Imaging brain development and organogenesis in zebrafish using immobilized embryonic explants

Live Imaging Transverse Sections of Zebrafish Embryo Explants

Vertebrate embryogenesis is a highly dynamic process involving coordinated cell and tissue movements that generate the final embryonic body plan. Many of these movements are difficult to image at high resolution because they occur deep within the embryo along the midline, causing light scattering and requiring longer working distances. Here, we present an explant-based method to image transverse cross sections of living zebrafish embryos. This method allows for the capture of all cell movements at high-resolution throughout the embryonic trunk, including hard-to-image deep tissues. This technique offers an alternative to expensive or computationally difficult microscopy methods. Key features • Generates intact zebrafish explants with minimal tissue disturbance. • Allows for live imaging of deep tissues normally obscured by common confocal microscopy techniques. • Immobilizes tissues for extended periods required for time-lapse imaging. • Utilizes readily available reagents and tools, which can minimize the time and cost of the procedure.

Flat mount preparation for whole-mount fluorescent imaging of zebrafish embryos

The zebrafish is a widely used model organism for biomedical research due to its ease of maintenance, external fertilization of embryos, rapid embryonic development, and availability of established genetic tools. One notable advantage of using zebrafish is the transparency of the embryos, which enables high-resolution imaging of specific cells, tissues, and structures through the use of transgenic and knock-in lines. However, as the embryo develops, multiple layers of tissue wrap around the lipid-enriched yolk, which can create a challenge to image tissues located deep within the embryo. While various methods are available, such as two-photon imaging, cryosectioning, vibratome sectioning, and micro-surgery, each of these has limitations. In this study, we present a novel deyolking method that allows for high-quality imaging of tissues that are obscured by other tissues and the yolk. Embryos are lightly fixed in 1% PFA to remove the yolk without damaging embryonic tissues and are then refixed in 4% PFA and mounted on custom-made bridged slides. This method offers a simple way to prepare imaging samples that can be subjected to further preparation, such as immunostaining. Furthermore, the bridged slides described in this study can be used for imaging tissue and organ preparations from various model organisms.

The sensitivity of the zebrafish embryo coiling assay for the detection of neurotoxicity by compounds with diverse modes of action

In the aim to determine neurotoxicity, new methods are being validated, including tests and test batteries comprising in vitro and in vivo approaches. Alternative test models such as the zebrafish (Danio rerio) embryo have received increasing attention, with minor modifications of the fish embryo toxicity test (FET; OECD TG 236) as a tool to assess behavioral endpoints related to neurotoxicity during early developmental stages. The spontaneous tail movement assay, also known as coiling assay, assesses the development of random movement into complex behavioral patterns and has proven sensitive to acetylcholine esterase inhibitors at sublethal concentrations. The present study explored the sensitivity of the assay to neurotoxicants with other modes of action (MoAs). Here, five compounds with diverse MoAs were tested at sublethal concentrations: acrylamide, carbaryl, hexachlorophene, ibuprofen, and rotenone. While carbaryl, hexachlorophene, and rotenone consistently induced severe behavioral alterations by ~ 30 h post fertilization (hpf), acrylamide and ibuprofen expressed time- and/or concentration-dependent effects. At 37-38 hpf, additional observations revealed behavioral changes during dark phases with a strict concentration-dependency. The study documented the applicability of the coiling assay to MoA-dependent behavioral alterations at sublethal concentrations, underlining its potential as a component of a neurotoxicity test battery.

Deconstructing body axis morphogenesis in zebrafish embryos using robot-assisted tissue micromanipulation

Classic microsurgical techniques, such as those used in the early 1900s by Mangold and Spemann, have been instrumental in advancing our understanding of embryonic development. However, these techniques are highly specialized, leading to issues of inter-operator variability. Here we introduce a user-friendly robotic microsurgery platform that allows precise mechanical manipulation of soft tissues in zebrafish embryos. Using our platform, we reproducibly targeted precise regions of tail explants, and quantified the response in real-time by following notochord and presomitic mesoderm (PSM) morphogenesis and segmentation clock dynamics during vertebrate anteroposterior axis elongation. We find an extension force generated through the posterior notochord that is strong enough to buckle the structure. Our data suggest that this force generates a unidirectional notochord extension towards the tailbud because PSM tissue around the posterior notochord does not let it slide anteriorly. These results complement existing biomechanical models of axis elongation, revealing a critical coupling between the posterior notochord, the tailbud, and the PSM, and show that somite patterning is robust against structural perturbations.

