The lateral line microcosmos

Lef1 controls patterning and proliferation in the posterior lateral line system of zebrafish

The embryonic development of the posterior lateral line of zebrafish involves the migration from head to tail of a primordium comprising approximately 100 cells, and the deposition at regular intervals of presumptive mechanosensory organs (neuromasts). Migration depends on the presence of chemokine SDF1 along the pathway, and on the asymmetrical distribution of chemokine receptors CXCR4 and CXCR7 in the primordium. Primordium polarization depends on Wnt signaling in the leading region. Here, we examine the role of a major effector of Wnt signaling, lef1, in this system. We show that, although its inactivation has no overt effect on the expression of cxcr4b and cxcr7b, lef1 contributes to their control. We also show that cell proliferation, which ensures constant primordium size despite successive rounds of cell deposition, is reduced upon lef1 inactivation. Because of this defect, the primordium runs short of cells and vanishes before the line has been completed. We conclude that lef1-mediated Wnt signaling is involved in various aspects of primordium migration, although part of this implication is masked by a high level of developmental redundancy.

Compartmentalized Notch signaling sustains epithelial mirror symmetry

Bilateral symmetric tissues must interpret axial references to maintain their global architecture during growth or repair. The regeneration of hair cells in the zebrafish lateral line, for example, forms a vertical midline that bisects the neuromast epithelium into perfect mirror-symmetric plane-polarized halves. Each half contains hair cells of identical planar orientation but opposite to that of the confronting half. The establishment of bilateral symmetry in this organ is poorly understood. Here, we show that hair-cell regeneration is strongly directional along an axis perpendicular to that of epithelial planar polarity. We demonstrate compartmentalized Notch signaling in neuromasts, and show that directional regeneration depends on the development of hair-cell progenitors in polar compartments that have low Notch activity. High-resolution live cell tracking reveals a novel process of planar cell inversions whereby sibling hair cells invert positions immediately after progenitor cytokinesis, demonstrating that oriented progenitor divisions are dispensable for bilateral symmetry. Notwithstanding the invariably directional regeneration, the planar polarization of the epithelium eventually propagates symmetrically because mature hair cells move away from the midline towards the periphery of the neuromast. We conclude that a strongly anisotropic regeneration process that relies on the dynamic stabilization of progenitor identity in permissive polar compartments sustains bilateral symmetry in the lateral line.

Aquaporin 4 in the sensory organs of adult zebrafish (Danio rerio)

The aquaporins (AQPs) are a family (AQP-AQP10) of transmembrane channel proteins that mediate the transport of water, ions, gases, and small molecules across the cell membrane, thus regulating cell homeostasis. AQP4 has the highest water permeability and it is involved in hearing and vision in mammals. Here, we used immunohistochemistry to map the presence of AQP4 in the sensory organs of adult zebrafish. The antibody used detected by Western blot proteins of 34 kDa (equivalent to that of mammalian AQP4) and maps in the sensory cells of taste buds, the hair sensory cells of the neuromast and of the maculae, and cristae ampullaris of the inner ear. Moreover, the retinal photoreceptors display AQP4 immunoreactivity. The non-sensory cells in these organs were AQP4 negative. These results suggest that the AQP4 could play a role in the regulation of water balance and ion transport in the sensory cells of zebrafish, bringing new data for the utilizing of this experimental model in the biology of sensory system.

Whence directionality: guidance mechanisms in solitary and collective cell migration

As individual cells or groups of cells move through the complex environment of the body, their migration is affected by multiple external cues. Some cues are diffusible signaling molecules, and some are solid biophysical features. How do cells respond appropriately? This perspective discusses the relationship between guidance input and the cellular output, considering effects from classical chemotaxis to contact-dependent guidance. The influences of membrane trafficking and of imposed constraints on directional movement are also considered. New insights regarding guidance and dynamic cell polarity have emerged from examining new cell migration models and from re-examining well known ones with new approaches and new tools.

Onset and dynamic expression of S100 proteins in the olfactory organ and the lateral line system in zebrafish development

In the zebrafish olfactory epithelium, three morphologically distinct olfactory neurons express different marker proteins. We utilize this feature to access developmental dynamics of one of the neuron types, the crypt cells, to determine whether they are differentiated at a stage similar to other olfactory neurons. Immunohistochemical studies using an S100 antibody that specifically recognizes crypt cells showed that S100-positive cells appear in olfactory rosettes as early as at 2day postfertilization (dpf). However, some of the rosettes did not have any S100-positive cells until 4 dpf. The number of S100-positive cells in individual rosettes increased steadily over the next 3days before it decreased significantly. There were 7.8 S100-positive cells per rosettes on average in larvae at 7 dpf. The number reduced to 2.2 at 9 dpf. A recovery to a pre-reduction level was detected in 12 dpf larvae. We also observed S100-positive cells in neuromasts of the lateral line system in 2 dpf larvae, suggesting that the crypt cells and sensory cells in the neuromasts have similar onsets of differentiation. Our data have provided a time line of differentiation of crypt cells in development of the olfactory system and demonstrated that this type of cell is differentiated at a stage similar to ciliated and microvillous olfactory neurons. A nonlinear growth trajectory of the crypt cell population in the first nine days of zebrafish development implicates a possible functional significance of crypt cells in early life stages of zebrafish.

