Sensory Dendrites Research Articles

The Ontogeny of Olfactory Epithelium: An Ultrastructural Study in Sheep

Kocianova I., M. Buchtova, I. Goro‰ova, S. Holubcova, P. âiIek:The Ontogeny of the Olfactory Epithelium: An Ultrastructural Study in Sheep. Acta Vet. Brno 2003, 72: 3-10. Ultrastructure of the olfactory epithelium was studied in ovine foetuses aged 74 and 79 days, in newborn and 30-day-old lambs, and in a 730-day-old sheep. Differentiation of apical parts of supporting cells, cilia on sensory cell dendrite endings, redistribution of centrioles from dendrite into endings and presence of a large amount of glycogen inclusions in supporting cells were apparent at day 74 of the intrauterine development. Supporting cells represented the major portion of the olfactory epithelium at day 79. The epithelial surface was covered by a dense network of cilia, but distal segments were seen only sporadically. The amount of microtubuli increased in dendrite cytoplasm. At birth, the cytoplasm of olfactory endings contained numerous light vesicles, mitochondria and centrioles. The free surface of supportive cells was furrowed into numerous microvilli. Distal segments of olfactory cilia were observed at the age of 30 days only. The segments contained a reduced number of axial microtubuli and contacted microvilli of supporting cells. The pattern and ultrastructure of olfactory cells of an adult sheep did not differ from those of a 30-day-old lamb. Degenerating olfactory cells were observed only sporadically. Supranuclear cytoplasm of supporting cells contained glycogen inclusions and bundles of tonofibrils. Sense organs, transmission electron microscopy, olfactory epithelium, ontogenesis

Moth pheromone binding proteins contribute to the excitation of olfactory receptor cells.

Pheromone binding proteins (PBPs) occur in high concentrations in the sensillum lymph surrounding the sensory dendrites of moth pheromone-sensitive sensilla. They were shown to transport the lipophilic odorants through the aqueous sensillum lymph to the receptor cells. The sensilla trichodea of the silkmoth Antheraea polyphemus are supplied with three types of receptor cells responding specifically to three pheromone components. The sensillum lymph of these sensilla contains three different types of PBPs. In this study, recombinant PBPs in various combinations with pheromone components were applied to the receptor cells via tip-opened sensilla during electrophysiological recordings. The responses of receptor cells were shown to depend on both the pheromone component and the PBP. Pheromone components artificially bound to particular PBPs elicited nerve impulses in receptor cell types which they do not activate under natural conditions. This is the first electrophysiological study to suggest that the PBPs contribute to the activation of receptor molecules.

Peripheral GABAergic inhibition of spider mechanosensory afferents

Spider mechanosensory neurons receive an extensive network of efferent synapses onto their sensory dendrites, somata and distal axonal regions. The function of these synapses is unknown. Peripheral synapses are also found on crustacean stretch-receptor neurons but not on mechanosensory afferents of other species, although inhibitory GABAergic synapses are a common feature of centrally located axon terminals. Here we investigated the effects of GABA receptor agonists and antagonists on one group of spider mechanosensory neurons, the slit sense organ VS-3, which are accessible to current- and voltage-clamp recordings. Bath application of GABA activated an inward current that depolarized the membrane and increased the membrane conductance leading to impulse inhibition. VS-3 neuron GABA receptors were activated by muscimol and inhibited by picrotoxin but not bicuculline, and their dose-response relationship had an EC(50) of 103.4 microm, features typical for insect ionotropic GABA receptors. Voltage- and current-clamp analysis confirmed that, while the Na(+) channel inhibition resulting from depolarization can lead to impulse inhibition, the increase in membrane conductance (i.e. 'shunting') completely inhibited impulse propagation. This result argues against previous findings from other preparations that GABA-mediated inhibition is caused by a depolarization that inactivates Na(+) conductance, and it supports those findings that assign this role to membrane shunting. Our results show that GABA can rapidly and selectively inhibit specific mechanoreceptors in the periphery. This type of peripheral inhibition may provide spiders with a mechanism for distinguishing between signals from potential prey, predators or mates, and responding with appropriate behaviour to each signal.

Apical sensory organ in larvae of the patellogastropod Tectura scutum.

