In vitro analyses of interactions between olfactory receptor growth cones and glial cells that mediate axon sorting and glomerulus formation

During development, the axons of olfactory receptor neurons project to the CNS and converge on glomerular targets. For vertebrate and invertebrate olfactory systems, neuron-glia interactions have been hypothesized to regulate the sorting and targeting of olfactory receptor axons and the development of glomeruli. In the moth Manduca sexta, glial reduction experiments have directly implicated two types of central olfactory glia, the sorting zone- and neuropil-associated glia, in key events in olfactory development, including axon sorting and glomerulus stabilization. By using cocultures containing central olfactory glial cells and explants of olfactory receptor epithelium, we show that olfactory receptor growth cones elaborate extensively and cease advancement following contact with sorting zone- and neuropil-associated glial cells. These effects on growth cone behavior were specific to central olfactory glia; peripheral glial cells of the olfactory nerve failed to elicit similar responses in olfactory receptor growth cones. We propose that sorting zone- and neuropil-associated glial cells similarly modify axon behavior in vitro by altering the adhesive properties and cytoskeleton of olfactory receptor growth cones and that these in vitro changes may underlie functionally relevant changes in growth cone behavior in vivo.

Evolution and development of brain sensory organs in molgulid ascidians

The ascidian tadpole larva has two brain sensory organs containing melanocytes: the otolith, a gravity receptor, and the ocellus, part of a photoreceptor. One or both of these sensory organs are absent in molgulid ascidians. We show here that developmental changes leading to the loss of sensory pigment cells occur by different mechanisms in closely related molgulid species. Sensory pigment cells are formed through a bilateral determination pathway in which two or more precursor cells are specified as an equivalence group on each side of the embryo. The precursor cells subsequently converge at the midline after neurulation and undergo cell interactions that decide the fates of the otolith and ocellus. Molgula occidentalis and M. oculata, which exhibit a tadpole larva with an otolith but lacking an ocellus, have conserved the bilateral pigment cell determination pathway. Programmed cell death (PCD) is superimposed on this pathway late in development to eliminate the ocellus precursor and supernumerary pigment cells, which do not differentiate into either an otolith or ocellus. In contrast to molgulids with tadpole larvae, no pigment cell precursors are specified on either side of the M. occulta embryo, which forms a tailless (anural) larva lacking both sensory organs, suggesting that the bilateral pigment cell determination pathway has been lost. The bilateral pigment cell determination pathway and superimposed PCD can be restored in hybrids obtained by fertilizing M. occulta eggs with M. oculata sperm, indicating control by a zygotic process. We conclude that PCD plays an important role in the evolution and development of brain sensory organs in molgulid ascidians.

Development of the zebrafish lateral line

The lateral line system is simple (comprising six cell types), its sense organs form according to a defined and reproducible pattern, and its neurons are easily visualized. In the zebrafish, these advantages can be combined with a wealth of genetic tools, making this system ideally suited to a combined molecular, cellular and genetic analysis. Recent progress has taken advantage of these various qualities to elucidate the mechanism that drives the migration from head to tail of the sense organ precursor cells, and to approach the questions surrounding axonal guidance and target recognition.

The planar cell polarity protein Strabismus promotes Pins anterior localization during asymmetric division of sensory organ precursor cells in Drosophila

Cell fate diversity is generated in part by the unequal segregation of cell-fate determinants during asymmetric cell division. In the Drosophila bristle lineage, the sensory organ precursor (pI) cell is polarized along the anteroposterior (AP) axis by Frizzled (Fz) receptor signaling. We show here that Fz localizes at the posterior apical cortex of the pI cell prior to mitosis, whereas Strabismus (Stbm) and Prickle (Pk), which are also required for AP polarization of the pI cell, co-localize at the anterior apical cortex. Thus, asymmetric localization of Fz, Stbm and Pk define two opposite cortical domains prior to mitosis of the pI cell. At mitosis, Stbm forms an anterior crescent that overlaps with the distribution of Partner of Inscuteable (Pins) and Discs-large (Dlg), two components of the anterior Dlg-Pins-Galphai complex that regulates the localization of cell-fate determinants. At prophase, Stbm promotes the anterior localization of Pins. By contrast, Dishevelled (Dsh) acts antagonistically to Stbm by excluding Pins from the posterior cortex. We propose that the Stbm-dependent recruitment of Pins at the anterior cortex of the pI cell is a novel read-out of planar cell polarity.

