XB-ART-37171Dev Neurobiol March 1, 2008; 68 (4): 504-20.
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Evidences for tangential migrations in Xenopus telencephalon: developmental patterns and cell tracking experiments.
Extensive tangential cell migrations have been described in the developing mammalian, avian, and reptilian forebrain, and they are viewed as a powerful developmental mechanism to increase neuronal complexity in a given brain structure. Here, we report for the first time anatomical and cell tracking evidence for the presence of important migratory processes in the developing forebrain of the anamniote Xenopus laevis. Combining developmental gene expression patterns (Pax6, Nkx2.1, Isl1, Lhx5, Lhx9, and Dll3), neurotransmitter identity (GABA, NOS, ChAT), and connectivity information, several types of putative migratory cell populations and migration routes originating in the ventral pallium and the subpallium are proposed. By means of in vivo cell tracking experiments, pallio-subpallial and subpallio-pallial migrating neurons are visualized. Among them, populations of Nkx2.1(+) striatal interneurons and pallial GABAergic interneurons, which also express the migratory marker doublecortin, are identified. Finally, we find that these tangentially migrating pallial interneurons travel through an "isl1-free channel" that may guide their course through the subpallium. Our findings strongly suggest that the developing Xenopus telencephalon shares many similarities with amniotes in terms of neuronal specification and migrations. However, some differences are discussed, particularly with regard to the evolution of the pallium.
PubMed ID: 18214835
Article link: Dev Neurobiol
Species referenced: Xenopus laevis
Genes referenced: chat dcx dlc dlx5 emx1 emx1l isl1 lhx5 lhx8 lhx9 myh3 nkx2-1 nos1 nos3 pax6 tbx2 tff3.7
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|Figure 1 Schematic drawings of Xenopus developing brain. Schematic drawings of Xenopus brains illustrating the morphology at different developing stages (late embryonic, premetamorphic, and prometamorphic). Main regions are shown on lateral views of the brain (a–c), sagittal sections (d–f), and transversal sections at rostral and caudal levels (g–l). Abbreviations for this and subsequent figures—BST, bed nucleus of the stria terminalis; CeA, central amygdala; Dth, dorsal thalamus; Hyp, hypothalamus; LA, lateral amygdala; LGE, lateral ganglionic eminence; LP, lateral pallium; MeA, medial amygdala; MGE, medial ganglionic eminence; MP, medial pallium; mz, marginal zone; OB, olfactory bulb; OT, optic tectum; P1, prosomere 1; P2, prosomere 2; P3, prosomere 3; P4, prosomere 4; Pa, pallium; pf, postfertilization; POa, anterior preoptic area; S, septum; Sd, septum dorsalis; Sl, septum lateralis; Str, striatum; SP, subpallium; v, ventricle; VP, sentral pallium; vz, ventricular zone.|
|Figure 2 x-Pax6, x-Lhx9, x-Lhx5, and x-Dll3 expressions are dynamic during Xenopus forebrain development. Simple and double in situ hybridization photomicrographs of sagittal (a, c, k–m) and transverse (b, d–j) sections at the indicated stages. (a–f) x-Pax6 expression. Filled arrowheads point to Pax6-expressing cell masses in the subpallium. In (d), empty arrowheads point to Pax6-expressing cells in the prospective septum. In (b), dotted lines indicate the x-Pax6 streams of cells. (g–j) Expression of x-Lhx5 and x-Lhx9, at embryonic (g, h) and larval (i, j) stages. Filled arrowheads in (i) and (j) point to xLhx5/9-expressing cells in the medial amygdala (i, j). (k–m) Expression of x-Dll3/x-Lhx7 (k), x-Emx1 (l), and x-Lhx9/7 (m) on sagittal sections. In this and subsequent figures, see Figure 1 for section levels and orientation and for anatomical abbreviations, and Table 1 for number of animals used for each experimental procedure. Scale bar ¼ 100 lm (a, f–m) and 50 lm (b–e).|
|Figure 3 Nkx2.1 and GABA patterns are dynamic during Xenopus forebrain development. Transverse sections after single or double immunofluorescence for Isl1, Nkx2.1, and GABA at the indicated stages. In the doubly immunostained sections the use of red or green fluorochromes is indicated for each case. In (a–c), arrowheads show Nkx2.1+-MGE originated-positive cells into the Isl1-positive mz of the LGE. In (d), arrowheads show GABAergic interneurons in the pallium. In (e), empty and filled arrowheads indicate Nkx2.1-expressing cells that are detected inside or outside the Isl1-positive striatum, respectively. In (f) arrowheads show GABAergic pallial interneurons. Scale bars ¼ 100 lm.|