The apical sensory organ in veliger larvae of a patellogastropod, a basal clade of gastropod molluscs, was studied using ultrastructural and immunohistochemical techniques. Immediately before veligers of Tectura scutum undergo ontogenetic torsion, the apical sensory organ consists of three large cells that generate a very long apical ciliary tuft, two cells that generate a bilateral pair of shorter ciliary tufts, and a neural ganglion (apical ganglion). Putative sensory neurons forming the ganglion give rise to dendrites that extend to the apical surface of the larva and to basal neurites that contribute to a neuropil. The ganglion includes only one ampullary neuron, a distinctive neuronal type found in the apical ganglion of other gastropod veligers. Serotonin immunoreactivity is expressed by a medial and two lateral neurons, all having an apical dendrite, and also by neurites within the neuropil and by peripheral neurites that run beneath the ciliated prototrochal cells that power larval swimming. The three cells generating the long apical ciliary tuft are lost soon after ontogenetic torsion, and the medial serotonergic cell stops expressing serotonin antigenicity in late-stage veligers. The lateral ciliary tuft cells of T. scutum may be homologs of lateral ciliary tuft cells in planktotrophic opisthobranch veligers. A tripartite arrangement of sensory dendrites, as described previously for veligers of other gastropod clades, can be recognized in T. scutum after loss of the apical ciliary tuft cells.

Frequency response functions and information capacities of paired spider mechanoreceptor neurons.

Pseudorandom white-noise stimulation followed by direct spectral estimation was used to obtain linear frequency response and coherence functions from paired, but dynamically different, spider mechanosensory neurons. The dynamic properties of the two neuron types were similar with either mechanical or electrical stimulation, showing that action potential encoding dominates the dynamics. Phase-lag data indicated that action potential initiation occurs more rapidly during mechanical stimulation, probably in the distal sensory dendrites. Total information capacity, calculated from coherence, as well as information per action potential, were both similar in the two types of neurons, and similar to the few available estimates from other spiking neurons. However, information capacity and information per action potential both depended strongly on neuronal firing rate, which has not been reported before.

A conserved mechanism of synaptogyrin localization.

We have studied the localization of synaptogyrin family members in vivo. Both native and green fluorescent protein (GFP)-tagged Caenorhabditis elegans synaptogyrin (SNG-1) are expressed in neurons and synaptically localized. Deletion and mutational analysis with the use of GFP-tagged SNG-1 has defined a 38 amino acid sequence within the C terminus of SNG-1 and a single arginine in the cytoplasmic loop between transmembrane domain 2 and 3 that are required for SNG-1 localization. These domains may represent components of signals that target synaptogyrin for endocytosis from the plasma membrane and direct synaptogyrin to synaptic vesicles, respectively. In chimeric studies, these regions were sufficient to relocalize cellugyrin, a nonneuronal form of synaptogyrin, from nonsynaptic regions such as the sensory dendrites and the cell body to synaptic vesicles. Furthermore, GFP-tagged rat synaptogyrin is synaptically localized in neurons of C. elegans and in cultured hippocampal neurons. Similarly, the C-terminal domain of rat synaptogyrin is necessary for localization in hippocampal neurons. Our study suggests that the mechanisms for synaptogyrin localization are likely to be conserved from C. elegans to vertebrates.

Membrane potential and its electrode-recorded counterpart in an electrical model of an olfactory sensillum.

Insect receptor neurons are surrounded with auxiliary cells and encased in a hair. Their electrical activity is usually recorded with an electrode located at the tip of the hair. Analytical expressions giving the membrane potential along the sensory dendrite and the tip-recorded potential are derived for a neuron in steady-state conditions. They formally close the gap between theoretical models and experimental measurements, when transduction mechanisms and active membrane properties are not taken into account. It is shown that the tip-recorded potential reflects correctly the relative variations of the dendritic membrane potential as a function of stimulus intensity over a large range of parameters. The geometric and electrical characteristics of the sensillum that need be known to compute the dendritic membrane potential from the tip-recorded potential are given.

Evidence that collapsin response mediator protein-2 is involved in the dynamics of microtubules.

