Gradients in the Brain: The Control of the Development of Form...

Gradients in the Brain: The Control of theDevelopment of Form and Function in theCerebral Cortex

Stephen N. Sansom and Frederick J. Livesey

Gurdon Institute and Department of Biochemistry, University of Cambridge, Tennis Court Road,Cambridge, CB2 1QN

Correspondence: [email protected]

In the developing brain, gradients are commonly used to divide neurogenic regions intodistinct functional domains. In this article, we discuss the functions of morphogen andgene expression gradients in the assembly of the nervous system in the context of the devel-opment of the cerebral cortex. The cerebral cortex is a mammal-specific region of theforebrain that functions at the top of the neural hierarchy to process and interpret sensoryinformation, plan and organize tasks, and to control motor functions. The mature cerebralcortex is a modular structure, consisting of anatomically and functionally distinct areas.Those areas of neurons are generated from a uniform neuroepithelial sheet by two formsof gradients: graded extracellular signals and a set of transcription factor gradients operatingacross the field of neocortical stem cells. Fgf signaling from the rostral pole of the cerebralcortex sets up gradients of expression of transcription factors by both activating andrepressing gene expression. However, in contrast to the spinal cord and the earlyDrosophila embryo, these gradients are not subsequently resolved into molecularly distinctdomains of gene expression. Instead, graded information in stem cells is translated into dis-crete, region-specific gene expression in the postmitotic neuronal progeny of the stem cells.

In the developing brain, gradients are com-monly used to divide neurogenic regions

into distinct functional domains. Examples ofsuch gradients are the sonic hedgehog (Shh)gradient responsible for specifying dorsoventralfates in the spinal cord (Ericson et al. 1997), andthe double inverted gradients of the transcrip-tion factors En1/2 and Pax2/5, induced bymorphogens secreted from the isthmic organ-izer that pattern the midbrain and hindbrain(Sato et al. 2004). In this article, we discuss

the functions of morphogen and geneexpression gradients in the assembly of thenervous system in the context of the develop-ment of the cerebral cortex. The neocortex isthe mammal-specific region of the forebrainthat functions at the top of the neural hierarchyto process and interpret sensory information,plan and organize tasks, and to control motorfunctions. A key aspect of the functionalanatomy of the cerebral cortex is that it is amodular structure and, as is discussed, those

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modules of neurons are generated from a neu-roepithelial sheet by two forms of gradients:graded extracellular signals and a set of tran-scription factor gradients operating across thefield of neocortical stem cells.

THE NEOCORTEX: A SIX-LAYERED SHEETOF NEURONS DIVIDED INTOFUNCTIONAL AREAS

In humans, the neocortex accounts for nearlythree-quarters of the brain volume andcontains more than twenty billion neurons(Mountcastle 1998). The neocortex is com-posed of six layers of neurons, and those layersdiffer markedly in neuronal cell type com-position, cell density, and connectivity. Theneurons of the neocortex are characteristicallyarrayed into vertical groups that span the corti-cal layers. In humans, neocortical columns areapproximately 2 mm tall, have a diameter of0.5 mm, and contain approximately 60,000neurons (Rakic 2008). Neurons within col-umns are highly interconnected and share acommon function. At a finer scale, neocorticalcolumns can be subdivided into mini or micro-columns, which contain tens to hundreds ofneurons, and have been proposed to comprisethe basic unit of the neocortex (Mountcastle1998; Jones 2000; Rakic 2008). At the simplestlevel of description, the deepest layers of corticalneurons (layers 5 and 6) contain projectionneurons that connect areas of the cortex toone another or to subcortical structures. Layer4 is the layer within which most extracorticalinputs arrive, particularly from the sensorythalamus, whereas the superficial layers arecomposed mainly of local circuits that formreciprocal connections with the deep layers(Fig. 1A) (Thomson and Lamy 2007).

