli form a template for the mature glomerular array, but an early role for projection neurons in establishing the template has not been excluded. We examined with the confocal laser scanning microscope the morphological development of the uniglomerular projection neurons during the stages in which glomeruli are constructed. Groups of projection neurons were stained with the lipophilic dye DiI to assess the development of the population as a whole; individual neurons were filled intracellularly with Lucifer Yellow to examine in detail the development of shape. In some preparations, sensory axons and glial cells also were labeled by using different fluorescent dyes to reveal possible interactions between projection neuron dendrites and sensory axons or glial cells. Protoglomeruli form in a wave beginning at the entry point of the antennal nerve and proceeding across the lobe to the opposite pole. A second wave follows in which projection neurons become tufted and innervate the newly formed glomeruli, sometimes extending into the glial border surrounding the protoglomeruli. In animals deprived of sensory axons, some projection neurons still form tufted dendritic trees and, in one region of the neuropil, a glomerulus-like structure. The early presence of projection neuron processes in the protoglomeruli and the formation of at least one glomerulus-like structure in unafferented lobes suggest that uniglomerular projection neurons play an active role in the construction of olfactory glomeruli.

In vitro analyses of neurite outgrowth indicate a potential role for tenascin-like molecules in the development of insect olfactory glomeruli

Tenascin-like material is associated with glial cells that form borders around developing glomerular units in the olfactory (antennal) lobe of the moth Manduca sexta and is present at critical stages of glomerulus formation (Krull et al., 1994, J. Neurobiol. 25:515-534). Tenascin-like immunoreactivity declines in the mature lobe, coincident with a wave of synapse formation within the glomeruli and glomerulus stabilization. Tenascin-like molecules associated with neuropilar glia are in the correct position to influence the branching patterns of growing neurites by constraining them to glomeruli. In this study, we examine the growth of cultured moth antennal-lobe neurons in response to mouse CNS tenascin. Uniform tenascin provides a poor substrate for cell-body attachment and neurite outgrowth. Neuronal cell bodies provided with a striped substratum consisting of tenascin and concanavalin-A (con-A)/laminin attach preferentially to con-A/laminin lanes. Most neurons restrict their branching to con-A/laminin lanes both at early and later times in culture but others send processes across multiple tenascin and con-/laminin lanes in an apparently indiscriminate manner. Tenascin can inhibit the neuritic outgrowth of most antennal-lobe neurons, and this raises the possibility that the tenascin-like molecules associated with neuropilar glia in vivo act to constrain growing neurites to glomeruli. Thus, glial cells, acting in concert with olfactory axons, might act to promote glomerular patterns of branching by antennal-lobe neurons.

Spatiotemporal pattern of expression of tenascin-like molecules in a developing insect olfactory system

During the development of the olfactory (antennal) lobe of the moth Manduca sexta, olfactory sensory axons induce glomerular branching patterns in their target neurons. Glial cells, by surrounding the developing glomerular template, are thought to mediate the developmental influence of olfactory axons on these branching patterns. Previous studies have demonstrated that, in the absence of glia, neurons in the antennal lobe branch in an aglomerular fashion, even in the presence of competent antennal axons (Oland and Tolbert, 1988, J. Comp. Neurol. 278:377-387; Oland et al., 1988, J. Neurosci. 8:353-367). We have begun to explore the molecular basis by which glial cells could influence patterns of neurite branching. For this work, we have utilized immunocytochemical techniques and a partial biochemical analysis to demonstrate that molecules antigenically similar and comparable in size to mammalian tenascin are localized on the neuropil-associated glial cells that form borders around glomeruli in the developing antennal lobe. These tenascin-like molecules associated with neuropilar glia are present at critical stages of glomerulus development; tenascin-like immunoreactivity declines after glomeruli form and become stabilized. Neither the arrival nor the absence of antennal axons in the lobe induces changes in either the molecular forms or the amounts of tenascin-like molecules. The spatiotemporal pattern of expression of tenascin-like molecules suggests that they are in a position to participate in the formation of a glomerular neuropil and could form a molecular barrier that constrains neurite outgrowth strictly to glomeruli.

Mesenchyme cells in Fasciola hepatica (platyhelminthes): primitive glia?

Mesenchyme cells and their processes are found in the cerebral ganglia of the parasitic flatworm, Fasciola hepatica. The mesenchyme cell processes are found in two specialized associations within the ganglion: (i) as lamellae-like multilayer sheaths encircling the cerebral ganglia and separating it from the surrounding parenchyma cells, and (ii) invaginated into the surface of large diameter ('giant') nerve processes to form trophospongium-like relationships. Based on morphological criteria, these mesenchyme cells resemble general invertebrate glial cells suggesting that the mesenchyme cells of these flatworms may represent the earliest glial-like cell.