Collapsin response mediator protein-2 (CRMP-2) is a member of the CRMP/TOAD/Ulip/DRP family of cytosolic phosphoproteins involved in neuronal differentiation and axonal guidance. CRMP-2 mediates the intracellular response to collapsin 1/semaphorin 3A, a repulsive extracellular guidance cue for axonal outgrowth. The mutation of UNC-33, a Caenorhabditis elegans homolog of CRMP-2, results in abnormality of microtubules in neurites, but the mechanism of CRMP-2 action remains to be clarified. Here, we report that overexpression of human CRMP-2 in Neuro2a cells, a mouse neuroblastoma cell line, results in blebbing of the cytoplasm. Furthermore, some cells exhibited intranuclear inclusions, which were labeled with antibodies to CRMP-2 and tubulin. CRMP-2 was found to be associated with microtubule bundles in the spindles at the metaphase and in the midbodies at the late telophase in mitotic cells. Thus, it is most likely that failure of complete disassembly of the spindle microtubules during mitosis is responsible for the formation of these intranuclear inclusions. We suggest that CRMP-2 functions by regulating the dynamics of microtubules.

Atlas of olfactory organs of Drosophila melanogaster: 1. Types, external organization, innervation and distribution of olfactory sensilla

The third antennal segment (funiculus) and the maxillary palp are the main and accessory olfactory sense organs of Drosophila melanogaster. Cryofixed antennae and palps revealed a superior preservation of the sensory dendrites and other cellular details as compared to conventional chemical fixation. Extensive cross-section series through funiculus and palp were studied in order to obtain as complete an evaluation as possible of the sensillar complement on these appendages. About 75% of all sensilla on the male and female funiculus were individually studied and their position on the antennal surface mapped. Dimensions of the cuticular apparatus of the various types of sensilla are provided as well as the number of innervating receptor neurons. Particular attention has been paid to the cuticular pores important for stimulus transport and to the sensory dendrites. On the funiculus surface, all sensilla have multiple wall pores: sensilla (s.) trichodea and s. basiconica are single-walled, s. coeloconica are double-walled. The distribution of s. trichodea and s. basiconica follows opposing gradients along a diagonal axis parallel to the axis of the arista from proximo-medial to disto-lateral. In this disto-lateral direction the density of s. trichodea increases while that of the s. basiconica decreases. S. trichodea occur in three subtypes with one, two or three receptor neurons. Basiconic sensilla can be subdivided into three subtypes of large s. basiconica (with two or four receptor neurons), three subtypes of thin s. basiconica (with mostly two, rarely four neurons), and one subtype of small s. basiconica with two receptor neurons. Large s. basiconica occur only in the most proximal region (the ‘LB-zone’); thin s. basiconica are most common in a belt that borders the LB-zone distally, while small s. basiconica are most numerous even further distally along the mentioned diagonal axis in between the s. trichodea. S. intermedia are single-walled, multiporous sensilla which combine features of s. trichodea and s. basiconica; they are found in two subtypes with two or three receptor neurons, in the same region where s. trichodea abound. The s. coeloconica are irregularly distributed over the funicular surface, and occur in two subtypes with two or three receptor neurons. Sexual dimorphism on the antenna is moderate, the female funiculus is a bit longer and carries a larger number of small s. basiconica and large s. basiconica of the LB-I subtype; the male funiculus, however, has more s. trichodea than the female. On the maxillary palp, besides mechanoreceptive s. chaetica, there are only s. basiconica with two receptor neurons. According to the fine structure of their sensory dendrites, three subtypes can be discriminated which are distributed in a random pattern. The functional significance of the described structures and distribution awaits future immunocytochemical and electrophysiological experiments.

1PA087 昆虫細胞感覚突起には新しい糖作動性情報変換チャネルが存在する

1PA087 昆虫細胞感覚突起には新しい糖作動性情報変換チャネルが存在する

Adaptation properties of two types of sensory neurons in a spider mechanoreceptor organ.

The VS-3 slit-sense organs of the tropical wandering spider Cupiennius salei contain two types of mechanosensory neurons with similar morphology but different adaptation properties. We measured the changes in membrane potential produced by mechanical stimulation and by electric current stimulation in a large number of neurons of both types. No significant differences were found between the passive membrane properties of the two groups, but there were significant differences in the extent and time course of receptor potential adaptation between the two types of neurons. These data, combined with the responses to suprathreshold electrical stimuli, indicate that adaptational differences exist at several stages in these neurons but that active membrane conductances dominate the overall behavior. The passive membrane measurements also indicate that effective voltage clamp of the receptor current at the tips of the sensory dendrites is possible in these neurons.