Groups of neocortical columns are orga-nized into functionally distinct neocorticalareas, such that the cortex is a patchwork offunctional modules, including areas for motorcontrol, vision, and hearing (Fig. 1B). Withsome notable exceptions, neocortical areaslack obvious anatomical boundaries, beingdistinguishable only by differences in cyto-architecture, chemoarchitecture, connectivity,

and gene expression. In the first accuratehistological analysis, Brodmann divided thehuman neocortex into areas based on serial sec-tioning to produce a map of cortical areas(Brodmann 2006). Brodmann’s maps havebeen largely confirmed by subsequent investi-gations, including studies involving electro-physical perturbation in live subjects andneurosurgical procedures on conscious patientsunder local anesthesia (Mountcastle 1998). Thefunctional importance of neocortical areas isalso known from the examination of subjectsthat have suffered loss or damage to particularareas—for example, damage to Broca’s orWernicke’s areas in the left hemisphere resultin an inability to process language and generatespeech.

In all species, the neocortex contains threemajor areas along the rostrocaudal axis(Fig. 1B): the rostral motor cortex, the medialsomatosensory cortex, and the caudal visualcortex; although the relative sizes of thoseareas differs markedly in different species.Although the broad organization of neocorticalareas shows remarkable similarity in differentmammals, the evolution of the neocortex ismarked by a dramatic increase in corticalvolume and in the total number of corticalareas. The neocortex of a mammal such as theshrew has a surface area a thousand-foldsmaller than that of a human and has 10-foldfewer functional areas (Striedter 2005). Under-standing the mechanisms that pattern the neo-cortex into the adult areas represents a majorchallenge and focus of modern developmentalneurobiology.

DEVELOPMENT OF THE NEOCORTEX—THE RADIAL UNIT HYPOTHESIS

In mice, a model organism commonly used forresearch in this area, development of the neo-cortex begins at embryonic day 9.5 (E9.5) withthe appearance of the cerebral vesicles fromthe dorsal surface of the rostral neural tube.Initially, the neocortical primordium is com-prised of an apparently homogenous poolof neural stem cells. The first postmitoticneurons of the neocortex, the Cajal-Retzius

S.N. Sansom and F.J. Livesey

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cells, appear at E10.5 to form a transient struc-ture known as the marginal zone that laterbecomes layer 1. The subsequent generation ofthe glutamatergic projection neurons of layers2–6 by neocortical stem cells takes place fromE11 until approximately E17, with neurons ofdeep layers (layer 6) produced before those ofthe outer layers (2/3) (Fig. 2A). Postmitoticlayer neurons born in the VZ migrate radiallyoutwards to form the cortical layers thattogether comprise the cortical plate. The out-wards migration of neurons of layers VI to IItakes place along the processes of radial glialcells that span the width of the developing neo-cortex (Fig. 2B). Neurons of layer 6 are first toleave the ventricular zone and migrate radiallyto form the nascent cortical plate. Neurons oflayer V to II then migrate past those of layerVI and adopt successively superficial positions(Fig. 2A).

The radial migration of the neuronalprogeny of neocortical stem cells suggestedthat they might remain in spatial register withtheir mother cell, and this important idea wasproposed as the radial unit hypothesis(McConnell 1988; Rakic 1988b; Rakic 1990).Initial efforts to test the hypothesis, in whichretroviral labeling was used to mark and

visualize clones of neurons in the developingneocortex, failed to provide support for thismodel, instead finding evidence for both hori-zontal and radial clones of cells (Price andThurlow 1988; Walsh and Cepko 1992; Walshand Cepko 1993). However, after some debate,subsequent studies of the subcortical originsof inhibitory interneurons of the cortex andfurther lineage analyses confirmed the accuracyof the radial unit hypothesis (Tan and Breen1993; Soriano et al. 1995; Anderson et al.1997). These studies revealed that the gluta-mergic neuronal progeny of neocortical stemcells form radial columns that span the corticalplate (Tan and Breen 1993; Soriano et al.1995) (Fig 2C). By contrast, the horizontalclones observed in the initial retroviral studieswere found to represent the tangential dis-persion of inwardly migrating inhibitoryGABAergic interneurons arriving from theganglionic eminences of the ventral forebrain(O’Rourke et al. 1995; Anderson et al. 1997;Wonders and Anderson 2006). Thus, the post-mitotic neuronal progeny of neocortical stemcells migrate radially upward to form thelayers and areas of the adult neocortex.Importantly, the radial unit hypothesis gaverise to the idea that a spatial pattern in

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Figure 1. The arrangement of neocortical circuits and areas in the adult mouse brain. (A) The basic corticalcircuit. Major extracortical inputs terminate in layer 4 and to a lesser degree in layer 6. Layer 4 neuronsproject to layers 2 and 3, which in turn innervate layers 5 and 6, the major output layers of the cortex. (B)Dorsal view of the adult mouse brain, with the functional roles of histologically defined areas labeled. Areamaps redrawn from Wree et al. 1983.

