The lateral-line neuromast of the zebrafish displays a restricted, consistent pattern of innervation that facilitates the comparison of microcircuits across individuals, developmental stages, and genotypes. We used serial blockface scanning electron microscopy to determine from multiple specimens the neuromast connectome, a comprehensive set of connections between hair cells and afferent and efferent nerve fibers. This analysis delineated a complex but consistent wiring pattern with three striking characteristics: each nerve terminal is highly specific in receiving innervation from hair cells of a single directional sensitivity; the innervation is redundant; and the terminals manifest a hierarchy of dominance. Mutation of the canonical planar-cell-polarity gene vangl2, which decouples the asymmetric phenotypes of sibling hair-cell pairs, results in randomly positioned, randomly oriented sibling cells that nonetheless retain specific wiring. Because larvae that overexpress Notch exhibit uniformly oriented, uniformly innervating hair-cell siblings, wiring specificity is mediated by the Notch signaling pathway.

In this work we characterize in detail the bifurcation leading to an excitable regime mediated by localized structures in a dissipative nonlinear Kerr cavity with a homogeneous pump. Here we show how the route can be understood through a planar dynamical system in which a limit cycle becomes the homoclinic orbit of a saddle point ͑saddle-loop bifurcation͒. The whole picture is unveiled, and the mechanism by which this reduction occurs from the full infinite-dimensional dynamical system is studied. Finally, it is shown that the bifurcation leads to an excitability regime, under the application of suitable perturbations. Excitability is an emergent property for this system, as it emerges from the spatial dependence since the system does not exhibit any excitable behavior locally.

The development of mechanosensory epithelia, such as those of the auditory and vestibular systems, results in the precise orientation of mechanosensory hair cells. After division of a precursor cell in the zebrafish's lateral line, the daughter hair cells differentiate with opposite mechanical sensitivity. Through a combination of theoretical and experimental approaches, we show that Notch1a-mediated lateral inhibition produces a bistable switch that reliably gives rise to cell pairs of opposite polarity. Using a mathematical model of the process, we predict the outcome of several genetic and chemical alterations to the system, which we then confirm experimentally. We show that Notch1a downregulates the expression of Emx2, a transcription factor known to be involved in polarity specification, and acts in parallel with the planar-cell-polarity system to determine the orientation of hair bundles. By analyzing the effect of simultaneous genetic perturbations to Notch1a and Emx2, we infer that the gene-regulatory network determining cell polarity includes an undiscovered polarity effector.

Dysfunctions of hearing and balance are often irreversible in mammals owing to the inability of cells in the inner ear to proliferate and replace lost sensory receptors. To determine the molecular basis of this deficiency we have investigated the dynamics of growth and cellular proliferation in a murine vestibular organ, the utricle. Based on this analysis, we have created a theoretical model that captures the key features of the organ’s morphogenesis. Our experimental data and model demonstrate that an elastic force opposes growth of the utricular sensory epithelium during development, confines cellular proliferation to the organ’s periphery, and eventually arrests its growth. We find that an increase in cellular density and the subsequent degradation of the transcriptional cofactor Yap underlie this process. A reduction in mechanical constraints results in accumulation and nuclear translocation of Yap, which triggers proliferation and restores the utricle’s growth; interfering with Yap’s activity reverses this effect.DOI: http://dx.doi.org/10.7554/eLife.25681.001

Stem cells of the small and large intestine are marked by expression of the Wnt target gene LGR5, a leucine-rich-repeat-containing G protein-coupled receptor. Previous studies reported increased expression of LGR5 in human colorectal cancer (CRC) compared to normal tissue either by immunohistochemistry or in situ hybridization (ISH). However, as these studies were semi-quantitative they did not provide a numerical estimate of the magnitude of this effect. While we confirm that LGR5 cells are exclusively located at the base of normal human small and large intestinal crypts, representing approximately 6% of total crypt cells, we show this cell population is 10-fold expanded in all grades of CRC, representing as much as 70% of the cells of tumor crypt-like structures. This expansion of the LGR5 compartment coincides with maintenance of crypt-like glandular structure (adenomas, and well and moderately differentiated adenocarcinomas), and is reduced in poorly differentiated CRC, where crypt-like glandular architecture is lost, accompanied by reduced epithelial terminal differentiation. Altogether these results indicate that LGR5 cell expansion is a hallmark of CRC tumorigenesis occurring during progression to adenoma, supporting CRC as a stem cell disease with implications for CRC therapy.