Postembryonic development of gamma-aminobutyric acid-like immunoreactivity in the brain of the sphinx moth Manduca sexta

We have investigated the distribution of immunocytochemical staining for the neurotransmitter gamma-aminobutyric acid (GABA) in the brain of the sphinx moth Manduca sexta during larval, pupal, and adult development. In the larval brain, about 300 neurons are GABA-immunoreactive. All neuropil areas except the mushroom bodies and central complex show intense immunostaining. Only minor changes in the pattern of immunoreactivity occur during larval development. During metamorphosis, changes in immunostaining occur in two phases. Beginning in wandering fifth-instar larvae (stage W2), immunoreactivity appears in numerous neurons of the central body and optic lobe and becomes more intense during early pupal stages. At the same time, GABA-like immunoreactivity disappears in most neuropil areas of the brain and becomes faint in many immunoreactive somata. Neurons with arborizations in the ventrolateral protocerebrum, however, continue to exhibit intense immunostaining during this period, and strongly immunolabeled fibers connect these areas with the ventral nerve cord. The second phase of transformation begins around pupal stage P5/P6, when faint immunostaining appears in many previously nonimmunoreactive somata and most neuropil areas of the brain. In subsequent stages (P8-P10), this immunoreactivity disappears again in most somata, but in certain cell groups, it becomes more intense and gradually develops to the adult pattern. Most larval GABA-immunoreactive neurons appear to survive through metamorphosis into the adult. Neurons in the midbrain that acquire GABA-like immunoreactivity during metamorphosis usually lie adjacent to larval immunostained neurons, suggesting common lineages. The onsets of the two developmental phases of GABA-like immunoreactivity correlate with sharp rises in hemolymph titers of ecdysteroid hormones, suggesting a role for ecdysteroids in the regulation of GABA synthesis. We hypothesize that the disappearance of GABA in many areas of the brain starting 2 days prior to pupation dramatically alters its functional circuitry and thus may account for profound changes in the behavior of the animal.

Effects of the steroid hormone 20-hydroxyecdysone and prior sensory input on the survival and growth of moth central olfactory neurons in vitro

Neurons in the developing (antennal) olfactory lobe of the moth Manduca sexta undergo a period of extensive process outgrowth and branching that coincides temporally with both a rising titer of the steroid hormone 20-hydroxyecdysone and the ingrowth of sensory axons from receptors in the antenna. To evaluate the contribution of these two influences to the morphological development of antennal-lobe neurons, we placed the neurons in cell culture. Antennal-lobe neurons were dissociated from normal and chronically unafferented lobes at different stages of development and were exposed to different doses of hormone. Six neuronal cell types with distinctive and stable morphologies appeared in cultures from all stages of pupal development. Morphological changes in these neuronal types were examined quantitatively by comparison of the total length and number of branches. We found that 20-hydroxyecdysone had little direct effect on the morphological development of antennal-lobe neurons, but brief exposure to sensory axons in vivo prior to dissociation significantly enhanced subsequent outgrowth in culture.

Perturbed glial scaffold formation precedes axon tract malformation in Drosophila mutants

The longitudinal glia (LG), progeny of a single glioblast, form a scaffold that presages the formation of longitudinal tracts in the ventral nerve cord (VNC) of the Drosophila embryo. The LG are used as a substrate during the extension of the first axons of the longitudinal tract. I have examined the differentiation of the LG in six mutations in which the longitudinal tracts were absent, displaced, or interrupted to determine whether the axon tract malformations may be attributable to disruptions in the LG scaffold. Embryos mutant for the gene prospero had no longitudinal tracts, and glial differentiation remained arrested at a preaxonogenic state. Two mutants of the Polycomb group also lacked longitudinal tracts; here the glia failed to form an oriented scaffold, but cytological differentiation of the LG was unperturbed. The longitudinal tracts in embryos mutant for slit fused at the VNC midline and scaffold formation was normal, except that it was medially displaced. Longitudinal tracts had intersegmental interruptions in embryos mutant for hindsight and midline. In hindsight, there were intersegmental gaps in the glial scaffold. In midline, the glial scaffold retracted after initial extension. LG morphogenesis during axonogenesis was abnormal in midline. Commitment to glial identity and glial differentiation also occurred before scaffold formation. In all mutants examined, the early distribution of the glycoprotein neuroglian was perturbed. This was indicative of early alterations in VNC pattern present before LG scaffold formation began. Therefore, some changes in scaffold formation may have reflected changes in the placement and differentiation of other cells of the VNC. In all mutants, alterations in scaffold formation preceded longitudinal axon tract formation.