Lateral Line Receptors: Where Do They Come from Developmentally and Where Is Our Research Going?

The lateral line system is composed of both mechanoreceptors, which exhibit little variation in structure between taxonomic groups, and electroreceptors, which exhibit considerably more variation. Cathodally sensitive ampullary electroreceptors are the primitive condition and are found in agnathans, chondrichthyans, and most osteichthyans. Aquatic amphibians also have ampullary electroreceptors for at least part of their life cycle. The more recently evolved anodally sensitive ampullary electroreceptors and tuberous electroreceptors are only found in four groups of teleost fishes. The basic ontogenetic unit of lateral line development is the dorsolateral placode. Primitively, there are six pairs of placodes, which pass through sequential stages of development into lateral line receptors. There is no question about the origin of primitive mechanoreceptors or electroreceptors, however, we do not have a good understanding of the origin of teleost mechanoreceptors and their ampullary or tuberous electroreceptors; do they come exclusively from dorsolateral placodes or from neural crest or even general ectoderm? A second intriguing lateral line question is how certain teleost fish groups evolved tuberous electroreceptors. Electroreception appears to have re-evolved at least twice in teleosts after being lost during the neopterygian radiation. It has been suggested that the development of tuberous electroreceptors might be due to changes in placodal patterning or a change in the general ectoderm that placodes arise from. Unfortunately, our understanding of lateral line origins in fishes is very sketchy, and, if we are to answer such an evolutionary question, we first need more complete information about lateral line development in a variety of fishes, which can then be combined with gene expression data to better interpret lateral line receptor development.

Integration of complex larval chemosensory organs into the adult nervous system ofDrosophila

The sense organs of adult Drosophila, and holometabolous insects in general, derive essentially from imaginal discs and hence are adult specific. Experimental evidence presented here, however, suggests a different developmental design for the three largely gustatory sense organs located along the pharynx. In a comprehensive cellular analysis, we show that the posteriormost of the three organs derives directly from a similar larval organ and that the two other organs arise by splitting of a second larval organ. Interestingly, these two larval organs persist despite extensive reorganization of the pharynx. Thus, most of the neurons of the three adult organs are surviving larval neurons. However, the anterior organ includes some sensilla that are generated during pupal stages. Also, we observe apoptosis in a third larval pharyngeal organ. Hence, our experimental data show for the first time the integration of complex, fully differentiated larval sense organs into the nervous system of the adult fly and demonstrate the embryonic origin of their neurons. Moreover, they identify metamorphosis of this sensory system as a complex process involving neuronal persistence, generation of additional neurons and neuronal death. Our conclusions are based on combined analysis of reporter expression from P[GAL4] driver lines, horseradish peroxidase injections into blastoderm stage embryos, cell labeling via heat-shock-induced flip-out in the embryo, bromodeoxyuridine birth dating and staining for programmed cell death. They challenge the general view that sense organs are replaced during metamorphosis.

[Cell polarity and asymmetric division in the peripheral nervous system of Drosophila]

During metazoan development, cell fate diversity is generated in part by asymmetric cell divisions, in which mother cells divide to produce two daughter cells with distinct developmental potentials. Adoption of different cell fates often relies on the polarised distribution and unequal segregation of cell-fate determinants. Unequal segregation of cell-fate determinants requires that the mother cell becomes polarised prior to mitosis. In response to this polarisation, cell-fate determinants localise asymmetrically and the mitotic spindle lines up with the pole to which cell-fate determinants accumulate, thereby leading to their unequal partitioning upon cytokinesis. I review here the regulatory mechanisms that establish cell asymmetry and orient this asymmetry relative to the body axis in the sensory organ lineages of Drosophila.