Elucidating the developmental process of an organism will require the complete cartography of cellular lineages in the spatial, temporal, and molecular domains. We present Zebrahub, a comprehensive dynamic atlas of zebrafish embryonic development that combines single-cell sequencing time course data with light-sheet microscopy-based lineage reconstructions. Zebrahub is a foundational resource to study developmental processes at both transcriptional and spatiotemporal levels. It is publicly accessible as a web-based resource, providing an open-access collection of datasets and tools. Using this resource we shed new light on the pluripotency of Neuro-Mesodermal Progenitors (NMPs). We find that NMPs are pluripotent only during early axis elongation before becoming exclusively mesodermal progenitors. We attribute this restriction in NMP cell fate to emerging morphodynamic features that compartmentalize tissue motion.

Actively regulated symmetry breaking, which is ubiquitous in biological cells, underlies phenomena such as directed cellular movement and morphological polarization. Here we investigate how an organ-level polarity pattern emerges through symmetry breaking at the cellular level during the formation of a mechanosensory organ. Combining theory, genetic perturbations, and in vivo imaging, we study the development and regeneration of the fluid-motion sensors in the zebrafish’s lateral line. We find that two interacting symmetry-breaking events — one mediated by biochemical signaling and the other by cellular mechanics — give rise to precise rotations of cell pairs, which produce a mirror-symmetric polarity pattern in the receptor organ.

The hair bundle, an apical specialization of the hair cell composed of several rows of regularly organized stereocilia and a kinocilium, is essential for mechanotransduction in the ear. Its precise organization allows the hair bundle to convert mechanical stimuli to electrical signals; mutations that alter the bundle's morphology often cause deafness. However, little is known about the proteins involved in the process of morphogenesis and how the structure of the bundle arises through interactions between these molecules. We present a mathematical model based on simple reaction-diffusion mechanisms that can reproduce the shape and organization of the hair bundle. This model suggests that the boundary of the cell and the kinocilium act as signaling centers that establish the bundle's shape. The interaction of two proteins forms a hexagonal Turing patterna periodic modulation of the concentrations of the morphogens, sustained by local activation and long-range inhibition of the reactantsthat sets a blueprint for the location of the stereocilia. Finally we use this model to predict how different alterations to the system might impact the shape and organization of the hair bundle.auditory system | development | hair cell | Turing pattern | vestibular system H air cells, which occur in the sensory epithelia of hearing and balance organs of vertebrates, are responsible for mechanotransduction in the inner ear. The specialized mechanoreceptive organelle of each such cell, the hair bundle, is a cluster of 10-300 actin-filled cylinders called stereocilia (1) that occur in a hexagonal pattern on a well-defined, bounded region of the apical cellular surface (Fig. 1). The stereocilia display a monotonic gradient in length along one of the hexagonal axes; at the tall edge stands a single true cilium termed the kinocilium. The mechanical forces initiated by sounds or movements owing to accelerations deflect the hair bundle, bending the stereocilia at their bases. This deflection opens ion channels located at the stereociliary tips and depolarizes the cell, transducing a mechanical stimulus into an electrical output.The process of hair-bundle morphogenesis starts after a prospective hair cell has differentiated from a population of precursors (2-5). The kinocilium migrates from the center of the apical surface to one edge, providing the first morphological evidence of planar polarity. Microvilli on the apical surface then grow into stereocilia and establish the height gradient of the hair bundle. The numbers, positions, and lengths of the stereocilia are well controlled, producing consistent bundle shapes. In different individuals of the same species the number and dimensions of the stereocilia at a specific location of the cochlea vary by less than 5% (3). A hair bundle is accordingly among the most precisely specified organelles in a vertebrate organism.The structure of the hair bundle, the basic features of which are similar in the organs of all vertebrate species, is essential for its proper functioning as a mechanotransdu...