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Final Activity and Management Report Summary - TISSUE POLARITY (Establishment and maintenance of cell polarity and tissue architecture in the lateral-line system of the zebrafish)

The zebrafish senses directional water movements with a functionally sophisticated, but anatomically simple organ called the lateral line (LL). This organ is formed by a collection of isolated units called neuromasts, each of them composed by a core of mechanosensory hair cells (HC) surrounded by supporting cells and innervated by afferent and efferent neurons. LL formation starts at 19hours-post-fertilisation (hpf), when a primordium located near the otic vesicle starts migrating along the horizontal myoseptum of the fish, depositing clusters of cells that will form the neuromasts. During migration, cells in the primordium divide, change shape and differentiate. At 40 hpf, 7 to 9 neuromasts can be observed along the body of the fish. Sensory HC in the neuromasts display a characteristic planar cell polarity (PCP) pattern, evidenced by the asymmetric localisation of the kinocilium and its associated stereocilia. The axis of morphological polarity of the stereocilia corresponds to the direction of excitability of the hair cells.

Each neuromast harbours two populations of sensory HC, equal in number, whose stereocilia are oriented at 180º relative to each other. Afferent neurons innervating them can discriminate their polarity and form stable synapses with cells of identical orientation. Trilobite (tri) mutants, that lack the PCP protein Vangl2, present randomised HC polarisation in the neuromast. In this project we use transgenic lines, molecular markers and SPIM and confocal mircroscopy to investigate the mechanisms governing the establishment of HC polarisation and innervation, in wild type animals and tri mutants.

We observe that early events of LL development- primordium migration, rosette formation, neuromast deposition and the axis of HC-precursor division- are not altered in these mutants. Preliminary, single-neuron analyses show that afferents contact HC, but their arborisation is more complex and synapses establishment are more dynamic than in wild type fish. In the context of this project we also screened a collection of Gal4 enhancer-trap lines and selected those driving Gal4 expression in the pLL organ of the fish. By using specific antibodies and time lapse analyses I characterised four new stable transgenic lines allowing expression of UAS elements in HC, HC progenitors, LL ganglion and lateralis glia.

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