Magnetic resonance histology: in situ single cell imaging of receptor cells in an invertebrate (Lolliguncula brevis, cephalopoda) sense organ

Utilizing contrast-enhanced MR histology, individual cell bodies were identified in situ and compared one-to-one with conventional histology. The squid Lolliguncula brevis served as a model where the receptor cells of the proprioceptive neck receptor organ were labeled with paramagnetic cobalt(II) ions by conventional cobalt iontophoresis. Stimulated echo images were obtained using a 9.4 T magnet and followed by conventional histologic treatment and light microscopy. Images obtained from both these techniques match well and validate MR histology.

[The development of the nervous system: from fly to fish, from fish to man....]

The nervous system of vertebrates is more complex and less tractable than that of current model organisms such as the fly and the nematode. Here we present a vertebrate sensory system which is structurally simple, experimentally accessible and genetically suitable: the lateral line of the zebra-fish. We review our recent work on the development of this system, with a particular emphasis on the migration events that shape the pattern of sense organs. Some of the factors involved in these migration events turn out to be similar to the factors that direct the formation of metastases in specific types of human cancers, illustrating once again the remarkable conservation of developmental mechanisms and genes throughout the animal kingdom.

Olfactory activation patterns in the antennal lobe of the sphinx moth, Manduca sexta

The sphinx moth Manduca sexta is a well-studied insect with regard to central olfactory functions. Until now, the innervation patterns of olfactory receptor neurons into the array of olfactory glomeruli in the antennal lobe have, however, been unclear. Using optical imaging to visualize calcium dynamics within the antennal lobe we demonstrate specific patterns elicited by sex pheromone components and plant-derived odours. These patterns mainly reflect receptor neuron activity. Within the male-specific macroglomerular complex the two major pheromone components evoke stereotyped activity in either of two macroglomerular complex glomeruli. Based on previous knowledge of output neuron specificity, our results suggest a matching of information between input and output in the macroglomerular complex. Plant odours evoked activity in the sexually isomorphic glomeruli. Two major results were obtained: (1). terpenes and aromatic compounds activate different clusters of glomeruli with only minor overlapping, and (2). the position of certain key glomeruli is fixed in both males and females, which suggests that host-plant related odorants are processed in a similar way in both sexes.

Low rates of change enhance effect of humidity on the activity of insect hygroreceptors

The inability to measure humidity during stimulation has so far prevented us from understanding the contribution of moist cells and dry cells to orientation in a gradient of humidity. The problem was solved in the present study by means of a UV-absorption hygrometer that made it possible to monitor humidity at a rate of 100 Hz. The antennal moist and dry cells of the cockroach were exposed to humidities alternatively falling or rising at low rates between -1% RH s(-1) and +1% RH s(-1) (relative humidity). Impulse frequency of both types of cells depended simultaneously on instantaneous humidity and its rate of change. High frequencies of the moist cells signal high humidity. But at a given humidity, the response frequency is higher still when humidity is also rising. Conversely, high frequencies of the dry cell signal low humidity, and frequency is higher still at a given humidity when humidity is also falling. These responses ensure that the cockroach spent a minimum time in environments where desiccation or hydration occur and may thus protect the animal from emerging accidentally from under cover into moving air. In the constant-humidity retreat of the cockroach, fluctuating or even drifting discharge frequencies could serve as an early warning: return!

Effects of 20-hydroxyecdysone and serotonin on neurite growth and survival rate of antennal lobe neurons in pupal stage of the silk moth Bombyx mori in vitro

Effects of 20-hydroxyecdysone and serotonin on the morphological development and the survival of antennal lobe neurons from day-2 pupal brains of the silk moth Bombyx mori were investigated in vitro. Four morphologically distinct neuronal types could be identified in the cultured antennal lobe neurons: unipolar, bipolar, multi-polar and projection neurons. Antennal lobe neurons in culture with 20-hydroxyecdysone and serotonin showed different patterns of the morphological development from those described in Manduca sexta. Projection neurons extend their neurites remarkably by 20-hydroxyecdysone in B. mori, but there is no extension from antennal lobe neurons in M. sexta. Multi-polar neurons conspicuously increase only formation of new branches from their primary neurites by serotonin in B. mori, but there are both extension and branching of the neurites in M. sexta. On day-5, antennal lobe neurons in lower titers of 20-hydroxyecdysone had significantly higher survival rates than those in higher titers. Neurons cultured for 7 days at different levels of 20-hydroxyecdysone generally showed significantly lower survival rates than neurons cultured for 5 days under the same conditions.