Transposon-mediated enhancer detection reveals the location, morphology and development of the cupular organs, which are putative hydrodynamic sensors, in the ascidian Ciona intestinalis

The adult of the ascidian Ciona intestinalis has cupular organs, i.e., putative hydrodynamic sensors, at the atrial epithelium. The cupular organ consists of support cells and sensory neurons, and it extends a gelatinous matrix, known as a cupula, toward the atrial cavity. These characteristics are shared with sensory hair cells in the vertebrate inner ear and lateral line neuromasts in fish and amphibians, which suggests an evolutionary link between the cupular organ and these vertebrate hydrodynamic sensors. In the present study, we have isolated and investigated two transposon-mediated enhancer detection lines that showed GFP expression in support cells of the cupular organs. Using the enhancer detection lines and neuron marker transgenic lines, we describe the position, morphology, and development of the cupular organs. Cupular organs were found at the atrial epithelium, but not in the branchial epithelium. We found that cupular organs are also present along the dorsal fold and the gonoducts. The cells lining the pre-atrial opening in juveniles are presumably precursor cells of the cupular organ. To our knowledge, the present study is the first precise description of the ascidian cupular organ, providing evidence that may help to resolve discrepancies among previous studies on the organ.

Somatosensory mechanisms in zebrafish lacking dorsal root ganglia

Dorsal root ganglion (DRG) sensory neurons transmit all somatosensory information from the trunk region of the body. erbb3 mutant zebrafish do not form DRG neurons because the neural crest cells that generate them migrate aberrantly. Here we report that homozygous erbb3 mutants appear to swim and feed normally, and that they survive through adulthood, despite never forming DRG neurons. The source of sensory compensation in adult erbb3 mutants remains unknown, although it may be from lateral line ganglion neuromasts which are reduced, but present, in erbb3 mutants. We also provide new information about the development of DRG neurons in wild-type juvenile zebrafish.

New insights into signaling during myelination in zebrafish

Myelin is a vertebrate adaptation that allows for the rapid propagation of action potentials along axons. Specialized glial cells-oligodendrocytes in the central nervous system (CNS) and Schwann cells in the peripheral nervous system (PNS)-form myelin by repeatedly wrapping axon segments. Debilitating diseases result from the disruption of myelin, including multiple sclerosis and Charcot-Marie-Tooth peripheral neuropathies. The process of myelination involves extensive communication between glial cells and the associated neurons. The past few years have seen important progress in understanding the molecular basis of the signals that coordinate the development of these fascinating cells. This review highlights recent advances in myelination deriving from studies in the zebrafish model system, with a primary focus on the PNS. While Neuregulin1-ErbB signaling has long been known to play important roles in peripheral myelin development, work in zebrafish has elucidated its roles in Schwann cell migration and radial sorting of axons in vivo. Forward genetic screens in zebrafish have also uncovered new genes required for development of myelinated axons, including gpr126, which encodes a G-protein coupled receptor required for Schwann cells to progress beyond the promyelinating stage. In addition, work in zebrafish uncovered new roles for Schwann cells themselves, including in regulating the boundary between the PNS and CNS and positioning a nerve after its initial outgrowth.

Molecular dissection of the migrating posterior lateral line primordium during early development in zebrafish

Background

Development of the posterior lateral line (PLL) system in zebrafish involves cell migration, proliferation and differentiation of mechanosensory cells. The PLL forms when cranial placodal cells delaminate and become a coherent, migratory primordium that traverses the length of the fish to form this sensory system. As it migrates, the primordium deposits groups of cells called neuromasts, the specialized organs that contain the mechanosensory hair cells. Therefore the primordium provides both a model for studying collective directional cell migration and the differentiation of sensory cells from multipotent progenitor cells.

Results

Through the combined use of transgenic fish, Fluorescence Activated Cell Sorting and microarray analysis we identified a repertoire of key genes expressed in the migrating primordium and in differentiated neuromasts. We validated the specific expression in the primordium of a subset of the identified sequences by quantitative RT-PCR, and by in situ hybridization. We also show that interfering with the function of two genes, f11r and cd9b, defects in primordium migration are induced. Finally, pathway construction revealed functional relationships among the genes enriched in the migrating cell population.

Conclusions

Our results demonstrate that this is a robust approach to globally analyze tissue-specific expression and we predict that many of the genes identified in this study will show critical functions in developmental events involving collective cell migration and possibly in pathological situations such as tumor metastasis.

atp2b1a regulates Ca(2+) export during differentiation and regeneration of mechanosensory hair cells in zebrafish

The molecular mechanisms of development of mechanosensory hair cells have been tackled successfully due to in vivo studies in the zebrafish lateral line. The enhancer trap (ET) transgenic line, SqET4 was instrumental in these studies even despite a lack of a link of its GFP expression pattern to a particular gene(s). We mapped the Tol2 transposon insertion of the SqET4 transgenics onto Chr. 4 next to a gene encoding Atp2b1a (Pmca1) - one of the four PMCAs acting to export Ca(2+) from a cell. atp2b1a expression recapitulates that of GFP during the development of mechanoreceptors of the inner ear and lateral line. atp2b1a expression correlates with the regeneration of these cells. Thus, SqET4 represents the Tg:atp2b1a-GFP line, which links Ca(2+) metabolism and the differentiation of mechanoreceptors. The morpholino-mediated knockdown of atp2b1a blocks Ca(2+) export and affects the division of hair cell progenitors, resulting in their accumulation. Under the control of a master gene of hair cells, Atoh1a, Atp2b1a functions during progenitor cell proliferation and hair cell differentiation. Given the similarity between the phenotypes of atp2b1a morphants and embryos treated with the pan-PMCA inhibitor 5(6)-carboxyeosin, Atp2b1a emerges as member of the Atp2b family responsible for Ca(2+) export during the development of hair cells.

