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x-Pax6 gene expression has been previously demonstrated in the Xenopus forebrain at early developmental stages and, in particular, it was analyzed in relation to putative migratory cells in the primordial septum and olfactory bulbs. Here we investigated the pattern of x-Pax6 expression during embryonic and larval stages and in the adult. To gain more insight into the exact localization of x-Pax6 labeled cells, we used combinations of x-Pax6 with other telencephalic markers of the Xenopus forebrain such as Islet-1, Nkx2.1, and tyrosine hydroxylase. The results, analyzed within the frame of the current prosomeric model of the vertebrate forebrain, demonstrated robust expression in the pallium and the prethalamic diencephalic segment, from early embryonic stages. In addition, starting at the end of the embryonic period, x-Pax6 was detected in the subpallial mantle (but not in the proliferating cells of the ventricular/subventricular zones). The pattern of distribution in the pallium narrows through larval development and in the adult, whereas in the subpallium the expression was gradually followed and identified into the dorsal part of the septum, corroborating in Xenopus a mixed pallial/subpallial origin of the neurons in this region. The findings show that the strongly conserved features of x-Pax6 expression through forebrain development shared by all amniote vertebrates are also present in the anamniote amphibians as a common characteristic of the forebrain organization of tetrapods.
Fig. 1.
x-Pax6 expression in the developing and adult Xenopus forebrain. Schematic drawings of lateral views of the brain (a, e, and i) at the indicated period of Xenopus development, where x-Pax-6 expression is indicated in blue (anterior is left, dorsal is up). Photomicrographs of transverse and sagittal sections through the telencephalon, at embryonic and larval stages, after in situ hybridization for x-Pax6, whose level is indicated on the corresponding drawing. (b–c). Sagittal (a) and transverse (c,d) sections through the brain of a late embryo showing x-Pax-6 expression pattern at rostral telencephalic (c) and diencephalic levels (d). (f–h). Sagittal (f) and transverse (g,h) sections through the brain of a premetamorphic larva showing x-Pax-6 expression pattern at telencephalic (g) and diencephalic levels (h). (j–l). Transverse sections through the brain of a prometamorphic larva showing x-Pax-6 expression pattern at telencephalic (j,k) and diencephalic levels (l). Schematic drawings of a transverse hemisection (o,q) and photomicrographs (p,r) of transverse sections through the adult Xenopus forebrain, showing x-Pax-6 expression in the telencephalon (p) and diencephalon (r). Sections levels are indicated in the schematic drawing of an adult brain in lateral view (m). Scale bars = 100 μm. Abbreviations: Acc, nucleus accumbens; BST, bed nucleus of the stria terminalis; CeA, central amygdala; DP, dorsal pallium; Hb, habenula; Hyp and hypo, hypothalamus; LA, lateralamygdala; lfb, lateralforebrain bundle; LGE, lateral ganglionic eminence; LP, lateralpallium; MGE, medial ganglionic eminence; MP, medialpallium; OB, olfactory bulb; oc, optic chiasm; OT, optic tectum; Pa, pallium; PTh, prethalamus; P1–P4, prosomeres 1–4; S, septum; SP, subpallium; SC, nucleus suprachiasmatic; Sd, septum dorsalis; Sl, septum lateralis; Str, striatum; Sv, septum ventralis; VL, ventrolateral nucleus of the thalamus; VM, ventromedial nucleus of the thalamus; VP, ventralpallium.
Fig. 2.
x-Pax6 cells are located in subpallial Isl-1 positive territories. Photomicrographs of transversals (a,b,c) and sagittal (d,e,f) sections through the telencephalon at embryonic and larval stages, after in situ hybridization for x-Pax6 (red) combined with Isl1 immunohistofluorescence (green). The merge images (c,f) show x-Pax6 cells which clearly enter the Isl1-postive striatal territory. Scale bar = 50 µm.
Fig. 3.
x-Pax6 expression in relation to TH and Nkx2.1. Photomicrographs of sagittal sections through the Xenopus telencephalon at prometamorphic stages, after in situ hybridization for x-Pax6 (blue) combined with TH (a,c) and Nkx2.1 (b,d) immunohistochemistry (brown). Dashed boxes in a and b indicate the amplified image of c and d respectively. Scale bar = 100 µm.
