Spatial distribution and characterization of non-apical progenitors in the zebrafish embryo central nervous system

Studies of non-apical progenitors (NAPs) have been largely limited to the developing mammalian cortex. They are postulated to generate the increase in neuron numbers that underlie mammalian brain expansion. Recently, NAPs have also been reported in the retina and central nervous system of non-mammalian species; in the latter, however, they remain poorly characterized. Here, we characterize NAP location along the zebrafish central nervous system during embryonic development, and determine their cellular and molecular characteristics and renewal capacity. We identified a small population of NAPs in the spinal cord, hindbrain and telencephalon of zebrafish embryos. Live-imaging analysis revealed at least two types of mitotic behaviour in the telencephalon: one NAP subtype retains the apical attachment during division, while another divides in a subapical position disconnected from the apical surface. All NAPs observed in spinal cord lost apical contact prior to mitoses. These NAPs express HuC and produce two neurons from a single division. Manipulation of Notch activity reveals that neurons and NAPs in the spinal cord use similar regulatory mechanisms. This work suggests that the majority of spinal NAPs in zebrafish share characteristics with basal progenitors in mammalian brains.