Gradients in the Brain

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neocortical stem cells is transferred to theneurons of the cortical plate.

NEOCORTICAL AREA FORMATION

Areas of the neocortex cannot be distinguishedby cytoarchitecture until postnatal day 2 (P2) inthe mouse, although at birth several genes showregion-specific expression in the cortical platein partial correspondence to the emerging areaboundaries. These include cell adhesionmolecules (Cad6, Cad8), the orphan nuclearreceptor RORß, the HLH transcription factorId2, a neurotrophin receptor (p75), and mole-cules involved in cell migration and axon guid-ance, such as EphA7 and ephrin-A5 (Bishopet al. 2002). Remarkably, these genes showdifferent rostrocaudal extents of expression ineach layer of the neocortex. For example,although Id2 has a defined border of expression

in layer-5 neurons that corresponds to thefuture boundary between somatosensory area1 (S1) and motor area 1 (M1), in layers 2 and3, Id2 has a graded expression that continuesacross the entire tangential extent of the neo-cortex (Rubenstein et al. 1999; Garel et al.2003). At present, there is little evidence forgene expression that is cleanly area-specific.

The question of neocortical area specifica-tion gave rise to two competing hypotheses:the protocortex hypothesis and the protomaphypothesis. In the protocortex hypothesis (vander Loos and Woolsey 1973; O’Leary 1989), itwas suggested that area pattern in the develop-ing neocortex was extrinsically specified by theinnervating thalamocortical afferents (TCAs),sensory afferent neurons that project from dis-tinct nuclei of the thalamus to specific corticalareas. These axons begin innervating the neo-cortex from E14.5, and by E15.5 have begun to

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Figure 2. Cortical stem cells are multipotent, generating neurons for each layer in a fixed temporal order.(A) Layer-specific neurons are generated in a fixed temporal order in a classic inside-out pattern over 6 daysin the mouse cortex. (B) Neurons (blue) and generated by radial glia stem cells (green) in the ventricularzone and subsequently migrate radially outwards into the cortical plate along the processes of the radial gliacells that span the width of the developing neocortex. (C) Cortical stem cells generate radially arrangedclones of neurons in mice and primates. Examples of retrovirally labeled clones are redrawn from Kornackand Rakic 1995 and Yu et al. 2009.





extend through the cortical plate. In contrast,the protomap hypothesis proposed that neocor-tical areas were patterned from a map of spatialidentity intrinsic to neocortical stem cells(Rakic 1988a). Thus, while the protocortexhypothesis advanced the idea that outsidefactors patterned a naıve cortical primordiumor “tabula rasa,” the protomap hypothesisargued that neocortical area pattern was intrin-sically established by neocortical stem cells.

Several lines of evidence now show that theprotomap hypothesis best describes the initialstages of neocortical patterning. First, hetero-topic transplantation studies have shown thatneocortical stem cells become regionallyspecified between E11 and E12, before thearrival of the thalamocortical afferents(Cohen-Tannoudji et al. 1994; Gitton et al.1999a,b; Gaillard et al. 2003). Second, in bothGbx-2 and Mash 1 mutant mice, in which theTCA projection is respectively impaired orabsent, region-specific gene expression is stillobserved in the cortical plate (Miyashita-Linet al. 1999; Liu et al. 2000). Finally, additionalsupport for the protomap hypothesis camefrom the identification of genes differentiallyexpressed across the neocortical primordiumbefore the arrival of the subcortical afferents.Notable among those genes are transcriptionfactors expressed in opposing rostral-caudalgradients. For example, Pax6 is expressed in arostrolateral high to caudomedial low gradient;Emx2 has a caudomedial high to rostrolaterallow gradient; and COUP-TFI displays a caudo-lateral high to rostromedial low gradient(Walther and Gruss 1991; Simeone et al. 1992;Stoykova and Gruss 1994; Gulisano et al.1996; Mallamaci et al. 1998; Liu et al. 2000).Other classes of genes commonly associatedwith development, such as the Ephs andEphrins, and Cadherins, were also found to beexpressed in gradients in neocortical stem cells(Mackarehtschian et al. 1999; Nakagawa et al.1999). These gradients of gene expression pro-vided the means by which a neocortical proto-map might be encoded in neocortical stem cells.