|Figure 4 Isl1 labels both projection neurons and interneurons. Transverse sections through the developing prosencephalon of Xenopus. (a,b) Retrogradely labeled cells after application of BDA (red) in the lateral forebrain bundle (a) or the hypothalamus (b), combined with Isl1 immunofluorescence staining (green). (c–h) Double (c–e, g) and triple (f, h) immunofluorescence with the indicated markers. Empty arrowheads show single labeled cells and filled arrowheads show doubly labeled cells. Scale bar ¼ 100 lm (a, b, d–h) and 25 lm (c).|
|Figure 5 The striatal Isl1-free-channel contains Nkx2.1 and GABA expressing cells. Transverse sections through the developing telencephalon at stage 56, after immunoflurescence for Isl1 (green) and GABA or Nkx2.1 (red). The boxed areas in (a–c) are shown at higher magnification in (a0–c0). A ‘‘channel’’ free of Isl1-expressing cells is delineated by dotted lines in (a0), and contains Nkx2.1-positive cells (filled arrowheads in c0). Empty arrowheads in (c0) point to Nkx2.1-expressing cells which enter the Isl1- expressing striatum. A schematic representation is given in (d) for comparison with the ‘‘Isl1-positive corridor’’ described in mammals (modified from Lo´pez-Bendito et al., 2006). Scale bars ¼ 100 lm.|
|Figure 6 Doublecortin telencephalic expression: double labeling with Nkx2.1 and GABA. Transverse sections after single (a, b, d), double (c, f–j) and triple (e) labeling for DCX, Isl1, Nkx2.1, and GABA at the indicated stages. In case of double/triple immunofluorescence, the use of red or green fluorochromes is indicated for each case. In (a), arrows point to the DCX chains. In (b), dotted lines delineate a high DCX expression zone in the internal part of the striatum. In (c), the white box indicates the area enlarged in (d) and (e). In (e), arrowhead shows a DCX/Nkx2.1 double-labeled cell in the Isl1-negative area, and empty arrowhead shows a Nkx2.1 cell in the Isl1- positive striatum. In (f), the white boxes indicate the areas enlarged in (g) and (h). In (g–j) the arrowheads point to doubly labeled cells. Scale bars ¼ 25 lm (b, e, g, f) and 50 lm (a, c, d, f, i, j).|
|Figure 7 In vivo electroporation and CMFDA applications demonstrate cell movements in the telencephalon. (a, h) show the targeted area for cell tracking experiments. (b–c) GFP immunofluorescence 48 h after electroporation. Asterisks indicate the electroporated area and arrowheads show cells in the subpallium. (d–g) Photomicrographs after small (d, f, g, i, j) or large (e) CMFDA applications into the ventral pallium (d–g) and the subpallium (i, j) at the indicated stages. Arrowheads show the presence of labeled cells outside of the zone of injection. In (d), the asterisk indicates the injection site and the dotted line the two courses followed by labeled cells. Scale bar ¼ 100 lm.|
|Figure 8 CMFDA applications in the subpallium demonstrate tangentially migrating Nkx2.1 and GABAergic cells. Transverse sections through the developing telencephalon. After combined CMFDA and immunolabeling for Nkx2.1 (a,d) and GABA (c, e–i), double-labeled cells (arrowheads) were detected leaving the application area. In (b, e, g) boxes indicate enlarged area in the next panel. Arrowheads in (b) and (e) indicate CMFDA cells out of the injection site. Scale bars ¼ 100 lm (a, b, d, e, g, i) and 25 lm (c, f, h).|
|Figure 9 Summary drawing for cell specification and migrations in Xenopus and mouse forebrain. The markers used are color-coded and represented on transverse schematic sections at developing stages (the development timing is not compared). In (a), results from the present and previous studies are summarized: (1) present results; (2) data from Gonza´lez et al., 2002a,b; (3) data from Endepols et al., 2007; (4) data from Bachy and Re´taux, 2006. The resulting cell types are indicated in the right column, where the distinction is made between projection neurons and interneuron types. In (b), the migratory streams observed in this study are compared to those known in mammals. The Xenopus data are from the present and previous studies (Bachy et al., 2001, 2002a,b; Brox et al., 2003; Moreno et al., 2003; 2004; Bachy and Re´taux, 2006). The mammalian data are compiled from Marı´n and Rubenstein (2001), Tole et al. (2005), and Carney et al. (2006).|