A 3-D Tail Explant Culture to Study Vertebrate Segmentation in Zebrafish

Vertebrate embryos pattern their major body axis as repetitive somites, the precursors of vertebrae, muscle, and skin. Somites progressively segment from the presomitic mesoderm (PSM) as the tail end of the embryo elongates posteriorly. Somites form with regular periodicity and scale in size. Zebrafish is a popular model organism as it is genetically tractable and has transparent embryos that allow for live imaging. Nevertheless, during somitogenesis, fish embryos are wrapped around a large, rounding yolk. This geometry limits live imaging of PSM tissue in zebrafish embryos, particularly at higher resolutions that require a close objective working distance. Here, we present a flattened 3-D tissue culture method for live imaging of zebrafish tail explants. Tail explants mimic intact embryos by displaying a proportional slowdown of axis elongation and shortening of rostrocaudal somite lengths. We are further able to stall axis elongation speed through explant culture. This, for the first time, enables us to untangle the chemical input of signaling gradients from the mechanistic input of axial elongation. In future studies, this method can be combined with a microfluidic setup to allow time-controlled pharmaceutical perturbations or screening of vertebrate segmentation without any drug penetration concerns.

The frontier of live tissue imaging across space and time

What you see is what you get-imaging techniques have long been essential for visualization and understanding of tissue development, homeostasis, and regeneration, which are driven by stem cell self-renewal and differentiation. Advances in molecular and tissue modeling techniques in the last decade are providing new imaging modalities to explore tissue heterogeneity and plasticity. Here we describe current state-of-the-art imaging modalities for tissue research at multiple scales, with a focus on explaining key tradeoffs such as spatial resolution, penetration depth, capture time/frequency, and moieties. We explore emerging tissue modeling and molecular tools that improve resolution, specificity, and throughput.

Novel roles for the radial spoke head protein 9 in neural and neurosensory cilia

Cilia are cell surface organelles with key roles in a range of cellular processes, including generation of fluid flow by motile cilia. The axonemes of motile cilia and immotile kinocilia contain 9 peripheral microtubule doublets, a central microtubule pair, and 9 connecting radial spokes. Aberrant radial spoke components RSPH1, 3, 4a and 9 have been linked with primary ciliary dyskinesia (PCD), a disorder characterized by ciliary dysmotility; yet, radial spoke functions remain unclear. Here we show that zebrafish Rsph9 is expressed in cells bearing motile cilia and kinocilia, and localizes to both 9 + 2 and 9 + 0 ciliary axonemes. Using CRISPR mutagenesis, we show that rsph9 is required for motility of presumptive 9 + 2 olfactory cilia and, unexpectedly, 9 + 0 neural cilia. rsph9 is also required for the structural integrity of 9 + 2 and 9 + 0 ciliary axonemes. rsph9 mutant larvae exhibit reduced initiation of the acoustic startle response consistent with hearing impairment, suggesting a novel role for Rsph9 in the kinocilia of the inner ear and/or lateral line neuromasts. These data identify novel roles for Rsph9 in 9 + 0 motile cilia and in sensory kinocilia, and establish a useful zebrafish PCD model.

Persistence, period and precision of autonomous cellular oscillators from the zebrafish segmentation clock

In vertebrate development, the sequential and rhythmic segmentation of the body axis is regulated by a “segmentation clock”. This clock is comprised of a population of coordinated oscillating cells that together produce rhythmic gene expression patterns in the embryo. Whether individual cells autonomously maintain oscillations, or whether oscillations depend on signals from neighboring cells is unknown. Using a transgenic zebrafish reporter line for the cyclic transcription factor Her1, we recorded single tailbud cells in vitro. We demonstrate that individual cells can behave as autonomous cellular oscillators. We described the observed variability in cell behavior using a theory of generic oscillators with correlated noise. Single cells have longer periods and lower precision than the tissue, highlighting the role of collective processes in the segmentation clock. Our work reveals a population of cells from the zebrafish segmentation clock that behave as self-sustained, autonomous oscillators with distinctive noisy dynamics.

DOI:http://dx.doi.org/10.7554/eLife.08438.001

The timing and pattern of gene activity in cells can be very important. For example, precise gene activity patterns in 24-hour circadian clocks help to set daily cycles of rest and activity in organisms. In such scenarios, cells often communicate with each other to coordinate the activity of their genes. To fully understand how the behavior of the population emerges, scientists must first understand the gene activity patterns in individual cells.