Fine structure of the dioptric apparatus in the stalk eye of Onchidium verruculatum (Gastropoda, Stylommatophora): a distinct lamellar substructure of the lens

The dioptric apparatus of the stalk eye in Onchidium verruculatum, including a tentacular epidermis, a cornea, and a lens, was examined using transmission electron microscopy. The tentacular epidermis was formed by columnar epidermal cells, sensory dendrites, and glandular cells. The cornea was an anterior part of the eye vesicle and consisted of corneal cells which contained abundant glycogen particles but no dark pigment granules in their cytoplasm. An acellular, transparent, ellipsoidal lens was located in the center of the eye vesicle. The lens showed a marginal zone, an outer zone, a transitional zone, an inner zone, and a central region arranged concentrically. The outer zone was the most intense electron-dense region and was finely granular in structure. The marginal zone was also finely granular and surrounded the outer zone with many hair-like slender strands on the retinal side. Toward the center of the lens this homogeneous fine granularity gradually changed into globular or rod-like substructures, about 30 nm in diameter, and then abruptly transformed into a lamellar substructure of about 30 nm in thickness. The inner zone contained a mosaic of lamellar substructures which were arranged in a fingerprint pattern that was particularly enhanced with periodic acid methenamine silver proteinate staining. The center itself consisted of deformed lamellar substructures. The concentric arrangement of substructures inside the lens of the O. ver-ruculatum stalk eye is probably responsible for the concentration and/or refraction of light.

Functional morphology of a double-walled multiporous olfactory sensillum: the sensillum coeloconicum of Bombyx mori (Insecta, Lepidoptera)

The fine structure of coeloconic sensilla of Bombyx mori was studied in cryofixed specimens. These sensilla belong to the category of double-walled wall-pore sensilla. The pegs are ∼10 μm long, located in pits on the dorsal side of the antennal branches, and longitudinally grooved in their distal half (grooved surface ∼30 μm 2). The central lumen contains the outer dendritic segments of usually five receptor cells, and is surrounded by up to 15 partially fused cuticular fingers. The peripheral lumina of these cuticular fingers are filled with material resembling wax-canal filaments. Radial spoke channels (∼600 per peg), each 10–20 nm wide, connect the central lumen with the longitudinal groove channels. Groove and spoke channels are assumed to mediate the transport of odorant molecules from the outer epicuticular surface layers to the sensory dendrites. Thus the double-walled wall-pore sensilla represent a bauplan essentially different from single-walled wall-pore sensilla; the reason, however, why the two types are found together throughout the insect orders remains enigmatic. Other peculiar features of the coeloconic sensilla of the silkmoth are invaginations of the outer dendritic segments and direct contacts between the receptor cell somata. The latter may be the structural correlate to electrophysiological observations indicative of peripheral interaction between the receptor neurons. All three auxiliary cells have elaborately folded apical plasma membranes studded with portasomes and associated with an abundance of mitochondria; basally they often contact tracheal branches. As compared to the auxiliary cells of the single-walled olfactory sensilla of the same species, all the mentioned features are much more prominent and hint to a higher ion pumping activity at the border to the sensillum-lymph cavities.

Ripped pocket and pickpocket, novel Drosophila DEG/ENaC subunits expressed in early development and in mechanosensory neurons.

Drosophila melanogaster has proven to be a good model for understanding the physiology of ion channels. We identified two novel Drosophila DEG/ ENaC proteins, Pickpocket (PPK) and Ripped Pocket (RPK). Both appear to be ion channel subunits. Expression of RPK generated multimeric Na+ channels that were dominantly activated by a mutation associated with neurodegeneration. Amiloride and gadolinium, which block mechanosensation in vivo, inhibited RPK channels. Although PPK did not form channels on its own, it associated with and reduced the current generated by a related human brain Na+ channel. RPK transcripts were abundant in early stage embryos, suggesting a role in development. In contrast, PPK was found in sensory dendrites of a subset of peripheral neurons in late stage embryos and early larvae. In insects, such multiple dendritic neurons play key roles in touch sensation and proprioception and their morphology resembles human mechanosensory free nerve endings. These results suggest that PPK may be a channel subunit involved in mechanosensation.

Dendritic integration in olfactory sensory neurons: a steady-state analysis of how the neuron structure and neuron environment influence the coding of odor intensity.

