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Structure of the olfactory bulb in tadpoles of Xenopus laevis
No full textThe structure of the olfactory bulb in tadpoles of Xenopus laevis (stages 54-56) was studied using axon tracing (with biocytin or low-weight dextran) and immunocytochemical techniques. Filling the olfactory nerve with biocytin made the nerve layer and the glomeruli visible. Dye injections into the glomerular layer labeled the lateral olfactory tract. Vice versa, dye injections into the lateral olfactory tract made mitral cells and their glomerular branching patterns visible. Anti-GABA anti serum stained periglomerular and granule cells, while the olfactory nerve and mitral cells were labeled by antiglutamate antiserum. We describe the layering, the numbers of cells and glomeruli, and their localization in both the main and the accessory olfactory bulb
Tornaria of hemichordates and other dipleurula-type larvae: a comparison
No full textThe evolutionary origin of phylum Chordata is the subject of intensive discussion, with the most conflicting views prevalent. One popular theory advocates the separation of chordates from a dipleurula-like ancestor. Thus the dipleurula-type larvae (tornaria of enteropneusts, auricularia and bipinnaria of echinoderms) are considered to recapitulate the ancestral features and the direct evolutionary path from Echinodermata and Hemichordata to Chordata (i.e. Garstang 1894 Zool. Anzeiger 27, 122-125; Grobben 1908 Verb. Zool.-Bot. Ges. Wien 58, 491-511; Dillon 1965 Evolution 19, 436-446; Jollie 1973 Acta Zool. (Stockholm) 54, 81-100; Ivanova-Kazas and Ivanov 1987 Sov. J. Mar. Biol. 13, 67-80; Crowther and Whittaker 1992 J. Neurophysiol. 23, 280-292; Lacalli 1994 Am. Zool. 34, 533-541; Lacalli et al. 1999 Proc. R. Sec. Biol. Series B 266, 1461-1470; Nielsen 1999 Dev. Genes Evol. 209, 198-205). Comparison of the nervous system in enteropneust tornariae and echinoderm larvae has revealed however, striking differences in distribution of biogenic amines and cholinesterase activity. In tornariae, monoamine-containing cells concentrate in the aboral and oesophageal ganglia. In echinoderms, they are located along the ciliary bands throughout their length. The difference in distribution of cholinesterase activity in each group reasonably suggests that acetylcholine-dependent control of locomotion also differs. Our data do not support the homology of the dipleurula-type larvae. Therefore we believe in the course of adaptive evolution, larvae of certain marine invertebrates acquired a set of common morphological and behavioural characteristics, yet retained different physiological mechanisms of behavioural regulation. Thus, similarities in the dipleurula-type larvae (tornaria, auricularia or bipinnaria, and actinotrocha) may have originated from convergence rather then from a common dipreurula-type predecessor. In consequence we must call into question any attempt to trace the ancestors of Chordata to the dipleurula-type animal
Individual olfactory sensory neurons project into more than one glomerulus in Xenopus laevis tadpole olfactory bulb
No full textAn essential step in the coding of odorants is the way olfactory sensory neurons (OSNs) convey their information to the olfactory bulb. This projection determines how the specificities of OSNs are mapped onto the spatial activity patterns of the olfactory bulb (OB). Despite the fact that virtually nothing is known about how individual OSN axons project to glomeruli, it is generally believed that OSNs always project to one glomerulus each. Our recent findings in tadpoles of Xenopus laevis challenge this view. By injection of a tracer into individual OSNs, we show for the first time that axons typically project into more than one glomerulus and that they do so in a characteristic way. Upon entering the olfactory bulb, an axon bifurcates to give two primary branches. Each of these branches bifurcates again to give two subbranches, thus resulting in four subbranches per OSN. The two subbranches of each primary branch project into two different glomeruli. Variations of this characteristic innervation pattern include the innervation of three and, occasionally, one glomerulus. In any case, and independent of the number of glomeruli innervated by an OSN, each glomerulus receives at least two axonal branches of the same OSN. (C) 2004 Wiley-Liss, Inc
Structure of the nervous system in the tornaria larva of Balanoglossus proterogonius (Hemichordata : Enteropneusta) and its phylogenetic implications
No full textThe tornaria larva of hemichordates occupies a central position in phylogenetic discussions on the relationships between Echinodermata, Hemichordata, and Chordata. Dipleurula-type larvae (tornaria and echinoderm larvae) are considered to be primary in the life cycle and thus provide a model for the ancestral animal common to all three taxa (the theory of W. Garstang). If the similarities between tornaria and the larvae in Echinodermata result from homology, their nervous systems should be basically similar as well. The present study utilizes anti-serotonin and FMRFamide antisera together with laser scanning microscopy, and transmission electron microscopy, to describe in detail the nervous system of the tornaria of Balanoglossus proterogonius. Serotonin immunoreactive neurons were found in the apical and esophageal ganglia, and in the stomach epithelium. FMRFamide immunoreactive neurons, probably sensory in nature, were detected in the apical ganglion and in the equatorial region of the stomach epithelium. At the ultrastructural level, the apical organ consists of a columnar epithelium of monociliated cells and includes a pair of symmetrical eyespots. The apical ganglion is located at its base and has a well-developed neuropil. Different types of neurons are described in the apical organ, esophagus, and stomach. Comparison with larvae in Echinodermata shows several significant differences in the way the larval nervous system is organized. This calls into question the homology between tornariae and echinoderm larvae. The possibility of convergence between the two larval types is discussed
Novel, posterior sensory organ in the trochophore larva of Phyllodoce maculata (Polychaeta)
No full textA new posterior sensory organ (PSO), located at the dorsal midline of the hyposphere, is described by immunocytochemical detection of acetylated atubulin and serotonin (5-HT) in a laser-scanning microscope, as well as three-dimensional reconstructions after optical serial sectioning in the trochophore larva of the polychaete Phyllodoce maculata (Phyllodocidae). The impaired PSO consists of five bipolar sensory cells, two of them being 5-HT immunopositive, which send axons to the cerebral ganglion and prototroch nerve. The dendrites of these cells project to the surface and bear one cilium each. A single neuronal fibre from the apical sensory organ innervates the PSO
Slice culture of the olfactory bulb of Xenopus laevis tadpoles.