Defective glia in the Drosophila brain degeneration mutant drop-dead

To understand better the cellular basis of late-onset neuronal degeneration, we have examined the brain of the drop-dead mutant of Drosophila. This mutant carries an X-chromosomal recessive mutation that causes severe behavioral defects and brain degeneration, manifested a few days after emergence of the adult. Analysis of genetically mosaic flies has indicated that the focus of the drop-dead mutant phenotype is in the brain and that the gene product is non-cell autonomous. We examined the adult drop-dead mutant brain prior to onset of symptoms and found that many glial cells have stunted processes, whereas neuronal morphology is essentially normal. Adult mutant glial cells resemble immature glia found at an earlier stage of normal brain development. These observations suggest that defective glia in the drop-dead brain may disrupt adult nervous system function, contributing to progressive brain degeneration and death. The normal drop-dead gene product may prevent brain degeneration by providing a necessary glial function.

Synaptic organization and development of the antennal lobe in insects

Many insects possess a highly developed sense of smell. This paper summarizes the cellular and synaptic organization of the antennal (olfactory) lobe of the insect brain and then reviews morphological and fine-structural aspects of the development of the lobe. Visualization of synapses between classes of neurons identified by physiological, morphological, or transmitter-cytochemical properties has provided insights into arrangements of contacts and their possible roles in information processing. Studies of development have revealed the requirement for afferent axons from the antenna for the formation of olfactory glomeruli, where virtually all of the synapses in the lobe occur, and have suggested the possibility that glial cells play a role in the instructive influence of the axons on their target neurons in the lobe. The findings reviewed in this paper are primarily from one representative hemimetabolous insect, the American cockroach, and one representative holometabolous insect, a hawkmoth, and comparisons are made with vertebrate systems when appropriate.

Immunocytochemical demonstration of early appearing astroglial structures that form boundaries and pathways along axon tracts in the fetal brain

During normal development of the mammalian forebrain, the paired cerebral hemispheres are initially separated midsagittally by the connective tissue-filled longitudinal fissure. During subsequent stages, the hemispheres fuse as basal lamina is remodeled and fibroblasts are eliminated from the fissure to create new central nervous system (CNS) territory in the midline. Two axon pathways, the corpus callosum and dorsal callosal stria, eventually use this region as part of their pathway. In order to assess the possible role of glial cells in the fusion process and in the guidance of axons in this and several other areas of the forebrain, we have analyzed the developing brain in timed cat and mouse embryos with immunohistochemical and morphological techniques. With the use of astroglial-specific antibodies and electron microscopy, we have visualized two distinct, primitive astroglial structures associated with the cerebral midline, and seven more associated with other specific brain regions. The way in which one of these structures moves as a column along the hemispheric midline in synchrony with seam formation suggests the possibility that during morphogenesis of the telencephalon, astrocytes may aid in the fusion process. In addition, the compact assemblage, early appearance and location of this and the other glial structures in relation to well defined neuroanatomical landmarks or axon pathways suggest that they may transiently compartmentalize relatively large regions of the CNS and organize certain developing fiber systems by acting as guides or barriers at critical stages of ontogeny.

Age-related expression patterns of the CD15 epitope in the human lateral geniculate nucleus (LGN)

The age-related distribution of the trisaccharide epitope 3-fucosyl-N-acetyl-lactosamine (CD15) was evaluated in the human lateral geniculate nucleus (LGN). Coronal paraffin sections from individuals between the 12th week of gestation to 99 years of age were processed for immunohistochemistry using monoclonal antibodies against the CD15 epitope. CD15 immunoreactivity was present in the neuropil from the 14th week of gestation with a graded pattern along the anteroposterior and mediolateral axes of the LGN. Immunoreactivity then became preferentially located within the future cell layers, shortly before cellular segregation was visible in Cresyl Violet stained sections. Maximal CD15 expression occurred from the 22nd week of gestation until the beginning of visual experience (second week of postnatal life). During the subsequent period the spatial pattern of CD15 expression changed. Whereas immunoreactivity in the cell layers gradually disappeared, CD15 positive astrocytes became transiently concentrated in the intercellular layers. The staining within the interlaminar region was best developed at about one year of postnatal life. The adult pattern was found at around 10 years of age, when the LGN appeared almost unstained. Two stages of CD15 expression can thus be separated. The first is characterized by neuropil staining and is synchronized with the time profile of neuronal maturation and of formation of non-stabilized contacts. CD15 is at this time possibly correlated with structural instability and increased vulnerability but at the same time with a high degree of plasticity. The second, peri- and postnatal stage is characterized by CD15 positive astrocytes. These appeared when CD15 in the neuropil disappeared. This loss of CD15 expression in the neuropil occurs during the phase of experience-dependent establishment of the mature interconnectivity and probably heralds loss in plasticity. The time-related expression pattern of CD15 is therefore compatible with the idea that CD15 levels reflect different degrees of developmental determination of retino-geniculate interaction.