Projections to higher olfactory centers from subdivisions of the antennal lobe macroglomerular complex of the male silkmoth

The macroglomerular complex (MGC) is the first-order center for synaptic processing of olfactory information about the female sex pheromone in the male moth brain. We have investigated the MGC subdivisions of the male silkmoth Bombyx mori by use of three-dimensional reconstruction of the MGC from sequential series of confocal slice images. The B. mori MGC consists of three subdivisions similar to those of Manduca sexta: the cumulus, toroid and horseshoe. Intracellular recording and staining revealed that responses of MGC projection neurons to pheromonal stimulation correlate with their dendritic arborizations in the subdivisions of the MGC (the cumulus, toroid and horseshoe) and each subdivision specific projection neuron transmits information to different regions in the calyces of the mushroom body and the inferior lateral protocerebrum. We revealed that major pheromone component information is transferred to the medial part of the inferior lateral protocerebrum through three different antennocerebral pathways. Although it is generally accepted that the calyces of the mushroom body and the inferior lateral protocerebrum are the target sites for pheromone information from the MGC in moths, our results suggest that the medial part of the inferior lateral protocerebrum may be a more important processing site for major pheromonal information in B. mori.

The glial cell undergoes apoptosis in the microchaete lineage of Drosophila

Apoptosis plays a major role in vertebrate and invertebrate development. The adult Drosophila thoracic microchaete is a mechanosensory organ whose development has been extensively studied as a model of how cell division and cell determination intermingle. This sensory organ arises from a cell lineage that produces a glial cell and four other cells that form the organ. In this study, using an in vivo approach as well as fixed material, we show that the glial cell undergoes nucleus fragmentation shortly after birth. Fragmentation was blocked after overexpression of the caspase inhibitor p35 or removal of the pro-apoptotic genes reaper, hid and grim, showing that the glial cell undergoes apoptosis. Moreover, it seems that fragments are eliminated from the epithelium by mobile macrophages. Forcing survival of the glial cells induces precocious axonal outgrowth but does not affect final axonal patterning and connectivity. However, under these conditions, glial cells do not fragment but leave the epithelium by a mechanism that is reminiscent of cell competition. Finally, we present evidences showing that glial cells are committed to apoptosis independently of gcm and prospero expression. We suggest that apoptosis is triggered by a cell autonomous mechanism.

From stress and strain to spikes: mechanotransduction in spider slit sensilla

This review focuses on the structure and function of a single mechanoreceptor organ in the cuticle of spiders. Knowledge emerging from the study of this organ promises to yield general principles that can be applied to mechanosensation in a wide range of animal systems. The lyriform slit sense organ on the antero-lateral leg patella of the spider Cupiennius salei is unusual in possessing large sensory neurons, whose cell bodies are close to the sites of sensory transduction, and accessible to intracellular recording during mechanotransduction. This situation, combined with recent technical developments, has made it possible to observe and experiment with all the major stages of mechanosensation. Important findings include the approximate size, number and ionic selectivity of the ion channels responsible for mechanotransduction, the types of voltage-activated ion channels responsible for action potential encoding, and the mechanisms controlling the dynamic properties of transduction and encoding. Most recently, a complex efferent system for peripheral modulation of mechanosensation has been discovered and partially characterized. Much remains to be learned about mechanosensation, but the lyriform slit sense organ system continues to offer important opportunities to advance our understanding of this crucial sense.