Rhythmic gene activity is essential for the spinal column to form in fish and other vertebrate embryos. A group of cells that switch genes on/off in a coordinated pattern act like a clock to regulate the timing of the various steps in the process of backbone formation. However, it is not clear if each cell is able to maintain a rhythm of gene expression on their own, or whether they rely on messages from neighboring cells to achieve it.

Now, Webb et al. use time-lapse videos of individual cells isolated from the tail of zebrafish embryos to show that each cell can maintain a pattern of rhythmic activity in a gene called Her1. In the experiments, individual cells were removed from zebrafish and placed under a microscope to record and track the activity of Her1 over time using fluorescent proteins. These experiments show that each cell is able to maintain a rhythmic pattern of Her1 expression on its own.

Webb et al. then compared the Her1 activity patterns in individual cells with the Her1 patterns present in a larger piece of zebrafish tissue. The experiments showed that the rhythms in the individual cells are slower and less precise in their timing than in the tissue. This suggests that groups of cells must work together to create the synchronized rhythms of gene expression with the right precision and timing needed for the spinal column to be patterned correctly.

In the future, further experiment with these cells will allow researchers to investigate the genetic basis of the rhythms in single cells, and find out how individual cells work together with their neighbors to allow tissues to work properly.

DOI:http://dx.doi.org/10.7554/eLife.08438.002

Capturing tissue repair in zebrafish larvae with time-lapse brightfield stereomicroscopy

The zebrafish larval tail fin is ideal for studying tissue regeneration due to the simple architecture of the larval fin-fold, which comprises of two layers of skin that enclose undifferentiated mesenchyme, and because the larval tail fin regenerates rapidly within 2-3 days. Using this system, we demonstrate a method for capturing the repair dynamics of the amputated tail fin with time-lapse video brightfield stereomicroscopy. We demonstrate that fin amputation triggers a contraction of the amputation wound and extrusion of cells around the wound margin, leading to their subsequent clearance. Fin regeneration proceeds from proximal to distal direction after a short delay. In addition, developmental growth of the larva can be observed during all stages. The presented method provides an opportunity for observing and analyzing whole tissue-scale behaviors such as fin development and growth in a simple microscope setting, which is easily adaptable to any stereomicroscope with time-lapse capabilities.

Zebrafish WNK lysine deficient protein kinase 1 (wnk1) affects angiogenesis associated with VEGF signaling

The WNK1 (WNK lysine deficient protein kinase 1) protein is a serine/threonine protein kinase with emerging roles in cancer. WNK1 causes hypertension and hyperkalemia when overexpressed and cardiovascular defects when ablated in mice. In this study, the role of Wnk1 in angiogenesis was explored using the zebrafish model. There are two zebrafish wnk1 isoforms, wnk1a and wnk1b, and both contain all the functional domains found in the human WNK1 protein. Both isoforms are expressed in the embryo at the initiation of angiogenesis and in the posterior cardinal vein (PCV), similar to fms-related tyrosine kinase 4 (flt4). Using morpholino antisense oligonucleotides against wnk1a and wnk1b, we observed that wnk1 morphants have defects in angiogenesis in the head and trunk, similar to flk1/vegfr2 morphants. Furthermore, both wnk1a and wnk1b mRNA can partially rescue the defects in vascular formation caused by flk1/vegfr2 knockdown. Mutation of the kinase domain or the Akt/PI3K phosphorylation site within wnk1 destroys this rescue capability. The rescue experiments provide evidence that wnk1 is a downstream target for Vegfr2 (vascular endothelial growth factor receptor-2) and Akt/PI3K signaling and thereby affects angiogenesis in zebrafish embryos. Furthermore, we found that knockdown of vascular endothelial growth factor receptor-2 (flk1/vegfr2) or vascular endothelial growth factor receptor-3 (flt4/vegfr3) results in a decrease in wnk1a expression, as assessed by insitu hybridization and q-RT-PCR analysis. Thus, the Vegf/Vegfr signaling pathway controls angiogenesis in zebrafish via Akt kinase-mediated phosphorylation and activation of Wnk1 as well as transcriptional regulation of wnk1 expression.