Glial cell line-derived neurotrophic factor defines the path of developing and regenerating axons in the lateral line system of zebrafish

How the peripheral axons of sensory neurons are guided to distant target organs is not well understood. Here we examine this question in the case of the posterior lateral line (PLL) system of zebrafish, where sensory organs are deposited by a migrating primordium. Sensory neurites accompany this primordium during its migration and are thereby guided to their prospective target organs. We show that the inactivation of glial cell line-derived neurotrophic factor (GDNF) signaling leads to defects of innervation and that these defects are due to the inability of sensory axons to track the migrating primordium. GDNF signaling is also used as a guidance cue during axonal regeneration following nerve cut. We conclude that GDNF is a major determinant of directed neuritic growth and of target finding in this system, and we propose that GDNF acts by promoting local neurite outgrowth.

Atoh1a expression must be restricted by Notch signaling for effective morphogenesis of the posterior lateral line primordium in zebrafish

The posterior lateral line primordium (pLLp) migrates caudally, depositing neuromasts to establish the posterior lateral line system in zebrafish. A Wnt-dependent FGF signaling center at the leading end of the pLLp initiates the formation of `proneuromasts' by facilitating the reorganization of cells into epithelial rosettes and by initiating atoh1a expression. Expression of atoh1a gives proneuromast cells the potential to become sensory hair cells, and lateral inhibition mediated by Delta-Notch signaling restricts atoh1a expression to a central cell. We show that as atoh1a expression becomes established in the central cell, it drives expression of fgf10 and of the Notch ligand deltaD, while it inhibits expression of fgfr1. As a source of Fgf10, the central cell activates the FGF pathway in neighboring cells, ensuring that they form stable epithelial rosettes. At the same time, DeltaD activates Notch in neighboring cells, inhibiting atoh1a expression and ensuring that they are specified as supporting cells. When Notch signaling fails, unregulated atoh1a expression reduces Fgfr1 expression, eventually resulting in attenuated FGF signaling, which prevents effective maturation of epithelial rosettes in the pLLp. In addition, atoh1a inhibits e-cadherin expression, which is likely to reduce cohesion and contribute to fragmentation of the pLLp. Together, our observations reveal a genetic regulatory network that explains why atoh1a expression must be restricted by Notch signaling for effective morphogenesis of the pLLp.

Schwann cells reposition a peripheral nerve to isolate it from postembryonic remodeling of its targets

Although much is known about the initial construction of the peripheral nervous system (PNS), less well understood are the processes that maintain the position and connections of nerves during postembryonic growth. Here, we show that the posterior lateral line nerve in zebrafish initially grows in the epidermis and then rapidly transitions across the epidermal basement membrane into the subepidermal space. Our experiments indicate that Schwann cells, which myelinate axons in the PNS, are required to reposition the nerve. In mutants lacking Schwann cells, the nerve is mislocalized and the axons remain in the epidermis. Transplanting wild-type Schwann cells into these mutants rescues the position of the nerve. Analysis of chimeric embryos suggests that the process of nerve relocalization involves two discrete steps - the degradation and recreation of the epidermal basement membrane. Although the outgrowth of axons is normal in mutants lacking Schwann cells, the nerve becomes severely disorganized at later stages. In wild-type embryos, exclusion of the nerve from the epidermis isolates axons from migration of their targets (sensory neuromasts) within the epidermis. Without Schwann cells, axons remain within the epidermis and are dragged along with the migrating neuromasts. Our analysis of the posterior lateral line system defines a new process in which Schwann cells relocate a nerve beneath the epidermal basement membrane to insulate axons from the postembryonic remodeling of their targets.

Progressive neurogenesis defines lateralis somatotopy

Fishes and amphibians localize hydromechanical variations along their bodies using the lateral-line sensory system. This is possible because the spatial distribution of neuromasts is represented in the hindbrain by a somatotopic organization of the lateralis afferent neurons' central projections. The mechanisms that establish lateralis somatotopy are not known. Using BAPTI and neuronal tracing in the zebrafish, we demonstrate growth anisotropy of the posterior lateralis ganglion. We characterized a new transgenic line for in vivo imaging to show that although peripheral growth-cone structure adumbrates somatotopy, the order of neurogenesis represents a more accurate predictor of the position of a neuron's central axon along the somatotopic axis in the hindbrain. We conclude that progressive neurogenesis defines lateralis somatotopy.

Single-cell analysis of somatotopic map formation in the zebrafish lateral line system

The zebrafish lateral line is a simple sensory system comprising a small number of neurons in addition to their sensory organs, the neuromasts. We have adopted this system as a model for single-cell level analyses of topographic map formation and examined when and how the lateral line topographic map is established. Single-neuron labeling demonstrated that somatotopic organization of the ganglion emerges by 54 hr postfertilization, but also that this initial map is not as accurate as that observed at 6 days postfertilization. During this initial stage, individual neurons exhibit extensively diverse behavior and morphologies. We identified leader neurons, the axons of which are the first to reach the tail, and later-appearing axons that contribute to the initial map. Our data suggest that lateral line neurons are heterogeneous from the beginning of lateral line development, and that some of them are intrinsically fate determined to contribute to the somatotopic map.