Novel bridge of axon-like processes of epithelial cells in the aboral sense organ of ctenophores

We describe by light and electron microscopy a novel structure in the aboral sense organ (apical organ) of cydippid (Pleurobrachia) and lobate (Mnemiopsis) ctenophores. An elevated bundle of long, thin, microtubule-filled processes arises from the apical ends of two groups of epithelial cells located on opposite sides of the apical organ along the tentacular plane of the body. This bundle of axon-like processes arches over the epithelial floor like a bridge, with branches at both ends running toward opposing pairs of ciliary balancers that are motile pacemakers for the rows of locomotory ciliary comb plates. The bridge in Pleurobrachia is approximately 40 microm long and 3-4 microm wide and consists of approximately 60 closely packed processes, 0.2-0.8 microm thick, containing vesicles and numerous microtubules running parallel to their long axes. There are approximately 30 epithelial cells in each of the two groups giving rise to the bridge and each cell forms a single process, so roughly half of the processes in the bridge must originate from cells on one side and diverge into branches to a pair of balancers on the opposite side of the apical organ. The 150-200 cilia in each balancer arise from morphologically complex cellular projections with asymmetric lateral extensions directed towards a fork of the bridge. Presynaptic triad structures and vesicles are found in this region but clear examples of synaptic contacts between bridge processes and balancer cells have not yet been traced. Cydippid larvae of Mnemiopsis have a conspicuous bridge along the tentacular plane of the apical organ. Beroid ctenophores that lack tentacles at all stages do not have a bridge. We discuss the possibility that the bridge is an electrical conduction pathway to balancers that coordinates tentacle-evoked swimming responses of ctenophores, such as global ciliary excitation.

Binary cell death decision regulated by unequal partitioning of Numb at mitosis

An important issue in Metazoan development is to understand the mechanisms that lead to stereotyped patterns of programmed cell death. In particular, cells programmed to die may arise from asymmetric cell divisions. The mechanisms underlying such binary cell death decisions are unknown. We describe here a Drosophila sensory organ lineage that generates a single multidentritic neuron in the embryo. This lineage involves two asymmetric divisions. Following each division, one of the two daughter cells expresses the pro-apoptotic genes reaper and grim and subsequently dies. The protein Numb appears to be specifically inherited by the daughter cell that does not die. Numb is necessary and sufficient to prevent apoptosis in this lineage. Conversely, activated Notch is sufficient to trigger death in this lineage. These results show that binary cell death decision can be regulated by the unequal segregation of Numb at mitosis. Our study also indicates that regulation of programmed cell death modulates the final pattern of sensory organs in a segment-specific manner.

Numb: "Adapting" notch for endocytosis

During sensory organ precursor divisions in Drosophila, the numb gene product segregates asymmetrically into one of the two daughter cells, to which it confers a specific fate by inhibiting Notch signaling. In this issue of Developmental Cell, Berdnik et al. show that Numb recruits alpha-Adaptin and that this physical interaction plays a role in downregulating Notch, presumably by stimulating endocytosis of Notch.

The endocytic protein alpha-Adaptin is required for numb-mediated asymmetric cell division in Drosophila

During asymmetric cell division in Drosophila sensory organ precursor cells, the Numb protein localizes asymmetrically and segregates into one daughter cell, where it influences cell fate by repressing signal transduction via the Notch receptor. We show here that Numb acts by polarizing the distribution of alpha-Adaptin, a protein involved in receptor-mediated endocytosis. alpha-Adaptin binds to Numb and localizes asymmetrically in a Numb-dependent fashion. Mutant forms of alpha-Adaptin that no longer bind to Numb fail to localize asymmetrically and cause numb-like defects in asymmetric cell division. Our results suggest a model in which Numb influences cell fate by downregulating Notch through polarized receptor-mediated endocytosis, since Numb also binds to the intracellular domain of Notch.

Sensory neurons of the Atonal lineage pioneer the formation of glomeruli within the adult Drosophila olfactory lobe

The first centers for processing of odor information by animals lie in the olfactory lobe. Sensory neurons from the periphery synapse with interneurons in anatomically recognizable units, termed glomeruli, seen in both insects and vertebrates. The mechanisms that underlie the formation of functional maps of the odor-world in the glomeruli within the olfactory lobe remains unclear. We address the basis of sensory targeting in the fruitfly Drosophila and show that one class of sensory neurons, those of the Atonal lineage, plays a crucial role in glomerular patterning. Atonal-dependent neurons pioneer the segregation of other classes of sensory neurons into distinct glomeruli. Furthermore, correct sensory innervation is necessary for the arborization of projection neurons into glomeruli and for the elaboration of processes of central glial cells into the lobe.