However, although strong evidence sup-ports the presence of an intrinsic pattern orprotomap in the neocortical primordium, a

role for TCAs in the differentiation, refinement,and maintenance of area identity has also beenestablished (Frost and Schneider 1979; Sur et al.1988; Schlaggar and O’Leary 1991; Cohen-Tannoudji et al. 1994; Nothias et al. 1998;Gitton et al. 1999a,b). The protomap versusprotocortex debate is therefore now resolvedinto a synthesis. Spacial information is con-ferred on neurons of the cortical plate from aprotomap intrinsic to the neocortical primor-dium. This area pattern of the cortical plate isthen refined and maintained by the innervatingthalamocortical afferents. This model of neo-cortical area specification immediately raisestwo key questions: How is the neocortical pro-tomap established and how is it interpreted inneocortical stem cells to give rise to region-specific gene expression in the cortical plate?

SECRETED GROWTH FACTORS PATTERNTHE ROSTROCAUDAL AXIS OF THENEOCORTEX

The idea that secreted signaling factors mightpattern a neocortical protomap has existed fordecades (Creutzfeldt 1977; Rakic 1988a; Kuljisand Rakic 1990; Barbe and Levitt 1991; Rakic1991). The first demonstration that such apatterning mechanism is used to generate thearea map came from Fukuchi-Shimogori andGrove, who investigated the role of fibroblastgrowth factor (FGF) signaling in area specifica-tion (Fukuchi-Shimogori and Grove 2001). Theexpression of FGFs 3, 8, 17, 15, and 18 from E9.5until E12.5 at the rostral midline of the neo-cortex in the commissural plate and surround-ing tissue suggested the presence of a rostral,FGF-secreting signaling center (Bachler andNeubuser 2001).

In an elegant study, Fukuchi-Shimogori andGrove performed in vivo electroporationexperiments to investigate the role of FGF8 inpatterning the neocortex (Fukuchi-Shimogoriand Grove 2001). Electroporations were per-formed at E11.5, at the time when the gener-ation of the neuronal layers is just beginning.Augmenting the endogenous rostral FGF8source led to an expansion of rostral corticalareas at the expense of caudal areas, as assayed





by the expression of regional cortical platemarkers at P0. Conversely, it was found thatreduction of the rostral FGF signal by theelectroporation of a soluble FGF receptor(sFGFR3) caused a dramatic shrinkage ofrostral neocortical areas, and this phenotypewas also later observed in FGF8 hypomorphicmice (Fig. 3A) (Garel et al. 2003). Most

strikingly, Fukuchi-Shimogori and Grove(2001) found that the introduction of anectopic FGF8 source in the caudal cortexresulted in a partial mirror duplication of thewhisker barrel field of the more rostral somato-sensory cortex (Fig. 3A).

In addition to demonstrating that thesecreted factor FGF8 specifies a rostral identity

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Figure 3. The roles of FGF signaling and graded transcription factor expression in neocortical pattern formation.(A) Diagram of a dorsal view of the mouse cortex (ncx, neocortex; ob, olfactory bulb; M, medial; L, lateral;R, rostral; C, caudal), with the major axes and areas labeled (M, motor; S1, somatosensory; V1, primaryvisual). The effects of altering the levels and positions of FGF8 signaling are shown. Increasing FGF8 levelsrostrally increases the size of the rostral motor area at the expense of caudal areas. Conversely, antagonizingFGF8 signaling by expression of the extracellular face of an FGF receptor (sFGFR3) results in a reduction ofthe size of M1 and an increase in the size of caudal areas. Introduction of a new, caudal source of FGF8results in the generation of a mirror-image of the S1 area in caudal cortex. (B) The transcription factorsCOUP-TF1, Emx2, Sp8, and Pax6 are expressed in gradients along the rostrocaudal axis of the cortex asshown. The effects of null mutations in each transcription factor are shown (CKO, cortex-specific knockout).See text for details of each phenotype.





in the neocortex, these studies suggested that itmight function in the manner of a morphogen.In the classical model of a morphogen gradient,the morphogen is released from a localizedsource and acts in a concentration-dependentmanner to specify two or more cell fates. Inthe neocortex, FGFs are released from a local-ized source at the rostral midline and the pre-dictable shifts in regional gene expression onthe reduction or augmentation of this sourcesuggest that FGFs may act in a concentration-dependent manner to specify rostrocaudalpositional identity in neocortical stem cells.Specifically, these data suggest that a high con-centration of FGF specifies the rostral-mostmotor cortex, whereas lower concentrationsdirect somatosensory and visual cortical fates.