Identification of the hair cell soma-1 antigen, HCS-1, as otoferlin

Hair cells, the mechanosensitive receptor cells of the inner ear, are critical for our senses of hearing and balance. The small number of these receptor cells in the inner ear has impeded the identification and characterization of proteins important for hair cell function. The binding specificity of monoclonal antibodies provides a means for identifying hair cell-specific proteins and isolating them for further study. We have generated a monoclonal antibody, termed hair cell soma-1 (HCS-1), which specifically immunolabels hair cells in at least five vertebrate classes, including sharks and rays, bony fish, amphibians, birds, and mammals. We used HCS-1 to immunoprecipitate the cognate antigen and identified it as otoferlin, a member of the ferlin protein family. Mutations in otoferlin underlie DFNB9, a recessive, nonsyndromic form of prelingual deafness characterized as an auditory neuropathy. Using immunocytochemistry, we find that otoferlin is associated with the entire basolateral membrane of the hair cells and with vesicular structures distributed throughout most of the hair cell cytoplasm. Biochemical assays indicate that otoferlin is tightly associated with membranes, as it is not solubilized by alterations in calcium or salt concentrations. HCS-1 immunolabeling does not co-localize with ribeye, a constituent of synaptic ribbons, suggesting that otoferlin may, in addition to its proposed function in synaptic vesicle release, play additional roles in hair cells.

Origin and early development of the posterior lateral line system of zebrafish

The lateral line system of teleosts has recently become a model system to study patterning and morphogenesis. However, its embryonic origins are still not well understood. In zebrafish, the posterior lateral line (PLL) system is formed in two waves, one that generates the embryonic line of seven to eight neuromasts and 20 afferent neurons and a second one that generates three additional lines during larval development. The embryonic line originates from a postotic placode that produces both a migrating sensory primordium and afferent neurons. Nothing is known about the origin and innervation of the larval lines. Here we show that a "secondary" placode can be detected at 24 h postfertilization (hpf), shortly after the primary placode has given rise to the embryonic primordium and ganglion. The secondary placode generates two additional sensory primordia, primD and primII, as well as afferent neurons. The primary and secondary placodes require retinoic acid signaling at the same stage of late gastrulation, suggesting that they share a common origin. Neither primary nor secondary neurons show intrinsic specificity for neuromasts derived from their own placode, but the sequence of neuromast deposition ensures that neuromasts are primarily innervated by neurons derived from the cognate placode. The delayed formation of secondary afferent neurons accounts for the capability of the fish to form a new PLL ganglion after ablation of the embryonic ganglion at 24 hpf.

Variability of hair cells in the coronal organ of ascidians (Chordata, Tunicata)

The tunicate ascidians are nonvertebrate chordates that possess mechanoreceptor cells in the coronal organ in the oral siphon, which monitor the incoming water flow. Like vertebrate hair cells, the mechanoreceptor–coronal cells are secondary sensory (axonless) cells accompanied by supporting cells and they exhibit morphological diversities of apical specialisations: they are multiciliate in ascidians of the order Enterogona, whereas they are more complex and possess one or two cilia accompanied by stereovilli, also graded in length, in ascidians of the order Pleurogona. In morphology, embryonic origin, and arrangement, coronal sensory cells closely resemble vertebrate hair cells. We describe here the coronal organs of five ascidians ( Pyura haustor (Stimpson, 1864), Pyura stolonifera (Heller, 1878), Styela gibbsii (Stimpson, 1864), Styela montereyensis (Dall, 1872), and Polyandrocarpa zorritensis (Van Name, 1931)), belonging to Pleurogona, also comprising species of one family (Pyuridae), not yet considered, and thus completing our overview of the order. Each species possesses at least two kinds of secondary sensory cells, some of them characterized by stereovilli graded in length. In some species, the coronal sensory cells exhibit secretory activity; in P. haustor, a mitotic sensory cell has also been found. We compare the coronal organ in both ascidians and with other chordate sensory organs formed of secondary sensory cells, and discuss their possible homologies.

Multispectral four-dimensional imaging reveals that evoked activity modulates peripheral arborization and the selection of plane-polarized targets by sensory neurons

The polarity of apical stereocilia endows hair cells with directional excitability, which in turn enables animals to determine the vectorial component of a sound. Neuromasts of the lateral line of aquatic vertebrates harbor two populations of hair cells that are oriented at 180° relative to each other. The resulting sensory-vectorial ambiguity is solved by lateralis afferent neurons that discriminate between hair cells of opposite polarities to innervate only those with the same orientation. How neurons select identically oriented hair cells remains unknown. To gain insight into the mechanism that underlies this selection, we devised a simple method to gather dynamic morphometric information about axonal terminals in toto by four-dimensional imaging. Applying this strategy to the zebrafish allowed us to correlate hair cell orientation to single afferent neurons at subcellular resolution. Here we show that in zebrafish with absent hair cell mechanoreception, lateralis afferents arborize profusely in the periphery, display less stability, and make improper target selections. Central axons, however, show no dynamic changes and establish normal contacts with the Mauthner cell, a characteristic second-order target in the hindbrain. We propose that the hardwired developmental mechanisms that underlie peripheral arborization and target recognition are modulated by evoked hair cell activity. This interplay between intrinsic and extrinsic cues is essential for plane-polarized target selection by lateralis afferent neurons.