Postembryonic development of the posterior lateral line in zebrafish

We examine how the posterior lateral line of the zebrafish grows and evolves from the simple midbody line present at the end of embryogenesis into the complex adult pattern. Our results suggest that secondary neuromasts do not form through budding from the embryonic line, but rather new waves of neuromasts are added anteroposteriorly. We propose that the developmental module that builds the embryonic pattern of neuromasts is used repeatedly during postembryonic development and that additional (secondary) primordia generate the additional neuromasts. We show that differentiated neuromasts migrate ventrally, and eventually generate ‘stitches’ by successive bisections. We also examine the repatterning of the terminal neuromasts, which anticipates the up-bending of the tail leading to the highly asymmetrical caudal fin of the adult (which develops exclusively from the ventral part of the tail). Because terminal repatterning affects all aspects of tail formation, including its sensory development, we speculate that terminal axis bending may have become intimately associated with the terminal Hox genes before the appearance of the tetrapod lineage.

Cell migration in the postembryonic development of the fish lateral line

We examine at the cellular level the postembryonic development of the posterior lateral line in the zebrafish. We show that the first wave of secondary neuromasts is laid down by a migrating primordium, primII. This primordium originates from a cephalic region much like the primordium that formed the primary line during embryogenesis. PrimII contributes to both the lateral and the dorsal branches of the posterior lateral line. Once they are deposited by the primordium, the differentiating neuromasts induce the specialisation of overlying epidermal cells into a pore-forming annulus, and the entire structure begins to migrate ventrally across the epithelium. Thus the final two-dimensional pattern depends on the combination of two orthogonal processes: anteroposterior waves of neuromast formation and dorsoventral migration of individual neuromasts. Finally, we examine how general these migratory processes can be by describing two fish species with very different adult patterns, Astyanax fasciatus (Mexican blind cavefish) and Oryzias latipes (medaka). We show that their primary patterns are nearly identical to that observed in zebrafish embryos, and that their postembryonic growth relies on the same combination of migratory processes that we documented in the case of the zebrafish.

Bazooka is required for localization of determinants and controlling proliferation in the sensory organ precursor cell lineage in Drosophila

Asymmetric divisions with two different division orientations follow different polarity cues for the asymmetric segregation of determinants in the sensory organ precursor (SOP) lineage. The first asymmetric division depends on frizzled function and has the mitotic spindle of the pI cell in the epithelium oriented along the anterior-posterior axis, giving rise to pIIa and pIIb, which divide in different orientations. Only the pIIb division resembles neuroblast division in daughter-size asymmetry, spindle orientation along the apical-basal axis, basal Numb localization, and requirement for inscuteable function. Because the PDZ domain protein Bazooka is required for spindle orientation and basal localization of Numb in neuroblasts, we wondered whether Bazooka plays a similar role in the pIIb in the SOP lineage. Surprisingly, Bazooka controls asymmetric localization of the Numb-anchoring protein Pon, but not spindle orientation, in pI and all subsequent divisions. Bazooka also regulates cell proliferation in the SOP lineage; loss of bazooka function results in supernumerary cell divisions and apoptotic cell death.

Statoconia formation in molluscan statocysts

The gravity sensors of all molluscs phylogenetically below the cephalopods are spherical organs called statocysts. The wall of the sphere contains mechanosensory cells whose sensory cilia project into the lumen of the cyst. The lumen is filled with fluid and dense "stones", the statoconia or statoliths, which sink under the influence of gravity to load, and stimulate, those receptor cells which are at the bottom. The statoconia of Aplysia californica are shown to be calcified about a lamellar arrangement of membranes. Similar lamellar membrane arrangements are seen within the receptor cells, and their possible role in the formation of the statoconia is discussed. SEM of unfixed statoconia reveals plate-like crystallization on their surface. Elemental analysis shows a relatively high Sr content, which is of interest, since others have recently reported that Sr is required in the culture medium of several laboratory reared molluscs in order for the statoconia to develop.