Response properties of the receptor potential at steady state were analyzed in a biophysical model of an olfactory sensory neuron embedded in a multicell environment. The neuron structure was described as a set of several identical dendrites (or cilia) bearing the transduction mechanisms, joined to a nonsensory part--dendritic knob, soma, and axon. The different ionic compositions of the media surrounding the neuron sensory and nonsensory parts and the extraneuronal voltage sources, which both result from the presence of auxiliary cells, were also taken into account. Analytical solutions were found to describe how the receptor potential at the nonsensory part responds to a uniform change in the odorant-dependent conductance resulting from odorant stimulation of the sensory dendrites. We investigated the influence of various geometrical and electrical parameters on the receptor-potential response in the classical model neuron within a homogeneous environment and in the model neuron surrounded with auxiliary cells. First, it was found that the maximum amplitude of the receptor potential is independent of the neuron structure in the absence of auxiliary cells but not in their presence. In the latter case, the amplitude decreases with the length and number of sensory dendrites and with the input resistance of the nonsensory part. Second, the sensitivity (as measured by the increase in membrane conductance at half-maximum response) of the neuron model in the absence of auxiliary cells is higher, but its dynamic range is narrower than in their presence. The dynamic range is wide and the sensitivity low when the input resistance of the nonsensory part is small and the sensory dendrite is unbranched. Both sensitivity and dynamic range are higher for a longer dendrite. These results help understand the morphology of insect olfactory sensilla and can be generalized to other neuron types.

Implication of nmda type glutamate receptors in neural regeneration and neoformation of synapses after excitotoxic injury in the guinea pig cochlea

In the adult mammalian cochlea, the ability of nerve fibres to regenerate has been observed following disruption of the organ of Corti by various means, or transsection of the cochlear nerve in the internal auditory meatus. Based upon the implication of glutamate as a neurotransmitter at synapses between sensory hair cells and terminal dendrites of the auditory nerve in the mammalian cochlea, we have developed, in a previous study, an in vivo model of neural regeneration and formation of synapses after the destruction of the afferent nerve endings by local application of the glutamate agonist α-amino-3-hydroxy-5-methyl-isoxazol-propionic acid (AMPA). In situ hybridization experiments performed during the re-innervation process revealed an overexpression of mRNA coding for NR1 subunit of N-methyl-d-aspartate (NMDA) receptors in the spiral ganglion neurons, suggesting that these receptors are implicated in neural regenerative processes. The present study has been designed to study the functional implication of NMDA receptors in the regrowth and synaptic repair of auditory dendrites in the guinea pig cochlea, by blocking the NMDA receptors during the period of normal functional recovery. In a first set of experiments, we recorded compound action potential after acute perilymphatic perfusion of cumulative doses (0.03–10 mM) of dl 2-amino-5-phosphonovalerate (d-AP5), a NMDA antagonist, to determine the efficiency of the drug. In a second set of experiments, the auditory dendrites were destroyed by local application of the glutamate agonist AMPA. The blockage of NMDA by the antagonist d-AP5 applied with an osmotic micropump delayed the functional recovery and the regrowth of auditory dendrites. The findings of our study support the hypothesis that, in addition to acting as a fast transmitter, glutamate has a neurotrophic role via the activation of NMDA receptors.

Cytoskeletal elements in insect sensilla

Insect sensilla have evolved prominent cytoskeletal elements as part of their functional specialization. The cytoskeleton present in sensory cells as well as in auxiliary cells may play an important role in sensilla function. The scolopale, the characteristic cytoskeletal component in the innermost auxiliary cell of mechanosensitive scolopidia and thermo-/hygrosensitive sensilla, is mainly composed of bundles of 10 nm filaments. Cytochemical approaches for light and electron microscopy identified these structures as actin filaments that exhibited a unique filament orientation and uniform filament polarity. None of these approaches has provided evidence for the presence of myosins in the scolopale. In contrast, tropomyosin and the microtubule-associated protein 2 are associated with the actin filament bundles in the scolopale of scolopidia. All data taken together suggest that the actin filaments of scolopale have a stabilizing rather than a contractile function. In scolopidia, in addition to cellular stabilization, filament elasticity would appear to be important during stimulation. Owing to the high number of microtubules, the scolopale in thermo-/hygrosensitive sensilla seems more rigid than in scolopidia and may protect sensory dendrites from mechanical forces. In sensory cells of scolopidia, regularly cross-striated ciliary rootlets are additional prominent cytoskeletal structures. Immunohistochemistry reveals that these rootlets contain the Ca 2+-binding protein centrin, which forms contractile filaments in other systems, e.g., unicellular green algae. Accordingly, contractions of ciliary rootlets may also be part of the filament function in insect sensilla.