Estrogen receptor ESR1 controls cell migration by repressing chemokine receptor CXCR4 in the zebrafish posterior lateral line system

The primordium that generates the embryonic posterior lateral line of zebrafish migrates from the head to the tip of the tail along a trail of SDF1-producing cells. This migration critically depends on the presence of the SDF1 receptor CXCR4 in the leading region of the primordium and on the presence of a second SDF1 receptor, CXCR7, in the trailing region of the primordium. Here we show that inactivation of the estrogen receptor ESR1 results in ectopic expression of cxcr4b throughout the primordium, whereas ESR1 overexpression results in a reciprocal reduction in the domain of cxcr4b expression, suggesting that ESR1 acts as a repressor of cxcr4b. This finding could explain why estrogens significantly decrease the metastatic ability of ESR-positive breast cancer cells. ESR1 inactivation also leads to extinction of cxcr7b expression in the trailing cells of the migrating primordium; this effect is indirect, however, and due to the down-regulation of cxcr7b by ectopic SDF1/CXCR4 signaling in the trailing region. Both ESR1 inactivation and overexpression result in aborted migration, confirming the importance of this receptor in the control of SDF1-dependent migration.

Dermal morphogenesis controls lateral line patterning during postembryonic development of teleost fish

The lateral line system displays highly divergent patterns in adult teleost fish. The mechanisms underlying this variability are poorly understood. Here, we demonstrate that the lateral line mechanoreceptor, the neuromast, gives rise to a series of accessory neuromasts by a serial budding process during postembryonic development in zebrafish. We also show that accessory neuromast formation is highly correlated to the development of underlying dermal structures such as bones and scales. Abnormalities in opercular bone morphogenesis, in endothelin 1-knockdown embryos, are accompanied by stereotypic errors in neuromast budding and positioning, further demonstrating the tight correlation between the patterning of neuromasts and of the underlying dermal bones. In medaka, where scales form between peridermis and opercular bones, the lateral line displays a scale-specific pattern which is never observed in zebrafish. These results strongly suggest a control of postembryonic neuromast patterns by underlying dermal structures. This dermal control may explain some aspects of the evolution of lateral line patterns.

Lighting up developmental mechanisms: how fluorescence imaging heralded a new era

Embryology and genetics have given rise to a mechanistic framework that explains the architecture of a developing organism. Until recently, however, such studies suffered from a lack of quantification and real-time visualization at the subcellular level, limiting their ability to monitor the dynamics of developmental processes. Live imaging using fluorescent proteins has overcome these limitations, uncovering unprecedented insights that call many established models into question. We review how the study of patterning, cell polarization and morphogenesis has benefited from this technology and discuss the possibilities offered by fluorescence imaging and by the contributions of quantitative disciplines.

Collective cell migration

For all animals, cell migration is an essential and highly regulated process. Cells migrate to shape tissues, to vascularize tissues, in wound healing, and as part of the immune response. Unfortunately, tumor cells can also become migratory and invade surrounding tissues. Some cells migrate as individuals, but many cell types will, under physiological conditions, migrate collectively in tightly or loosely associated groups. This includes invasive tumor cells. This review discusses different types of collective cell migration, including sheet movement, sprouting and branching, streams, and free groups, and highlights recent findings that provide insight into cells' organization and behavior. Cells performing collective migration share many cell biological characteristics with independently migrating cells but, by affecting one another mechanically and via signaling, these cell groups are subject to additional regulation and constraints. New properties that emerge from this connectivity can contribute to shaping, guiding, and ultimately ensuring tissue function.

Making senses development of vertebrate cranial placodes

Cranial placodes (which include the adenohypophyseal, olfactory, lens, otic, lateral line, profundal/trigeminal, and epibranchial placodes) give rise to many sense organs and ganglia of the vertebrate head. Recent evidence suggests that all cranial placodes may be developmentally related structures, which originate from a common panplacodal primordium at neural plate stages and use similar regulatory mechanisms to control developmental processes shared between different placodes such as neurogenesis and morphogenetic movements. After providing a brief overview of placodal diversity, the present review summarizes current evidence for the existence of a panplacodal primordium and discusses the central role of transcription factors Six1 and Eya1 in the regulation of processes shared between different placodes. Upstream signaling events and transcription factors involved in early embryonic induction and specification of the panplacodal primordium are discussed next. I then review how individual placodes arise from the panplacodal primordium and present a model of multistep placode induction. Finally, I briefly summarize recent advances concerning how placodal neurons and sensory cells are specified, and how morphogenesis of placodes (including delamination and migration of placode-derived cells and invagination) is controlled.

Physiological recordings from zebrafish lateral-line hair cells and afferent neurons

Sensory signal transduction, the process by which the features of external stimuli are encoded into action potentials, is a complex process that is not fully understood. In fish and amphibia, the lateral-line organ detects water movement and vibration and is critical for schooling behavior and the detection of predators and prey. The lateral-line system in zebrafish serves as an ideal platform to examine encoding of stimuli by sensory hair cells. Here, we describe methods for recording hair-cell microphonics and activity of afferent neurons using intact zebrafish larvae. The recordings are performed by immobilizing and mounting larvae for optimal stimulation of lateral-line hair cells. Hair cells are stimulated with a pressure-controlled water jet and a recording electrode is positioned next to the site of mechanotransduction in order to record microphonics--extracellular voltage changes due to currents through hair-cell mechanotransduction channels. Another readout of the hair-cell activity is obtained by recording action currents from single afferent neurons in response to water-jet stimulation of innervated hair cells. When combined, these techniques make it possible to probe the function of the lateral-line sensory system in an intact zebrafish using controlled, repeatable, physiological stimuli.