No full textWe report on the development of a slice culture of amphibian brain tissue. In particular, we cultured slices from Xenopus laevis tadpoles that contain the olfactory mucosae, the olfactory nerves, the olfactory bulb and the telencephalon. During 6 days in roller tubes the slices flattened, starting from 250 microm and decreasing to approximately 40 microm, corresponding to about three cell layers. Dendritic processes could be followed over distances as long as 200 microm. Neurons in the cultured slice could be recorded using the patch clamp technique and simultaneously imaged using an inverted laser scanning microscope. We characterized the main neuron types of the olfactory bulb, i.e. mitral cells and granule cells, by correlating their typical morphological features in the acute slice with the electrophysiological properties in both the acute slice and slice culture. This correlation allowed unambiguous identification of mitral cells and granule cells in the slice culture
Noradrenergic modulation of calcium currents and synaptic transmission in the olfactory bulb of Xenopus laevis tadpoles.
No full textNorepinephrine (NE) has various modulatory roles in both the peripheral and the central nervous systems. Here we investigate the function of the locus coeruleus efferent fibres in the olfactory bulb of Xenopus laevis tadpoles. In order to distinguish unambiguously between mitral cells and granule cells of the main olfactory bulb and the accessory olfactory bulb, we used a slice preparation. The two neuron types were distinguished on the basis of their location in the slice, their typical branching pattern and by electrophysiological criteria. At NE concentrations lower than 5 microM there was only one effect of NE upon voltage-gated conductances; NE blocked a high-voltage-activated Ca(2+)-current in mitral cells of both the main and the accessory olfactory bulbs. No such effect was observed in granule cells. The effect of NE upon mitral cell Ca(2+)-currents was mimicked by the alpha(2)-receptor agonists clonidine and alpha-methyl-NE. As a second effect, NE or clonidine blocked spontaneous synaptic activity in granule cells of both the main and the accessory olfactory bulbs. NE or clonidine also blocked the spontaneous synaptic activity in mitral cells of either olfactory bulb. The amplitude of glutamate-induced currents in granule cells was modulated neither by clonidine nor by alpha-methyl-NE. Taken together, the main effect of the noradrenergic, presynaptic, alpha(2)-receptor-mediated block of Ca(2)+-currents in mitral cells appeared to be a wide-spread disinhibition of mitral cells in the accessory olfactory bulb as well as in the main olfactory bulb
Slice culture of the olfactory bulb of Xenopus laevis tadpoles.
No full textWe report on the development of a slice culture of amphibian brain tissue. In particular, we cultured slices from Xenopus laevis tadpoles that contain the olfactory mucosae, the olfactory nerves, the olfactory bulb and the telencephalon. During 6 days in roller tubes the slices flattened, starting from 250 microm and decreasing to approximately 40 microm, corresponding to about three cell layers. Dendritic processes could be followed over distances as long as 200 microm. Neurons in the cultured slice could be recorded using the patch clamp technique and simultaneously imaged using an inverted laser scanning microscope. We characterized the main neuron types of the olfactory bulb, i.e. mitral cells and granule cells, by correlating their typical morphological features in the acute slice with the electrophysiological properties in both the acute slice and slice culture. This correlation allowed unambiguous identification of mitral cells and granule cells in the slice culture
Neuronal development in larval polychaete Phyllodoce maculata (Phyllodocidae)
No full textThe existing view on neuronal development in polychaetes, as undergoing neurogenesis beginning in the rudiments of central ganglia and then extending peripherally, has been contrasted with the latest findings in molluscs, their sister trochozoan group, which show a peripheral to central mode of neurogenesis. The current study addresses this issue by examining early neuronal development in the polychaete Phyllodoce maculata using immunolabeling against acetylated alpha-tubulin, serotonin, and the FMRFamide. The first nervous cell was detected 20 hours before hatching, at the early trochophore stage. A solitary serotonergic neuron was located at the posterior-dorsal extreme of the larva and issued anterior projecting fibers, which outline the future ventral nerve cords and prototroch nerve. Two more serotonergic dorsal peripheral cells and three peripheral FMRFamidergic cells appeared soon thereafter. The fibers of these early cells formed a scaffolding, which prefigured the future adult nervous system (cerebral ganglion, ventral cords, prototroch and esophageal nerve rings) in prehatched trochophores. Shortly before hatching, the larval nervous system developed, including the apical organ, meridianal nerves in the episphere, and posttrochal nerves that innervate the feeding apparatus. After hatching, the rudiments of the adult nervous system started to develop along the paths already established by the earliest peripheral neurons. Thus, the general strategy of neurogenesis in a representative polychaete trochophore appears to resemble that of molluscs. The first neuronal cells to appear are peripheral in origin, located near the posterior margins of the embryo. Their similar anatomical appearance suggests that they share a similar functional role in trochophore development and behavior
Alpha-2-receptors modulate calcium currents and synaptic transmission in the olfactory bulb of Xenopus laevis
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