Generation and early differentiation of glial cells in the first optic ganglion of Drosophila melanogaster

We have examined the generation and development of glial cells in the first optic ganglion, the lamina, of Drosophila melanogaster. Previous work has shown that the growth of retinal axons into the developing optic lobes induces the terminal cell divisions that generate the lamina monopolar neurons. We investigated whether photoreceptor ingrowth also influences the development of lamina glial cells, using P element enhancer trap lines, genetic mosaics and birthdating analysis. Enhancer trap lines that mark the differentiating lamina glial cells were found to require retinal innervation for expression. In mutants with only a few photoreceptors, only the few glial cells near ingrowing axons expressed the marker. Genetic mosaic analysis indicates that the lamina neurons and glial cells are readily separable, suggesting that these are derived from distinct lineages. Additionally, BrdU pulse-chase experiments showed that the cell divisions that produce lamina glia, unlike those producing lamina neurons, are not spatially or temporally correlated with the retinal axon ingrowth. Finally, in mutants lacking photoreceptors, cell divisions in the glial lineage appeared normal. We conclude that the lamina glial cells derive from a lineage that is distinct from that of the L-neurons, that glia are generated independently of photoreceptor input, and that completion of the terminal glial differentiation program depends, directly or indirectly, on an inductive signal from photoreceptor axons.

Formation of an olfactory glomerulus: morphological aspects of development and organization

We have studied the development of olfactory nerves in the rat from their first contact with the telencephalic vesicle until the formation of glomerular structures in the olfactory bulb at early postnatal period. The study is based on serial semithin and ultrathin sections of material prepared for electron microscopy and antibodies to label radial glial cells, glial fibrillary acidic protein and Rat-401. Beginning on embryonic day 12, developing olfactory axons from the olfactory placode are accompanied by migratory cells, also derived from the olfactory placode, that reach the prospective olfactory bulb by embryonic day 13. The mass of migratory cells accumulate superficial to the telencephalic vesicle. The cells increase in number by mitotic divisions. The majority of these cells represent precursor elements that will later develop into the ensheathing cells of the olfactory nerves and olfactory nerve layer of the adult. Some migratory cells penetrate into the prospective olfactory bulb early during development. The first synaptic contacts of olfactory axons with dendritic processes in the olfactory bulb were observed at embryonic day 18. Glomerular formation is initiated by penetration of cells from the migratory mass into the prospective glomerular layer by embryonic day 20 to postnatal day 0. These cells form walls surrounding zones of high synaptic density forming protoglomeruli. Postnatally, the peripheral processes of radial glial cells branch profusely delimiting glomerular formations and transform into periglomerular astrocytes. Rat-401 stains radial glial cells from embryonic day 14. Immunoreactivity becomes restricted to the olfactory glomeruli during the first postnatal weeks and it virtually disappears by the end of the first postnatal month. We conclude that the early penetration of cells from the migratory mass into the prospective olfactory bulb, observed immediately after the first synaptic contacts were established, initiates the formation of olfactory glomeruli which becomes completed by the transformation of radial glial cells into periglomerular astrocytes.

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.

Increase in a focal population of juxtaglomerular cells in the olfactory bulb associated with early learning

Young rats learn to approach an odor that had been experienced in the presence of reinforcing tactile stimulation. Subsequent presentation of the conditioned odor also evokes an enhanced focal uptake of 2-deoxyglucose (2-DG) in the glomerular layer of the olfactory bulb, and the glomerular-layer width of such foci increases in conditioned pups. In the present study, we determined whether an increase in the glomerular-layer cell population contributes to this structural and functional change. We therefore counted and measured glomerular-layer cells in Nissl-stained sections from focal regions of radiolabeled 2-DG uptake. While cell size did not differ between groups, conditioned pups had a 19% increase in the number of glomerular-layer cells associated with the 2-DG foci compared to controls. The increase in cell number may contribute to the enhanced 2-DG uptake in glomerular-layer foci.