Summary of the cerebellar progenitors and the progenitor niche in zebrafish. (A) During larval and juvenile stages the URL and a portion of the VZ together with a part of IVth ventricle (cerebellar recessus, CR) is displaced from the rest of the ventricle through tissue growth (red opposing arrows). The neural progenitors in the URL (dorsal part) remain active throughout life, while the VZ progenitors (ventral part) largely become quiescent in the adult. During larval and juvenile stages progenitors delaminate from the VZ into parenchyma. The highest density of proliferating VZ progenitors is found at the interface of the caudal lobe (best seen in C); (B) Schematic view of the cellular arrangement of the cerebellar progenitor niche. 1. Granule cell stem and progenitor cells (green) are found dorsal to the cerebellar recessus in the URL. The granule precursors migrate to the GL and differentiate into granule neurons. 2. Ventricular zone derived progenitors (orange) are found ventral to the recessus. Proliferating and differentiating VZ progenitors migrate to the cerebellar parenchyma during larval and juvenile stages. Most of the cells in VZ of the mature cerebellum express glial and ependymal markers and display epithelial-like morphology. Bergmann glia and inhibitory inter-neurons are produced at the interface between the URL and VZ. 3. Very rare progenitors reside in the cerebellar parenchyma. (C) Schematic parasagittal view of the cerebellum and the distribution of progenitors; (D) The URL derived progenitors remain active into adulthood, while the proliferative activity of VZ derived progenitors virtually is exhausted during juvenile stages. Purkinje neurons and Eurydendroid cells are produced up to 1 month. In sexually mature zebrafish only few inhibitory neurons and glia are produced. Production of granule neurons is maintained throughout life in zebrafish.