Dissection and lateral mounting of zebrafish embryos: analysis of spinal cord development

The zebrafish spinal cord is an effective investigative model for nervous system research for several reasons. First, genetic, transgenic and gene knockdown approaches can be utilized to examine the molecular mechanisms underlying nervous system development. Second, large clutches of developmentally synchronized embryos provide large experimental sample sizes. Third, the optical clarity of the zebrafish embryo permits researchers to visualize progenitor, glial, and neuronal populations. Although zebrafish embryos are transparent, specimen thickness can impede effective microscopic visualization. One reason for this is the tandem development of the spinal cord and overlying somite tissue. Another reason is the large yolk ball, which is still present during periods of early neurogenesis. In this article, we demonstrate microdissection and removal of the yolk in fixed embryos, which allows microscopic visualization while preserving surrounding somite tissue. We also demonstrate semipermanent mounting of zebrafish embryos. This permits observation of neurodevelopment in the dorso-ventral and anterior-posterior axes, as it preserves the three-dimensionality of the tissue.

Studying cell behavior in whole zebrafish embryos by confocal live imaging: application to hematopoietic stem cells

Confocal live imaging is a key tool for studying cell behavior in the whole zebrafish embryo. Here we provide a detailed protocol that is adaptable for imaging any progenitor cell behavior in live zebrafish embryos. As an example, we imaged the emergence of the first hematopoietic stem cells from the aorta. We discuss the importance of selecting the appropriate zebrafish transgenic line as well as methods for immobilization of embryos to be imaged. In addition, we highlight the confocal microscopy acquisition parameters required for stem cell imaging and the software tools we used to analyze 4D movies. The whole protocol takes 2 h 15 min and allows confocal live imaging from a few hours to several days.

Requirement for Pdx1 in specification of latent endocrine progenitors in zebrafish

Background

Insulin-producing beta cells emerge during pancreas development in two sequential waves. Recently described later-forming beta cells in zebrafish show high similarity to second wave mammalian beta cells in developmental capacity. Loss-of-function studies in mouse and zebrafish demonstrated that the homeobox transcription factors Pdx1 and Hb9 are both critical for pancreas and beta cell development and discrete stage-specific requirements for these genes have been uncovered. Previously, exocrine and endocrine cell recovery was shown to follow loss of pdx1 in zebrafish, but the progenitor cells and molecular mechanisms responsible have not been clearly defined. In addition, interactions of pdx1 and hb9 in beta cell formation have not been addressed.

Results

To learn more about endocrine progenitor specification, we examined beta cell formation following morpholino-mediated depletion of pdx1 and hb9. We find that after early beta cell reduction, recovery occurs following loss of either pdx1 or hb9 function. Unexpectedly, simultaneous knockdown of both hb9 and pdx1 leads to virtually complete and persistent beta cell deficiency. We used a NeuroD:EGFP transgenic line to examine endocrine cell behavior in vivo and developed a novel live-imaging technique to document emergence and migration of late-forming endocrine precursors in real time. Our data show that Notch-responsive progenitors for late-arising endocrine cells are predominantly post mitotic and depend on pdx1. By contrast, early-arising endocrine cells are specified and differentiate independent of pdx1.

Conclusions

The nearly complete beta cell deficiency after combined loss of hb9 and pdx1 suggests functional cooperation, which we clarify as distinct roles in early and late endocrine cell formation. A novel imaging approach permitted visualization of the emergence of late endocrine cells within developing embryos for the first time. We demonstrate a pdx1-dependent progenitor population essential for the formation of duct-associated, second wave endocrine cells. We further reveal an unexpectedly low mitotic activity in these progenitor cells, indicating that they are set aside early in development.

Tissue micromanipulation in zebrafish embryos

Although a common approach in large vertebrate embryos such as chick or frog, manipulation at the tissue level is only rarely applied to zebrafish embryos. Despite its relatively small size, the zebrafish embryo can be readily used for micromanipulations such as tissue and organ primordium transplantation, explantation, and microbead implantation, to study inductive tissue interactions and tissue autonomy of pleiotropic, mutant phenotypes or to isolate tissue for organotypic and primary cell culture or RNA isolation. Since this requires special handling techniques, tools, and tricks, which are rarely published and thus difficult to apply without hands-on demonstration, this article provides detailed instructions and protocols on tissue micromanipulation. The goal is to introduce a broader scientific audience to these surgical techniques, which can be applied to a wide range of questions and used as a starting point for many downstream applications in the genetically tractable zebrafish embryo.

