How astrocytes grow and integrate into neural circuits remains poorly defined. Zebrafish are well suited for such investigations, but bona fide astrocytes have not been described in this system. Here we characterize a zebrafish cell type that is remarkably similar to mammalian astrocytes that derive from radial glial cells and elaborate processes to establish their territories at early larval stages. Zebrafish astrocytes associate closely with synapses, tile with one another and express markers, including Glast and glutamine synthetase. Once integrated into circuits, they exhibit whole-cell and microdomain Ca²⁺ transients, which are sensitive to norepinephrine. Finally, using a cell-specific CRISPR-Cas9 approach, we demonstrate that fgfr3 and fgfr4 are required for vertebrate astrocyte morphogenesis. This work provides the first visualization of astrocyte morphogenesis from stem cell to post-mitotic astrocyte in vivo, identifies a role for Fgf receptors in vertebrate astrocytes and establishes zebrafish as a valuable new model system to study astrocyte biology in vivo.

Astrocytes are a large and diverse population of morphologically complex cells that exist throughout nervous systems of multiple species. Progress over the last two decades has shown that astrocytes mediate developmental, physiological, and pathological processes. However, a long-standing open question is how astrocytes regulate neural circuits in ways that are behaviorally consequential. In this regard, we summarize recent studies using Caenorhabditis elegans, Drosophila melanogaster, Danio rerio, and Mus musculus. The data reveal diverse astrocyte mechanisms operating in seconds or much longer timescales within neural circuits and shaping multiple behavioral outputs. We also refer to human diseases that have a known primary astrocytic basis. We suggest that including astrocytes in mechanistic, theoretical, and computational studies of neural circuits provides new perspectives to understand behavior, its regulation, and its disease-related manifestations.