The C. elegans tailless/Tlx homolog nhr-67 regulates a stage-specific program of linker cell migration in male gonadogenesis

Cell migration is a common event during organogenesis, yet little is known about how migration is temporally coordinated with organ development. We are investigating stage-specific programs of cell migration using the linker cell (LC), a migratory cell crucial for male gonadogenesis of C. elegans. During the L3 and L4 larval stages of wild-type males, the LC undergoes changes in its position along the migratory route, in transcriptional regulation of the unc-5 netrin receptor and zmp-1 zinc matrix metalloprotease, and in cell morphology. We have identified the tailless homolog nhr-67 as a cell-autonomous, stage-specific regulator of timing in LC migration programs. In nhr-67-deficient animals, each of the L3 and L4 stage changes is either severely delayed or never occurs, yet LC development before the early L3 stage or after the mid-L4 stage occurs with normal timing. We propose that there is a basal migration program utilized throughout LC migration that is modified by stage-specific regulators such as nhr-67.

Activity-independent specification of synaptic targets in the posterior lateral line of the larval zebrafish

The development of functional neural circuits requires that connections between neurons be established in a precise manner. The mechanisms by which complex nervous systems perform this daunting task remain largely unknown. In the posterior lateral line of larval zebrafish, each afferent neuron forms synaptic contacts with hair cells of a common hair-bundle polarity. We investigated whether afferent neurons distinguish hair-cell polarities by analyzing differences in the synaptic signaling between oppositely polarized hair cells. By examining two mutant zebrafish lines with defects in mechanoelectrical transduction, and by blocking transduction during the development of wild-type fish, we found that afferent neurons could form specific synapses in the absence of stimulus-evoked patterns of synaptic release. Asking next whether this specificity arises through intrinsically generated patterns of synaptic release, we found that the polarity preference persisted in two mutant lines lacking essential synaptic proteins. These results indicate that lateral-line afferent neurons do not require synaptic activity to distinguish hair-cell polarities and suggest that molecular labels of hair-cell polarity guide prepatterned afferents to form the appropriate synapses.

Chemokine signaling in embryonic cell migration: a fisheye view

Chemokines and their receptors were discovered about twenty years ago as mediators of leukocyte traffic. Over the past decade, functional studies of these molecules have revealed their importance for cell migration processes during embryogenesis, which, in addition to providing mechanistic insights into embryonic development, could complement information about chemokine function in the immune system. Here, we review the roles of the chemokine stromal cell-derived factor 1 (SDF-1/CXCL12) and its receptor CXCR4 during zebrafish and mouse embryonic development, and discuss their function in regulating the interactions of cells with their extracellular environment, in directing their migration, and in maintaining their location.

First detection of neuropeptide Y (NPY)-like immunoreactivity in the lateral line: presence and distribution in the neuromasts of the Antarctic notothenioid fish Trematomus bernacchii

The mechanosensory lateral line (LL) is involved in many fish and amphibian behaviors, however little is known about the molecules involved in the signal transmission. Neuropeptide Y (NPY) has a number of functions in vertebrate physiology and also plays important roles in different sensory systems. The Antarctic nototheniods are a monophyletic radiation of fishes that have evolved under the extreme environmental conditions of low light and cold, where non-visual sensory structures, such as LL, are of importance. In this study we describe the presence of NPY-like immunoreactivity (IR) in LL of the Antarctic nototheniod fish, Trematomus bernacchii Boulenger. Differences in size and cellular composition between the two neuromasts were in compliance with previous descriptions of these sensory organs. Despite structural and functional differences between canal and superficial neuromasts, the distribution of NPY-like IR was similar within both the receptors classes. In particular, NPY IR was observed in all three cell types which constitute these sensory organs, allowing us to hypothesize the involvement of this molecule in the processing of the sensory information.

Fgf and Sdf-1 pathways interact during zebrafish fin regeneration

The chemokine stromal cell-derived factor-1 (SDF1) was originally identified as a pre-B cell stimulatory factor but has been recently implicated in several other key steps in differentiation and morphogenesis. In addition, SDF1 as well as FGF signalling pathways have recently been shown to be involved in the control of epimorphic regeneration. In this report, we address the question of a possible interaction between the two signalling pathways during adult fin regeneration in zebrafish. Using a combination of pharmaceutical and genetic tools, we show that during epimorphic regeneration, expression of sdf1, as well as of its cognate receptors, cxcr4a, cxcr4b and cxcr7 are controlled by FGF signalling. We further show that, Sdf1a negatively regulates the expression of fgf20a. Together, these results lead us to propose that: 1) the function of Fgf in blastema formation is, at least in part, relayed by the chemokine Sdf1a, and that 2) Sdf1 exerts negative feedback on the Fgf pathway, which contributes to a transient expression of Fgf20a downstream genes at the beginning of regeneration. However this feedback control can be bypassed since the Sdf1 null mutants regenerate their fin, though slower. Very few mutants for the regeneration process were isolated so far, illustrating the difficulty in identifying genes that are indispensable for regeneration. This observation supports the idea that the regeneration process involves a delicate balance between multiple pathways.