Regionally autonomous segmentation within zebrafish presomitic mesoderm

Similar to chick, mouse, and Xenopus, a number of intercellular signaling pathways are required for zebrafish segmentation. However, the spatial scales over which these signaling pathways operate in zebrafish remain largely unknown. During zebrafish segmentation, waves of her1 transcription (a) initiate within the tailbud region, (b) propagate anteriorly through presomitic mesoderm (PSM), and (c) terminate in the anlage of newly-forming somites. These observations raise the question of whether the tailbud region serves as a "pacemaker" or "organizing center" for the initiation of propagating her1 expression waves. Microsurgical manipulations reveal that the anteriorly waves of her1 transcription are not perturbed by removal of the zebrafish tailbud. Furthermore, expression patterns of deltaD, paraxial protocadherin C (papc), and myoD within recently formed somites also appear to be relatively unperturbed by either removal of the tailbud, or by removal of lateral plate mesoderm. Although dynamic gene networks rapidly specify mesenchymal or epithelial cellular identities within forming somites, this specification is plastic. Time-lapse analysis has shown that the cellular progeny of mitotically-active epithelial border cells within newly formed somites can adopt different cellular identities than their precursor cells. Overall, these results indicate that sustained long-range intercellular communication with the tailbud, anterior somites, or lateral plate mesoderm is not necessary for segmentation or somitogenesis to proceed within the PSM. The basic segmentation and somitogenesis processes in zebrafish presomitic mesoderm appear to be largely regionally autonomous and governed by local morphogenetic cell behaviors.

Transient axonal glycoprotein-1 (TAG-1) and laminin-alpha1 regulate dynamic growth cone behaviors and initial axon direction in vivo

BACKGROUND

How axon guidance signals regulate growth cone behavior and guidance decisions in the complex in vivo environment of the central nervous system is not well understood. We have taken advantage of the unique features of the zebrafish embryo to visualize dynamic growth cone behaviors and analyze guidance mechanisms of axons emerging from a central brain nucleus in vivo.

RESULTS

We investigated axons of the nucleus of the medial longitudinal fascicle (nucMLF), which are the first axons to extend in the zebrafish midbrain. Using in vivo time-lapse imaging, we show that both positive axon-axon interactions and guidance by surrounding tissue control initial nucMLF axon guidance. We further show that two guidance molecules, transient axonal glycoprotein-1 (TAG-1) and laminin-alpha1, are essential for the initial directional extension of nucMLF axons and their subsequent convergence into a tight fascicle. Fixed tissue analysis shows that TAG-1 knockdown causes errors in nucMLF axon pathfinding similar to those seen in a laminin-alpha1 mutant. However, in vivo time-lapse imaging reveals that while some defects in dynamic growth cone behavior are similar, there are also defects unique to the loss of each gene. Loss of either TAG-1 or laminin-alpha1 causes nucMLF axons to extend into surrounding tissue in incorrect directions and reduces axonal growth rate, resulting in stunted nucMLF axons that fail to extend beyond the hindbrain. However, defects in axon-axon interactions were found only after TAG-1 knockdown, while defects in initial nucMLF axon polarity and excessive branching of nucMLF axons occurred only in laminin-alpha1 mutants.

CONCLUSION

These results demonstrate how two guidance cues, TAG-1 and laminin-alpha1, influence the behavior of growth cones during axon pathfinding in vivo. Our data suggest that TAG-1 functions to allow growth cones to sense environmental cues and mediates positive axon-axon interactions. Laminin-alpha1 does not regulate axon-axon interactions, but does influence neuronal polarity and directional guidance.

Semaphorin3D regulates axon axon interactions by modulating levels of L1 cell adhesion molecule

The decision of a growing axon to selectively fasciculate with and defasciculate from other axons is critical for axon pathfinding and target innervation. Fasciculation can be regulated by cell adhesion molecules that modulate interaxonal adhesion and repulsive molecules, expressed by surrounding tissues that channel axons together. Here we describe crosstalk between molecules that mediate these mechanisms. We show that Semaphorin3D (Sema3D), a classic repulsive molecule, promotes fasciculation by regulating L1 CAM levels and axon-axon interactions rather than by creating a repulsive surround. Knockdown experiments show that Sema3D and L1 genetically interact to promote fasciculation. Sema3D overexpression increases and Sema3D knockdown decreases levels of axonal L1 protein. Moreover, excess L1 rescues defasciculation caused by the loss of Sema3D. In vivo time-lapse imaging reveals that Sema3D or L1 knockdown cause identical defects in growth cone behaviors during axon-axon interactions, consistent with a loss of adhesion. These results reveal a novel mechanism by which a semaphorin promotes fasciculation and modulates axon-axon interactions by regulating an adhesion molecule.

