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Patterns of neurogenesis and amplitude of Reelin expression are essential for making a mammalian-type cortex.

Nomura T, Takahashi M, Hara Y, Osumi N - PLoS ONE (2008)

Bottom Line: We compared the neurogenesis in mammalian and avian pallium, focusing on subtype-specific gene expression, and found that the avian pallium generates distinct types of neurons in a spatially restricted manner.Furthermore, expression of Reelin gene is hardly detected in the developing avian pallium, and an experimental increase in Reelin-positive cells in the avian pallium modified radial fiber organization, which resulted in dramatic changes in the morphology of migrating neurons.Our results demonstrate that distinct mechanisms govern the patterns of neuronal specification in mammalian and avian pallial development, and that Reelin-dependent neuronal migration plays a critical role in mammalian type corticogenesis.

View Article: PubMed Central - PubMed

Affiliation: Division of Developmental Neuroscience, Center for Translational and Advanced Animal Research (CTTAR), Tohoku University School of Medicine, Sendai, Japan.

ABSTRACT
The mammalian neocortex is characterized as a six-layered laminar structure, in which distinct types of pyramidal neurons are distributed coordinately during embryogenesis. In contrast, no other vertebrate class possesses a brain region that is strictly analogous to the neocortical structure. Although it is widely accepted that the pallium, a dorsal forebrain region, is specified in all vertebrate species, little is known of the differential mechanisms underlying laminated or non-laminated structures in the pallium. Here we show that differences in patterns of neuronal specification and migration provide the pallial architectonic diversity. We compared the neurogenesis in mammalian and avian pallium, focusing on subtype-specific gene expression, and found that the avian pallium generates distinct types of neurons in a spatially restricted manner. Furthermore, expression of Reelin gene is hardly detected in the developing avian pallium, and an experimental increase in Reelin-positive cells in the avian pallium modified radial fiber organization, which resulted in dramatic changes in the morphology of migrating neurons. Our results demonstrate that distinct mechanisms govern the patterns of neuronal specification in mammalian and avian pallial development, and that Reelin-dependent neuronal migration plays a critical role in mammalian type corticogenesis. These lines of evidence shed light on the developmental programs underlying the evolution of the mammalian specific laminated cortex.

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Reelin controls directed growth of radial fibers.(A) Schematic illustration of slice culture. After GFP plasmid electroporation, quail embryonic slices (E7) are co-cultured with Reelin and/or RFP-expressing COS7 cells (magenta). (B and C) Patterns of radial glial fiber extension in slices with control cells (B) and with Reelin-expressing cells (C). Straight projection of GFP-labeled radial fibers is evident with Reelin-expressing cells compared with control cells. (D) Quantification of radial fiber orientation. The parallel index (PI) of radial fibers was calculated by dividing the maximum distance between adjacent radial fibers by the minimum distance. Differences in PI between slices with control and Reelin-expressing cells were analyzed statistically (F-test). PI is reduced in Reelin-treated slices compared with that in control slices. Asterisks indicate statistically significance (p<0.01, F-test). Scale bar, 20 µm.
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pone-0001454-g005: Reelin controls directed growth of radial fibers.(A) Schematic illustration of slice culture. After GFP plasmid electroporation, quail embryonic slices (E7) are co-cultured with Reelin and/or RFP-expressing COS7 cells (magenta). (B and C) Patterns of radial glial fiber extension in slices with control cells (B) and with Reelin-expressing cells (C). Straight projection of GFP-labeled radial fibers is evident with Reelin-expressing cells compared with control cells. (D) Quantification of radial fiber orientation. The parallel index (PI) of radial fibers was calculated by dividing the maximum distance between adjacent radial fibers by the minimum distance. Differences in PI between slices with control and Reelin-expressing cells were analyzed statistically (F-test). PI is reduced in Reelin-treated slices compared with that in control slices. Asterisks indicate statistically significance (p<0.01, F-test). Scale bar, 20 µm.