Phoenix is required for mechanosensory hair cell regeneration in the zebrafish lateral line

In humans, the absence or irreversible loss of hair cells, the sensory mechanoreceptors in the cochlea, accounts for a large majority of acquired and congenital hearing disorders. In the auditory and vestibular neuroepithelia of the inner ear, hair cells are accompanied by another cell type called supporting cells. This second cell population has been described as having stem cell-like properties, allowing efficient hair cell replacement during embryonic and larval/fetal development of all vertebrates. However, mammals lose their regenerative capacity in most inner ear neuroepithelia in postnatal life. Remarkably, reptiles, birds, amphibians, and fish are different in that they can regenerate hair cells throughout their lifespan. The lateral line in amphibians and in fish is an additional sensory organ, which is used to detect water movements and is comprised of neuroepithelial patches, called neuromasts. These are similar in ultra-structure to the inner ear's neuroepithelia and they share the expression of various molecular markers. We examined the regeneration process in hair cells of the lateral line of zebrafish larvae carrying a retroviral integration in a previously uncharacterized gene, phoenix (pho). Phoenix mutant larvae develop normally and display a morphologically intact lateral line. However, after ablation of hair cells with copper or neomycin, their regeneration in pho mutants is severely impaired. We show that proliferation in the supporting cells is strongly decreased after damage to hair cells and correlates with the reduction of newly formed hair cells in the regenerating phoenix mutant neuromasts. The retroviral integration linked to the phenotype is in a novel gene with no known homologs showing high expression in neuromast supporting cells. Whereas its role during early development of the lateral line remains to be addressed, in later larval stages phoenix defines a new class of proteins implicated in hair cell regeneration.

Feathers and fins: non-mammalian models for hair cell regeneration

Death of mechanosensory cells in the inner ear results in two profound disabilities: hearing loss and balance disorders. Although mammals lack the capacity to regenerate hair cells, recent studies in mice and other rodents have offered valuable insight into strategies for stimulating hair cell regeneration in mammals. Investigations of model organisms that retain the ability to form new hair cells after embryogenesis, such as fish and birds, are equally important and have provided clues as to the cellular and molecular mechanisms that may block hair cell regeneration in mammals. Here, we summarize studies on hair cell regeneration in the chicken and the zebrafish, discuss specific advantages of each model, and propose future directions for the use of non-mammalian models in understanding hair cell regeneration.

Afferent neurons of the zebrafish lateral line are strict selectors of hair-cell orientation

Hair cells in the inner ear display a characteristic polarization of their apical stereocilia across the plane of the sensory epithelium. This planar orientation allows coherent transduction of mechanical stimuli because the axis of morphological polarity of the stereocilia corresponds to the direction of excitability of the hair cells. Neuromasts of the lateral line in fishes and amphibians form two intermingled populations of hair cells oriented at 180° relative to each other, however, creating a stimulus-polarity ambiguity. Therefore, it is unknown how these animals resolve the vectorial component of a mechanical stimulus. Using genetic mosaics and live imaging in transgenic zebrafish to visualize hair cells and neurons at single-cell resolution, we show that lateral-line afferents can recognize the planar polarization of hair cells. Each neuron forms synapses with hair cells of identical orientation to divide the neuromast into functional planar-polarity compartments. We also show that afferent neurons are strict selectors of polarity that can re-establish synapses with identically oriented targets during hair-cell regeneration. Our results provide the anatomical bases for the physiological models of signal-polarity resolution by the lateral line.

Apical membrane maturation and cellular rosette formation during morphogenesis of the zebrafish lateral line

Tissue morphogenesis and cell sorting are major forces during organ development. Here, we characterize the process of tissue morphogenesis within the zebrafish lateral line primordium, a migratory sheet of cells that gives rise to the neuromasts of the posterior lateral line organ. We find that cells within this epithelial tissue constrict actin-rich membranes and enrich apical junction proteins at apical focal points. The coordinated apical membrane constriction in single Delta D-positive hair cell progenitors and in their neighbouring prospective support cells generates cellular rosettes. Live imaging reveals that cellular rosettes subsequently separate from each other and give rise to individual neuromasts. Genetic analysis uncovers an involvement of Lethal giant larvae proteins in the maturation of apical junction belts during cellular rosette formation. Our findings suggest that apical constriction of cell membranes spatially confines regions of strong cell-cell adhesion and restricts the number of tightly interconnected cells into cellular rosettes, which ensures the correct deposition of neuromasts during morphogenesis of the posterior lateral line organ.

Regulated adhesion as a driving force of gastrulation movements

Recent data have reinforced the fundamental role of regulated cell adhesion as a force that drives morphogenesis during gastrulation. As we discuss, cell adhesion is required for all modes of gastrulation movements in all organisms. It can even be instructive in nature, but it must be tightly and dynamically regulated. The picture that emerges from the recent findings that we review here is that different modes of gastrulation movements use the same principles of adhesion regulation, while adhesion molecules themselves coordinate the intra- and extracellular changes required for directed cell locomotion.