However, an FGF protein gradient across theneocortical ventricular zone has not beendirectly visualized, and the question of howsuch a gradient could form across the rostro-caudal axis of the neocortex remains unan-swered. At the beginning of neocorticaldevelopment (E9.5), the neocortical primor-dium is 300 mm in length, approximately thesame length as the syncytial blastoderm of thefruit fly, but then doubles in length within aday and reaches a length of 7000 mm in theadult mouse (Loncar and Singer 1995; Leinet al. 2007). FGFs are produced by stem cellsat the rostral midline and at the rostral pole ofeach cerebral hemisphere. One possibility isthat FGFs diffuse solely through the pseudo-stratified neuroepithelium. Alternatively, FGFsmay accumulate in the fluid-filled ventricles ofboth cerebral hemispheres, which are lined bythe apical surfaces of the cortical stem cells.Therefore, it may be that an FGF gradientforms within this cavity across the ventricularsurface of the developing neocortex.

Although definitive evidence for a gradientof FGF8 protein is lacking, evidence exists fora gradient of FGF signaling in the ventricularzone along the rostrocaudal axis. FGFs bind toand signal through four classical tyrosinekinase receptors (FGFR1–4), and three ofthese receptors are known to be expressed byneocortical stem cells. FGF receptors FGFR1,-2, and -3 are all expressed in neocortical stem

cells, with FGFR1 showing a high rostralto low caudal expression gradient, whereasFGFR2 and FGFR3 have low rostral to highcaudal gradient of expression (Hebert et al.2003). In mice, the forebrain-specific loss ofFGFR1 causes a rostral shift in area identity, inaddition to causing the loss of the olfactorybulbs, further supporting the idea that FGFfunctions as a rostrally secreted morphogen tospecify area identity (Hebert et al. 2003).

Downstream of the receptors, the activationof the intracellular components of the FGFsignaling pathway, such as the MAP kinasesErk1/2, has not been investigated in the cortex.However, the FGF inhibitors Sprouty1, -2, and-4, which are members of the FGF synexpressiongroup and direct targets of Ras-Erk signaling,are expressed in rostral-high gradients in theneocortex from E9.5 to E12.5, suggesting thepresence of a gradient of active MAP kinasesignaling in this tissue (Minowada et al. 1999;Mason et al. 2006). Furthermore, the expressiongradients of Sprouty1 and -2 are diminished byreduced levels of FGF signaling (Cholfin andRubenstein 2008).

The three members of the PEA3 group ofthe Ets domain transcription factors Erm(Etv5), Pea3 (Etv4), and Er81 (Etv1), aremembers of the FGF synexpession group andknown transcriptional targets of FGF signaling(Buchwalter et al. 2004; de Launoit et al.2006). In the developing neocortex, these tran-scription factors are expressed in nested rostral-high to caudal-low gradients and the expressionof these factors can be decreased or increased byreducing or augmenting the rostral FGF source(Fukuchi-Shimogori and Grove 2003). Finally,two other genes, the transcription factorSp8 and the imprinted gene Mest/Peg1, theexpression of which is induced by FGF signalingin the neocortex, are also expressed in rostral-high to caudal gradient across the neocorticalprimordium (Sansom et al. 2005; Sahara et al.2007; Zembrzycki et al. 2007).

Thus, FGFs pattern the cortex to form neo-cortical areas. Although it remains formallypossible that FGF signaling acts indirectly orthrough secondary signals to do so, strong evi-dence at many levels of the FGF signaling





pathway suggests that FGFs directly pat-tern the cortex in a concentration-dependentmanner. FGF signaling acts to determine pos-itional identity in the neocortex by controllinga cellular network that includes several inter-acting transcription factors, the elements ofwhich are considered below and summarizedin Figure 4.

