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Fig. 2. Key features of the cell biology of neural stem and progenitor cells in (A) D. melanogaster, (B) rodents and (C) primates. The apical plasma membrane and corresponding cortical domains are represented by blue lines, and the basolateral plasma membrane and corresponding cortical domains are represented by red lines. Interphase nuclei are shown in grey and representative sister chromatids are in dark blue. Black rectangles represent junctional complexes and yellow dots indicate centrosomes or mitotic-spindle poles. For clarity, only the astral microtubules of the mitotic spindle are depicted (black lines). (A) D. melanogaster neuroectodermal cells (bottom) divide with a cleavage plane that is parallel to their apicobasal axis (vertical cleavage), which results in symmetric (Sy) division. In neuroblasts (NB, top), Insc (green line) directs a 90° rotation of the mitotic spindle, aligning it along the apicobasal axis and thus generating a cleavage plane that is perpendicular to this axis (horizontal cleavage), which results in asymmetric (As) division. (Bi) In rodents, APs of the VZ exhibit apicobasal polarity (apical junctional complexes and apically located centrosomes in interphase). APs include neuroepithelial cells (not shown) and, after the onset of neurogenesis, short neural precursors (SNPs) and radial glia (RG) cells, the basal processes of which terminate at the basal side of the VZ and at the basal lamina, respectively, in interphase (two left-hand cells). APs divide at the apical surface with a vertical or nearly vertical cleavage plane that can result in either symmetric (Sy) or asymmetric (As) division (two right-hand cells; M-phase basal processes are not shown for clarity). (Bii) By contrast, BPs of the SVZ are known to have an apical process in interphase (left-hand cell), which is retracted before mitosis. Mitotic BPs (right-hand cell) are unpolarized, lack adherens junctions and divide in a basal location. BP-cell divisions are symmetric, with a random cleavage-plane orientation. (C) In primates, APs (Ci) and BPs (Cii) that are similar to those in rodents are also present, and BPs constitute the ISVZ. In addition, a novel neural progenitor that undergoes mitosis in a basal location, the OSVZ progenitor (Ciii), has evolved. Most of the cell-biological features of this progenitor are unknown. However, we hypothesize that interphase (left-hand cell) and mitotic (right-hand cell) OSVZ progenitors maintain epithelial characteristics, including radial processes and apical junctional complexes. Other features might distinguish OSVZ progenitors from APs, such as a perinuclear location of the centrosome in interphase (left-hand cell). Additionally, we hypothesize that symmetric cell divisions of polarized neural progenitors predominantly occur in APs (Ci) of the VZ (right-hand cell), whereas OSVZ progenitors (Ciii) might be restricted to asymmetric, differentiative divisions (right-hand cell).
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