Development of diverse lateral line patterns on the teleost caudal fin

The lateral line is composed of mechanoreceptors, the neuromasts, which are distributed over the body surfaces of fish. We examine the development of neuromast patterns on the caudal fins of medaka and zebrafish. In medaka, the terminal neuromast is established just prior to the caudal fin formation. The terminal neuromast subsequently gives rise to a cluster of accessory neuromasts. In zebrafish, the terminal neuromasts vary in terms of both number and position, and they achieve their final positions relative to the caudal fin structures through migration. Subsequently, they give rise to four lines of accessory neuromasts that extend along the caudal fin. We show that developmental processes similar to those observed in medaka and zebrafish may account for a large variety of patterns in other teleost species. These results establish terminal neuromast patterning as a new model for the study of the developmental mechanisms underlying diverse lateral line patterns.

Do vertebrate neural crest and cranial placodes have a common evolutionary origin?

Two embryonic tissues-the neural crest and the cranial placodes-give rise to most evolutionary novelties of the vertebrate head. These two tissues develop similarly in several respects: they originate from ectoderm at the neural plate border, give rise to migratory cells and develop into multiple cell fates including sensory neurons. These similarities, and the joint appearance of both tissues in the vertebrate lineage, may point to a common evolutionary origin of neural crest and placodes from a specialized population of neural plate border cells. However, a review of the developmental mechanisms underlying the induction, specification, migration and cytodifferentiation of neural crest and placodes reveals fundamental differences between the tissues. Taken together with insights from recent studies in tunicates and amphioxus, this suggests that neural crest and placodes have an independent evolutionary origin and that they evolved from the neural and non-neural side of the neural plate border, respectively.

In migrating cells, the Golgi complex and the position of the centrosome depend on geometrical constraints of the substratum

Although cells migrate in a constrained 3D environment in vivo, in-vitro studies have mainly focused on the analysis of cells moving on 2D substrates. Under such conditions, the Golgi complex is always located towards the leading edge of the cell, suggesting that it is involved in the directional movement. However, several lines of evidence indicate that this location can vary depending on the cell type, the environment or the developmental processes. We have used micro contact printing (μCP) to study the migration of cells that have a geometrically constrained shape within a polarized phenotype. Cells migrating on micropatterned lines of fibronectin are polarized and migrate in the same direction. Under such conditions, the Golgi complex and the centrosome are located behind the nucleus. In addition, the Golgi complex is often displaced several micrometres away from the nucleus. Finally, we used the zebrafish lateral line primordium as an in-vivo model of cells migrating in a constrained environment and observe a similar localization of both the Golgi and the centrosome in the leading cells. We propose that the positioning of the Golgi complex and the centrosome depends on the geometrical constraints applied to the cell rather than on a precise migratory function in the leading region.

FGF-dependent mechanosensory organ patterning in zebrafish

During development, organ primordia reorganize to form repeated functional units. In zebrafish (Danio rerio), mechanosensory organs called neuromasts are deposited at regular intervals by the migrating posterior lateral line (pLL) primordium. The pLL primordium is organized into polarized rosettes representing proto-neuromasts, each with a central atoh1a-positive focus of mechanosensory precursors. We show that rosettes form cyclically from a progenitor pool at the leading zone of the primordium as neuromasts are deposited from the trailing region. fgf3/10 signals localized to the leading zone are required for rosette formation, atoh1a expression, and primordium migration. We propose that the fibroblast growth factor (FGF) source controls primordium organization, which, in turn, regulates the periodicity of neuromast deposition. This previously unrecognized mechanism may be applicable to understanding segmentation and morphogenesis in other organ systems.

Dynamic Fgf signaling couples morphogenesis and migration in the zebrafish lateral line primordium

The collective migration of cells in the form of cohesive tissues is a hallmark of both morphogenesis and repair. The extrinsic cues that direct these complex migrations usually act by regulating the dynamics of a specific subset of cells, those at the leading edge. Given that normally the function of tissue migration is to lay down multicellular structures, such as branched epithelial networks or sensory organs, it is surprising how little is known about the mechanisms that organize cells behind the leading edge. Cells of the zebrafish lateral line primordium switch from mesenchyme-like leader cells to epithelial rosettes that develop into mechanosensory organs. Here, we show that this transition is regulated by an Fgf signaling circuit that is active within the migrating primordium. Point sources of Fgf ligand drive surrounding cells towards a ;non-leader' fate by increasing their epithelial character, a prerequisite for rosette formation. We demonstrate that the dynamic expression of Fgf ligands determines the spatiotemporal pattern of epithelialization underlying sensory organ formation in the lateral line. Furthermore, this work uncovers a surprising link between internal tissue organization and collective migration.

Tbx2b is required for the development of the parapineal organ

Structural differences between the left and right sides of the brain exist throughout the vertebrate lineage. By studying the zebrafish pineal complex, which exhibits notable asymmetries, both the genes and the cell movements that result in left-right differences can be characterized. The pineal complex consists of the midline pineal organ and the left-sided parapineal organ. The parapineal is responsible for instructing the asymmetric architecture of the bilateral habenulae, the brain nuclei that flank the pineal complex. Using in vivo time-lapse confocal microscopy, we find that the cells that form the parapineal organ migrate as a cluster of cells from the pineal complex anlage to the left side of the brain. In a screen for mutations that disrupted brain laterality, we identified a nonsense mutation in the T-box2b (tbx2b) gene, which encodes a transcription factor expressed in the pineal complex anlage. The tbx2b mutant makes fewer parapineal cells, and they remain as individuals near the midline rather than migrating leftward as a group. The reduced number and incorrect placement of parapineal cells result in symmetric development of the adjacent habenular nuclei. We conclude that tbx2b functions to specify the correct number of parapineal cells and to regulate their asymmetric migration.