Notch loss of function differentially affects neuroepithelial polarity in the brain and spinal cord. (a–h) In mib1 mutants neuroepithelial apico-basal polarity is maintained at the level of the midbrain-hindbrain boundary (b, n = 7), partially disrupted at the level of the hindbrain (f, arrowheads indicate residual polarized aPKC signal, n = 7) but completely lost in the dorso-medial spinal cord (d,h). (g,h) represent the most anterior and (c,d) the trunk spinal cord. (i–l) In WT siblings, the tp1bGlob:GFP (tp1:GFP) Notch reporter transgene indicates active signaling in a small number of cells in the anterior hindbrain (aHB, arrowheads in i”). More anteriorly, Notch activity is detected only in epidermal cells (arrow in i”) but not in the midbrain (MB) itself. mib1 mutants present enhanced levels of neurogenesis (j’) and a partial disruption of apical aPKC localisation (j) at the level of the anterior hindbrain (n = 9). Only few elavl3-positive neurons are detected in the midbrain, which maintains neuroepithelial organization (j,j’). (k,l) At the level of the anterior spinal cord, WT sibling embryos display widespread Notch reporter activity (k”), basally localized elavl3-positive neurons (k’) and polarized aPKC enrichment at the apical neural tube midline (k, n = 6). mib1 mutants present a loss of Notch reporter expression (l”) and a lack of polarized aPKC localization (l, n = 8). Quantification of the area of the neural tube occupied by elavl3 positive cells reveals an increase in neurogenesis in mib1 mutants (l’, 95.4 ± 2.6%) compared to WT siblings (k’, 31.7 ± 5.8%, p = 1.36E-07). Pictures (a,c), (b,d), (e,g), (f,h), (i,k) and (j,l) each represent the same embryo imaged at different antero-posterior locations. All images are dorsal views of 30 somites stage embryos, anterior left. Scalebars: 40 µm.
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