Mentions: In the developing mammalian cortex, Reelin plays a crucial role in radial neuronal migration and the formation of laminar structures (reviewed in [29], [30]). Reelin is a large extra-cellular protein that is secreted from the Cajal-Retzius cells [31], [32]. The mice compromising generation of functional Reelin exhibit severe abnormalities in an inside-out pattern of corticogenesis; thereby six-layered laminar structure is disorganized (reviewed in [29], [30]). Previous results indicated that Reelin expression is also detected in the avian pallium, although their expression is less prominent compared to that in the mammalian cortex (Figure 1 and [21]). Hence, it is possible that some, if not all, of the architectural differences between the mammalian and avian pallium, might be due to differences in the amplitude of Reelin signaling in these taxa. In order to test this hypothesis, we examined the effect of experimental amplification of Reelin signaling on avian pallial development, by co-culture of the quail telencephalon (E7) with COS7 cells transfected with a Reelin-expression vector (Fig. 5A). To trace immature neuronal progenitors and migrating neurons, a GFP expression vector was electroporated into the slice. Although we did not detect significant changes in neuronal migration patterns in this culture, we identified significant alterations in the attachment of radial glial cells (radial fibers) to Reelin-expressing cells. In the slices with control COS7 cells, GFP-labeled radial fibers did not extend straight, but exhibited curled morphology in the neuronal layer (Fig. 5B). We identified a similar projection pattern of radial fibers samples in vivo (Figure S4), indicating that this effect is not artifactual to the culture conditions. Labeling radial fibers with specific markers, DiI or GFP in fixed samples indicated a meandering extension of radial fibers in the quail pallium (Figure S4B–D). This is extremely different from the organization of mammalian radial glial fibers, which project straightly from the ventricular zone toward the pial surface (Figure S4A). However, when the quail slices were co-cultured with Reelin-expressing COS cells, GFP-labeled radial fibers became to extend long processes in highly parallel orientation towards the pial surface (Fig. 5C). To quantify the alteration in radial fiber organization, we determined the “parallel index” of fibers by calculating the ratio of the maximum to minimum distances between two fibers (Fig. 5D). In control cultures, the parallel index ranged from 1.2 to 13.1 (n = 22, 3 slices), indicating that radial processes were oriented randomly. In contrast, the parallel index was significantly lower in cultures containing Reelin-expressing cells (ranged from 1.09 to 5.34, n = 20, 3 slices), indicating that fibers extended with less directional variance than in controls (Fig. 5D). These data indicate that 1) projection patterns of radial glial fibers in the quail pallium is largely different from those in the mammalian cortex, and that 2) exogenous Reelin modified extension patterns of the quail radial fibers as those seen in the mammalian cortex.


Patterns of neurogenesis and amplitude of Reelin expression are essential for making a mammalian-type cortex.

Nomura T, Takahashi M, Hara Y, Osumi N - PLoS ONE (2008)

Reelin controls directed growth of radial fibers.(A) Schematic illustration of slice culture. After GFP plasmid electroporation, quail embryonic slices (E7) are co-cultured with Reelin and/or RFP-expressing COS7 cells (magenta). (B and C) Patterns of radial glial fiber extension in slices with control cells (B) and with Reelin-expressing cells (C). Straight projection of GFP-labeled radial fibers is evident with Reelin-expressing cells compared with control cells. (D) Quantification of radial fiber orientation. The parallel index (PI) of radial fibers was calculated by dividing the maximum distance between adjacent radial fibers by the minimum distance. Differences in PI between slices with control and Reelin-expressing cells were analyzed statistically (F-test). PI is reduced in Reelin-treated slices compared with that in control slices. Asterisks indicate statistically significance (p<0.01, F-test). Scale bar, 20 µm.
© Copyright Policy
Related In: Results  -  Collection

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getmorefigures.php?uid=PMC2175532&req=5