G-protein activation, identification and immunolocalization in pheromone-sensitive sensilla trichodea of moths.

Electrophysiological in situ recordings from pheromone-sensitive sensilla trichodea of Bombyx mori males with a recording pipette which contained G-protein-activating fluoride, showed receptor cell activity similar to that evoked by pheromone stimulation. This suggests that G-proteins might be physiologically active in olfactory sensilla of insects in situ. Biochemical experiments using specific antibodies revealed the presence of G-protein, belonging to the Gq family, in antennal preparations. Similar G-protein was identified in sensory hair preparations of Antheraea pernyi which contained only cuticle, sensillum lymph and dendritic material. Moreover, the absence of this G-protein in pure sensillum lymph preparations indicates its association with the receptive dendrites. This particular association could be shown by immunolabelling studies at the ultrastructural level. Strong specific labelling of membranes of receptor-cell dendrites was found in all types of olfactory sensilla present on the antenna of the silkmoths. Additional specific labelling of apical membranes of auxiliary cells, epidermal cells and membranes forming the axon/glia interface demonstrated that this G-protein is not restricted to the sensory dendrites and that other signal-transduction pathways could be present at these membranes. In summary, the experiments imply a participation of G-protein of the Gq family in signal transduction of olfactory receptor cells in moths.

First developmental signs of the scolopale (glial) cell and neuron comprising the chordotonal organ in theDrosophila embryo

The chordotonal (scolopidial) organ is a segmental stretch receptor (proprioceptor) and a four-celled organ of the peripheral nervous system of Drosophila. This organ has become a model in studies of embryogenesis involving molecular genetics and developmental biology. We determined how three glial cells and a bipolar neuron (the chordotonal organ) develop and assemble to become a sensitive stretch receptor. Our focus was on the scolopale cell-neuron association which is the core of this organ. The first anatomical appearance of these developing organs appears in Stage 12 embryos: a central fissure forms in the scolopale cell and two intracellular adherence loci redirect the cleft to wall off a cylindrical sector. This portion hollows out and the dendrite of a sensory neuron enters the cavity. Cytoplasmic walls of the cylinder then regress to leave a more prominent lymph space, within which is the cilary portion of the sensory dendrite. A cap (glial) cell then covers and connects the distal portion of the scolopale-neuron pair. Chordotonal organs are formed in about two hours and assembled throughout Stages 13 to 15. We have drawn schematics of these developmental phases with the resultant four-celled organ. Lanthanum tracer in the embryonic hemocoel never gained access to the neuron housed by the scolopale cell. Thus a blood-nerve barrier forms as early as Stage 13 in the peripheral nervous system. Paracellular clefts, sealed by occlusive septate junctions between accessory (glial) cells, are the probable basis for barrier properties.

First developmental signs of the scolopale (glial) cell and neuron comprising the chordotonal organ in the Drosophila embryo

The chordotonal (scolopidial) organ is a segmental stretch receptor (proprioceptor) and a four-celled organ of the peripheral nervous system of Drosophila. This organ has become a model in studies of embryogenesis involving molecular genetics and developmental biology. We determined how three glial cells and a bipolar neuron (the chordotonal organ) develop and assemble to become a sensitive stretch receptor. Our focus was on the scolopale cell-neuron association which is the core of this organ. The first anatomical appearance of these developing organs appears in Stage 12 embryos: a central fissure forms in the scolopale cell and two intracellular adherence loci redirect the cleft to wall off a cylindrical sector. This portion hollows out and the dendrite of a sensory neuron enters the cavity. Cytoplasmic walls of the cylinder then regress to leave a more prominent lymph space, within which is the cilary portion of the sensory dendrite. A cap (glial) cell then covers and connects the distal portion of the scolopale-neuron pair. Chordotonal organs are formed in about two hours and assembled throughout Stages 13 to 15. We have drawn schematics of these developmental phases with the resultant four-celled organ. Lanthanum tracer in the embryonic hemocoel never gained access to the neuron housed by the scolopale cell. Thus a blood-nerve barrier forms as early as Stage 13 in the peripheral nervous system. Paracellular clefts, sealed by occulsive septate junctions between accessory (glial) cells, are the probable basis for barrier properties. GLIA 19:269–274, 1997. © 1997 Wiley-Liss, Inc.