Patterning, morphogenesis, and neurogenesis of zebrafish cranial sensory placodes

Protocol to locally express cxcl12a during zebrafish olfactory organ development by combining IR-LEGO with live imaging

Temporal and spatial regulation of gene expression is crucial for proper embryonic development. Infrared laser-evoked gene operator (IR-LEGO) can provide information for various developmental processes. Here, we present a protocol to locally express cxcl12a during zebrafish olfactory organ development1 using a combination of IR-LEGO and live imaging. We describe steps for implementing IR-LEGO, biological sample preparation, live imaging, data collection, and analysis. This protocol can be applied to virtually any genetically modified experimental organism.

Intertissue mechanical interactions shape the olfactory circuit in zebrafish

While the chemical signals guiding neuronal migration and axon elongation have been extensively studied, the influence of mechanical cues on these processes remains poorly studied in vivo. Here, we investigate how mechanical forces exerted by surrounding tissues steer neuronal movements and axon extension during the morphogenesis of the olfactory placode in zebrafish. We mainly focus on the mechanical contribution of the adjacent eye tissue, which develops underneath the placode through extensive evagination and invagination movements. Using quantitative analysis of cell movements and biomechanical manipulations, we show that the developing eye exerts lateral traction forces on the olfactory placode through extracellular matrix, mediating proper morphogenetic movements and axon extension within the placode. Our data shed new light on the key participation of intertissue mechanical interactions in the sculpting of neuronal circuits.

Selective disruption of trigeminal sensory neurogenesis and differentiation in a mouse model of 22q11.2 deletion syndrome

22q11.2 Deletion Syndrome (22q11DS) is a neurodevelopmental disorder associated with cranial nerve anomalies and disordered oropharyngeal function, including pediatric dysphagia. Using the LgDel 22q11DS mouse model, we investigated whether sensory neuron differentiation in the trigeminal ganglion (CNgV), which is essential for normal orofacial function, is disrupted. We did not detect changes in cranial placode cell translocation or neural crest migration at early stages of LgDel CNgV development. However, as the ganglion coalesces, proportions of placode-derived LgDel CNgV cells increase relative to neural crest cells. In addition, local aggregation of placode-derived cells increases and aggregation of neural crest-derived cells decreases in LgDel CNgV. This change in cell-cell relationships was accompanied by altered proliferation of placode-derived cells at embryonic day (E)9.5, and premature neurogenesis from neural crest-derived precursors, reflected by an increased frequency of asymmetric neurogenic divisions for neural crest-derived precursors by E10.5. These early differences in LgDel CNgV genesis prefigure changes in sensory neuron differentiation and gene expression by postnatal day 8, when early signs of cranial nerve dysfunction associated with pediatric dysphagia are observed in LgDel mice. Apparently, 22q11 deletion destabilizes CNgV sensory neuron genesis and differentiation by increasing variability in cell-cell interaction, proliferation and sensory neuron differentiation. This early developmental divergence and its consequences may contribute to oropharyngeal dysfunction, including suckling, feeding and swallowing disruptions at birth, and additional orofacial sensory/motor deficits throughout life.

The evolutionary origins of the vertebrate olfactory system

Vertebrates develop an olfactory system that detects odorants and pheromones through their interaction with specialized cell surface receptors on olfactory sensory neurons. During development, the olfactory system forms from the olfactory placodes, specialized areas of the anterior ectoderm that share cellular and molecular properties with placodes involved in the development of other cranial senses. The early-diverging chordate lineages amphioxus, tunicates, lampreys and hagfishes give insight into how this system evolved. Here, we review olfactory system development and cell types in these lineages alongside chemosensory receptor gene evolution, integrating these data into a description of how the vertebrate olfactory system evolved. Some olfactory system cell types predate the vertebrates, as do some of the mechanisms specifying placodes, and it is likely these two were already connected in the common ancestor of vertebrates and tunicates. In stem vertebrates, this evolved into an organ system integrating additional tissues and morphogenetic processes defining distinct olfactory and adenohypophyseal components, followed by splitting of the ancestral placode to produce the characteristic paired olfactory organs of most modern vertebrates.

Diving into the streams and waves of constitutive and regenerative olfactory neurogenesis: insights from zebrafish

The olfactory system is renowned for its functional and structural plasticity, with both peripheral and central structures displaying persistent neurogenesis throughout life and exhibiting remarkable capacity for regenerative neurogenesis after damage. In general, fish are known for their extensive neurogenic ability, and the zebrafish in particular presents an attractive model to study plasticity and adult neurogenesis in the olfactory system because of its conserved structure, relative simplicity, rapid cell turnover, and preponderance of neurogenic niches. In this review, we present an overview of the anatomy of zebrafish olfactory structures, with a focus on the neurogenic niches in the olfactory epithelium, olfactory bulb, and ventral telencephalon. Constitutive and regenerative neurogenesis in both the peripheral olfactory organ and central olfactory bulb of zebrafish is reviewed in detail, and a summary of current knowledge about the cellular origin and molecular signals involved in regulating these processes is presented. While some features of physiologic and injury-induced neurogenic responses are similar, there are differences that indicate that regeneration is not simply a reiteration of the constitutive proliferation process. We provide comparisons to mammalian neurogenesis that reveal similarities and differences between species. Finally, we present a number of open questions that remain to be answered.

Cell-type heterogeneity in the early zebrafish olfactory epithelium is generated from progenitors within preplacodal ectoderm

The zebrafish olfactory epithelium comprises a variety of neuronal populations, which are thought to have distinct embryonic origins. For instance, while ciliated sensory neurons arise from preplacodal ectoderm (PPE), previous lineage tracing studies suggest that both Gonadotropin releasing hormone 3 (Gnrh3) and microvillous sensory neurons derive from cranial neural crest (CNC). We find that the expression of Islet1/2 is restricted to Gnrh3 neurons associated with the olfactory epithelium. Unexpectedly, however, we find no change in Islet1/2+ cell numbers in sox10 mutant embryos, calling into question their CNC origin. Lineage reconstruction based on backtracking in time-lapse confocal datasets, and confirmed by photoconversion experiments, reveals that Gnrh3 neurons derive from the anterior PPE. Similarly, all of the microvillous sensory neurons we have traced arise from preplacodal progenitors. Our results suggest that rather than originating from separate ectodermal populations, cell-type heterogeneity is generated from overlapping pools of progenitors within the preplacodal ectoderm.

Extrinsic mechanical forces mediate retrograde axon extension in a developing neuronal circuit

To form functional neural circuits, neurons migrate to their final destination and extend axons towards their targets. Whether and how these two processes are coordinated in vivo remains elusive. We use the zebrafish olfactory placode as a system to address the underlying mechanisms. Quantitative live imaging uncovers a choreography of directed cell movements that shapes the placode neuronal cluster: convergence of cells towards the centre of the placodal domain and lateral cell movements away from the brain. Axon formation is concomitant with lateral movements and occurs through an unexpected, retrograde mode of extension, where cell bodies move away from axon tips attached to the brain surface. Convergence movements are active, whereas cell body lateral displacements are of mainly passive nature, likely triggered by compression forces from converging neighbouring cells. These findings unravel a previously unknown mechanism of neuronal circuit formation, whereby extrinsic mechanical forces drive the retrograde extension of axons.How neuronal migration and axon growth coordinate during development is only partially understood. Here the authors use quantitative imaging to characterise the morphogenesis of the zebrafish olfactory placode and report an unexpected phenomenon, whereby axons extend through the passive movement of neuron cell bodies away from tethered axon tips.