pone-0001454-g005: Reelin controls directed growth of radial fibers.(A) Schematic illustration of slice culture. After GFP plasmid electroporation, quail embryonic slices (E7) are co-cultured with Reelin and/or RFP-expressing COS7 cells (magenta). (B and C) Patterns of radial glial fiber extension in slices with control cells (B) and with Reelin-expressing cells (C). Straight projection of GFP-labeled radial fibers is evident with Reelin-expressing cells compared with control cells. (D) Quantification of radial fiber orientation. The parallel index (PI) of radial fibers was calculated by dividing the maximum distance between adjacent radial fibers by the minimum distance. Differences in PI between slices with control and Reelin-expressing cells were analyzed statistically (F-test). PI is reduced in Reelin-treated slices compared with that in control slices. Asterisks indicate statistically significance (p<0.01, F-test). Scale bar, 20 µm.
Mentions: In the developing mammalian cortex, Reelin plays a crucial role in radial neuronal migration and the formation of laminar structures (reviewed in [29], [30]). Reelin is a large extra-cellular protein that is secreted from the Cajal-Retzius cells [31], [32]. The mice compromising generation of functional Reelin exhibit severe abnormalities in an inside-out pattern of corticogenesis; thereby six-layered laminar structure is disorganized (reviewed in [29], [30]). Previous results indicated that Reelin expression is also detected in the avian pallium, although their expression is less prominent compared to that in the mammalian cortex (Figure 1 and [21]). Hence, it is possible that some, if not all, of the architectural differences between the mammalian and avian pallium, might be due to differences in the amplitude of Reelin signaling in these taxa. In order to test this hypothesis, we examined the effect of experimental amplification of Reelin signaling on avian pallial development, by co-culture of the quail telencephalon (E7) with COS7 cells transfected with a Reelin-expression vector (Fig. 5A). To trace immature neuronal progenitors and migrating neurons, a GFP expression vector was electroporated into the slice. Although we did not detect significant changes in neuronal migration patterns in this culture, we identified significant alterations in the attachment of radial glial cells (radial fibers) to Reelin-expressing cells. In the slices with control COS7 cells, GFP-labeled radial fibers did not extend straight, but exhibited curled morphology in the neuronal layer (Fig. 5B). We identified a similar projection pattern of radial fibers samples in vivo (Figure S4), indicating that this effect is not artifactual to the culture conditions. Labeling radial fibers with specific markers, DiI or GFP in fixed samples indicated a meandering extension of radial fibers in the quail pallium (Figure S4B–D). This is extremely different from the organization of mammalian radial glial fibers, which project straightly from the ventricular zone toward the pial surface (Figure S4A). However, when the quail slices were co-cultured with Reelin-expressing COS cells, GFP-labeled radial fibers became to extend long processes in highly parallel orientation towards the pial surface (Fig. 5C). To quantify the alteration in radial fiber organization, we determined the “parallel index” of fibers by calculating the ratio of the maximum to minimum distances between two fibers (Fig. 5D). In control cultures, the parallel index ranged from 1.2 to 13.1 (n = 22, 3 slices), indicating that radial processes were oriented randomly. In contrast, the parallel index was significantly lower in cultures containing Reelin-expressing cells (ranged from 1.09 to 5.34, n = 20, 3 slices), indicating that fibers extended with less directional variance than in controls (Fig. 5D). These data indicate that 1) projection patterns of radial glial fibers in the quail pallium is largely different from those in the mammalian cortex, and that 2) exogenous Reelin modified extension patterns of the quail radial fibers as those seen in the mammalian cortex.

Bottom Line: We compared the neurogenesis in mammalian and avian pallium, focusing on subtype-specific gene expression, and found that the avian pallium generates distinct types of neurons in a spatially restricted manner.Furthermore, expression of Reelin gene is hardly detected in the developing avian pallium, and an experimental increase in Reelin-positive cells in the avian pallium modified radial fiber organization, which resulted in dramatic changes in the morphology of migrating neurons.Our results demonstrate that distinct mechanisms govern the patterns of neuronal specification in mammalian and avian pallial development, and that Reelin-dependent neuronal migration plays a critical role in mammalian type corticogenesis.

View Article: PubMed Central - PubMed

Affiliation: Division of Developmental Neuroscience, Center for Translational and Advanced Animal Research (CTTAR), Tohoku University School of Medicine, Sendai, Japan.

ABSTRACT
The mammalian neocortex is characterized as a six-layered laminar structure, in which distinct types of pyramidal neurons are distributed coordinately during embryogenesis. In contrast, no other vertebrate class possesses a brain region that is strictly analogous to the neocortical structure. Although it is widely accepted that the pallium, a dorsal forebrain region, is specified in all vertebrate species, little is known of the differential mechanisms underlying laminated or non-laminated structures in the pallium. Here we show that differences in patterns of neuronal specification and migration provide the pallial architectonic diversity. We compared the neurogenesis in mammalian and avian pallium, focusing on subtype-specific gene expression, and found that the avian pallium generates distinct types of neurons in a spatially restricted manner. Furthermore, expression of Reelin gene is hardly detected in the developing avian pallium, and an experimental increase in Reelin-positive cells in the avian pallium modified radial fiber organization, which resulted in dramatic changes in the morphology of migrating neurons. Our results demonstrate that distinct mechanisms govern the patterns of neuronal specification in mammalian and avian pallial development, and that Reelin-dependent neuronal migration plays a critical role in mammalian type corticogenesis. These lines of evidence shed light on the developmental programs underlying the evolution of the mammalian specific laminated cortex.

Show MeSH