In the Light of Evolution Volume VI: Brain and Behavior (2013) / Chapter Skim
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5 Expansion, Folding, and Abnormal Lamination of the Chick Optic Tectum After Intraventricular Injections of FGF2--Luke D. McGowan, Roula A. Alaama, Amanda C. Freise, Johnny C. Huang, Christine J. Charvet, and Georg F. Striedter
5 Expansion, Folding, and Abnormal Lamination of the Chick Optic Tectum After Intraventricular Injections of FGF2--Luke D. McGowan, Roula A. Alaama, Amanda C. Freise, Johnny C. Huang, Christine J. Charvet, and Georg F. Striedter
Pages 75-90
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From page 75... ...
By ED7, the tectum of FGF2-treated birds is abnormally thin and has a reduced postmitotic layer, consistent with a delay in neurogenesis. FGF2 treatment also increases tectal volume and ventricular surface area, disturbs tectal lamination, and creates small discontinuities in the pia mater overlying the tectum.
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From page 76... ...
One way to fill this gap in our knowledge is to experimentally recreate the key species differences in the laboratory by means of carefully selected developmental manipulations. A good example of this phenocopy approach was the creation of transgenic mice with a constitutively active form of β-catenin that prolongs proliferation, increases neocortical volume, and generates cortical folds (Chenn and Walsh, 2002)
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From page 77... ...
This mismatch causes tectal foliation and tears holes in pia, which then disrupt laminar development. The presumably maladaptive nature of these alterations probably explains why naturally occurring species differences in optic tectum size are based, as far as we know, on changes in brain patterning, rather than neurogenesis timing (McGowan et al., 2011)
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From page 78... ...
. Normalized tectum volume and tectal ventricular surface area are also larger in FGF2-treated birds than in controls, but tectal radial thickness is reduced.
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From page 79... ...
As mentioned earlier, FGF2-treated embryos on ED7 have an abnormally thin mantle zone in the optic tectum. This effect is most extreme in the lateral tectum, where the tectal surface approaches the developing skull.
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From page 80... ...
. However, double labeling with anti-PCNA and bisbenzimide, a fluorescent counterstain, reveals that the tops of the cellular mountains in the FGF2-treated tecta consist primarily of postproliferative cells (Fig.
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From page 81... ...
. Double-staining with anti-PCNA to label proliferating cells and bisbenzimide to label all cell nuclei reveals that the tops of the mountains in the FGF2-treated tecta contain mainly postproliferative cells (C and D)
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From page 82... ...
. Therefore, we refer to these protrusions as "volcanoes." Staining with antibodies against vimentin, an intermediate filament found mainly in radial glial cells, shows that the central channel of each volcano contains numerous radial glia processes (Fig.
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From page 83... ...
The volcano-like laminar disturbances at ED12 tend to be located in the lateral and dorsal tectum, where the tectal surface approaches the developing skull (McGowan et al., 2012, Movie S1)
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From page 84... ...
[Note: Figure can be viewed in color in the PDF version of this volume on the National Academies Press website, www.nap.edu.] Caudal to the volcanoes, the tectum of FGF2-treated embryos frequently exhibits folds resembling cortical gyri and sulci.
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From page 85... ...
Neuronal birth-dating studies to confirm the delay in neurogenesis are in progress. FGF2 treatment also affects neuronal migration in the lateral tectum, where the mountains and volcanoes are observed, but these effects are likely to be downstream consequences of the delay in tectal neurogenesis.
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From page 86... ...
Because the radial glia processes are attached to the pial surface (Halfter et al., 2002; Siegenthaler et al., 2009; Georges-Labouesse et al., 1998; Radakovits et al., 2009) , the differential tangential expansion creates laterally directed tension between the ventricular and pial surfaces.
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From page 87... ...
secreted by specialized cells near the pial surface. By delaying neurogenesis, exogenous FGF2 causes the ventricular surface of the developing tectum to expand tangentially more quickly than the pial surface can expand (B and C)
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From page 88... ...
However, our present findings suggest that the avian tectum is vulnerable to morphological disruptions if tectal neurogenesis is delayed dramatically. To create viable increases in tectum size by delaying neurogenesis, those delays would have to be small or coupled with increased pial proliferation.
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From page 89... ...
To examine whether FGF2 injections delay tectal neurogenesis, we measured the proliferative and postproliferative zones in FGF2-treated and control chickens at ED7. As development proceeds, cells exit the proliferative ventricular zone and form a postproliferative mantle zone.
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From page 90... ...
. For a more detailed analysis of the FGF2-induced morphological alterations, selected sections were stained with antibodies against laminin (clone 3H11; mouse; 1:20; Developmental Studies Hybridoma Bank)
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