Frizzled3a and Celsr2 function in the neuroepithelium to regulate migration of facial motor neurons in the developing zebrafish hindbrain

Migration of neurons from their birthplace to their final target area is a crucial step in brain development. Here, we show that expression of the off-limits/frizzled3a (olt/fz3a) and off-road/celsr2 (ord/celsr2) genes in neuroepithelial cells maintains the facial (nVII) motor neurons near the pial surface during their caudal migration in the zebrafish hindbrain. In the absence of olt/fz3a expression in the neuroepithelium, nVII motor neurons extended aberrant radial processes towards the ventricular surface and mismigrated radially to the dorsomedial part of the hindbrain. Our findings reveal a novel role for these genes, distinctive from their already known functions, in the regulation of the planar cell polarity (i.e. preventing integration of differentiated neurons into the neuroepithelial layer). This contrasts markedly with their reported role in reintegration of neuroepithelial daughter cells into the neuroepithelial layer after cell division.

Head size constrains forebrain development and evolution in ray-finned fishes

In ray-finned fishes, which comprise nearly half of all vertebrate species, the telencephalon does not evaginate, as it does in other vertebrates, but instead everts. No detailed explanation for this species difference has ever been offered. Here we propose that telencephalic eversion evolved because ray-finned fish embryos are so small that their telencephalon cannot evaginate but must, instead, squeeze into the space just dorsal to the developing nasal epithelia and rostral to the eyes-morphogenetic movements that amount to eversion. Evidence for this hypothesis derives from cladistic analyses, which show that early ray-finned fishes reduced their adult body size and adopted a novel reproductive strategy, based on the production of myriad minute young. Because body size tends to be inversely proportional to brain:body ratio, this phylogenetic reduction in body size implies that embryonic ray-finned fishes should have proportionately larger brains than embryos of species whose telencephalons evaginate. This prediction was confirmed by comparing serially sectioned heads of representative ray-finned and cartilaginous fish embryos at several stages of development. The brain, excluding its ventricles, occupies 36-46% of the cranial cavity in embryonic ray-finned fishes, but less than 20% in embryonic sharks. Moreover, three-dimensional reconstructions show that in embryonic ray-finned fishes the telencephalon has no room for a full-fledged evagination; instead, it spreads into the spaces just dorsal and caudal to the developing nasal epithelia. These morphogenetic movements, in conjunction with a thinning of the forebrain roof, generate telencephalic eversion.

Live imaging of fluorescent proteins in chordate embryos: from ascidians to mice

Although we have advanced in our understanding of the molecular mechanisms intrinsic to the morphogenesis of chordate embryos, the question of how individual developmental events are integrated to generate the final morphological form is still unresolved. Microscopic observation is a pivotal tool in developmental biology, both for determining the normal course of events and for contrasting this with the results of experimental and pathological perturbations. Since embryonic development takes place in three dimensions over time, to fully understand the events required to build an embryo we must investigate embryo morphogenesis in multiple dimensions in situ. Recent advances in the isolation of naturally fluorescent proteins, and the refinement of techniques for in vivo microscopy offer unprecedented opportunities to study the cellular and molecular events within living, intact embryos using optical imaging. These technologies allow direct visual access to complex events as they happen in their native environment, and thus provide greater insights into cell behaviors operating during embryonic development. Since most fluorescent protein probes and modes of data acquisition are common across species, we have chosen the mouse and the ascidian, two model organisms at opposite ends of the chordate clade, to review the use of some of the current genetically-encoded fluorescent proteins and their visualization in vivo in living embryos for the generation of high-resolution imaging data.

Lateral line hair cell maturation is a determinant of aminoglycoside susceptibility in zebrafish (Danio rerio)

Developmental differences in hair cell susceptibility to aminoglycoside-induced cell death has been observed in multiple species. Increased sensitivity to aminoglycosides has been temporally correlated with the onset of mechanotransduction-dependent activity. We have used in vivo fluorescent vital dye markers to further investigate the determinants of aminoglycoside induced hair cell death in the lateral line of zebrafish (Danio rerio). Labeling hair cells of the lateral line in vivo with the dyes FM 1-43, To-Pro-3, and Yo-Pro-1 served as reliable indicators of hair cell viability. Results indicate that hair cell maturation is a determinant of developmental differences in susceptibility. The age dependent differences in susceptibility to aminoglycosides are independent of the onset of mechanotransduction-dependent activity as measured by FM 1-43 uptake and independent of hair cell ability to take up fluorescently conjugated aminoglycosides.

Visualizing morphogenesis in transgenic zebrafish embryos using BODIPY TR methyl ester dye as a vital counterstain for GFP

Green fluorescent protein (GFP) technology is rapidly advancing the study of morphogenesis, by allowing researchers to specifically focus on a subset of labeled cells within the living embryo. However, when imaging GFP-labeled cells using confocal microscopy, it is often essential to simultaneously visualize all of the cells in the embryo using dual-channel fluorescence to provide an embryological context for the cells expressing GFP. Although various counterstains are available, part of their fluorescence overlaps with the GFP emission spectra, making it difficult to clearly identify the cells expressing GFP. In this study, we report that a new fluorophore, BODIPY TR methyl ester dye, serves as a versatile vital counterstain for visualizing the cellular dynamics of morphogenesis within living GFP transgenic zebrafish embryos. The fluorescence of this photostable synthetic dye is spectrally separate from GFP fluorescence, allowing dual-channel, three-dimensional (3D) and four-dimensional (4D) confocal image data sets of living specimens to be easily acquired. These image data sets can be rendered subsequently into uniquely informative 3D and 4D visualizations using computer-assisted visualization software. We discuss a variety of immediate and potential applications of BODIPY TR methyl ester dye as a vital visualization counterstain for GFP in transgenic zebrafish embryos.

Lineage restriction maintains a stable organizer cell population at the zebrafish midbrain-hindbrain boundary

The vertebrate hindbrain is subdivided into segments, termed neuromeres,that are units of gene expression, cell differentiation and behavior. A key property of such segments is that cells show a restricted ability to mix across segment borders – termed lineage restriction. In order to address segmentation in the midbrain-hindbrain boundary (mhb) region, we have analyzed single cell behavior in the living embryo by acquiring time-lapse movies of the developing mhb region in a transgenic zebrafish line. We traced the movement of hundreds of nuclei, and by matching their position with the expression of a midbrain marker, we demonstrate that midbrain and hindbrain cells arise from two distinct cell populations. Single cell labeling and analysis of the distribution of their progeny shows that lineage restriction is probably established during late gastrulation stages. Our findings suggest that segmentation as an organizing principle in early brain development can be extended to the mhb region. We argue that lineage restriction serves to constrain the position of the mhb organizer cell population.

Neuronal development and migration in zebrafish hindbrain explants

The zebrafish embryo is an excellent system for studying dynamic processes such as cell migration during vertebrate development. Dynamic analysis of neuronal migration in the zebrafish hindbrain has been hampered by morphogenetic movements in vivo, and by the impermeability of embryos. We have applied a recently reported technique of embryo explant culture to the analysis of neuronal development and migration in the zebrafish hindbrain. We show that hindbrain explants prepared at the somitogenesis stage undergo normal morphogenesis for at least 14 h in culture. Importantly, several aspects of hindbrain development such as patterning, neurogenesis, axon guidance, and neuronal migration are largely unaffected, inspite of increased cell death in explanted tissue. These results suggest that hindbrain explant culture can be employed effectively in zebrafish to analyze neuronal migration and other dynamic processes using pharmacological and imaging techniques.

Time-lapse microscopy of brain development

Zebrafish embryos represent an ideal vertebrate model organism for noninvasive intravital imaging because of their optical clarity, external embryogenesis, and fast development. Many different labeling techniques have been adopted from other model organisms or newly developed to address a wealth of different developmental questions directly inside the living organism. The parallel advancements in the field of optical imaging let us now observe dynamic processes at the cellular and subcellular resolution. Combined with the repertoire of available surgical and genetic manipulations, zebrafish embryos provide the powerful and almost unique possibility to observe the interplay of molecular signals with cellular, morphological, and behavioral changes directly within a living and developing vertebrate organism. A bright future for zebrafish is yet to come, let there be light.