FGF SIGNALING ESTABLISHES ANOPPOSING GRADIENT OF THECAUDALISING TRANSCRIPTIONFACTOR COUP-TFI

The orphan nuclear receptor COUP-TFI is atranscription factor expressed in a caudomedial-high to rostrolateral-low gradient in the devel-oping neocortex (Zhou et al. 2001) (Fig. 3B).The roles of COUP-TFI in cortical developmenthave been studied in germline and cortex-specific mutant mice, and have also been inves-tigated by in vivo overexpression (Zhou et al.2001; Armentano et al. 2007; Faedo et al.2008). In mice with a cortex-specific deletionof COUP-TFI, the expression of regional

cortical plate markers Cdh8, Id2, Fezf2, andEfna5 revealed that the balance of area patternwas changed such that the majority of thecortex was transformed into motor cortex,and the somatosensory and visual areas weregreatly reduced in size and located at thecaudal pole of the structure (Fig. 3B)(Armentano et al. 2007). COUP-TFI is thereforecrucial for promoting a caudal identity in theneocortex. The rostralization of the neocortexin the absence of COUP-TFI, and the estab-lished role for COUP-TFI as a transcriptionalrepressor, suggest that COUP-TFI acts tospecify caudal neocortical identity by repressinga rostral fate.

The gradient of COUP-TFI in the develop-ing neocortex lies in opposition to the rostral-high to caudal-low gradient of FGF signaling,and it is natural to ask whether these gradientsregulate one another. There is good evidencethat FGF signaling regulates COUP-TFIexpression. The sensitivity of the COUP-TFIgradient to levels of FGF signaling hasbeen shown both in vivo and in vitro.Augmentation of the rostral FGF source via in

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Figure 4. The outline of a cellular network for controlling neocortical area formation. FGF8, signaling throughFGF receptors, induces or increases expression of the rostrally expressed transcription factors, the ETS factorsPax6 and FGF8. Sp8 in turn increases expression of Fgf8 rostrally. The rostral and caudal transcriptionfactors show a degree of mutual cross-repression. Among the caudal transcription factors, COUP-TFIappears to be upstream of Emx2, as loss of Emx2 does not alter COUP-TFI expression, and COUP-TFI-nullcortices have a more severe patterning phenotype than that observed in Emx2 nulls.





utero electroporation results in a contraction ofthe COUP-TFI gradient, whereas reduction ofthe endogenous FGF source induces a rostralexpansion of COUP-TFI expression (Fukuchi-Shimogori and Grove 2003; Garel et al. 2003).In vitro experiments have shown that COUP-TFI is rapidly down-regulated in corticalexplants in response to elevated FGF levels(Sansom et al. 2005). Interestingly, bindingsites for the FGF target genes, the Ets transcrip-tion factors, have been identified in theCOUP-TFI promoter, suggesting a possiblemechanism for the regulation of COUP-TFIby FGF signaling (Salas et al. 2002). The regu-lation of FGF signaling by COUP-TFI is lesswell understood. However, reduced levels ofphospho-Erk1/2 have been reported in theneocortex of mice constitutively overexpressingCOUP-TFI, indicating that COUP-TFI cannegatively regulate FGF signaling (Faedo et al.2008).

The ability of FGF to repress the expressionof COUP-TFI indicates that the Fgf-signalinggradient is responsible for establishing thecaudal-high to rostral-low gradient of COUP-TFI in the neocortex, in a manner analogousto that of the repression of dorsally expressedclass II transcription factors in the spinal cordby the ventrally secreted morphogen sonichedgehog (Briscoe and Ericson 2001). This sug-gests a default caudal identity for neocorticalstem cells in the absence of Fgf signaling, andin support of this supposition, neocortical stemcells produced by directed differentiation fromembryonic stem cells express COUP-TFI(Gaspard et al. 2008).

GRADIENTS OF TRANSCRIPTION FACTORSREFINE AREA PATTERNING

Downstream of FGF signaling and COUP-TFI,gradients of transcription factor expressionacross the field of neocortical stem cells areimportant for refining area pattern. Expressedin rostral-high to caudal-low gradients, thetranscription factors Pax6 and Sp8 promoterostral area formation, whereas the transcrip-tion factor Emx2 is expressed in an opposinggradient and promotes a caudal identity. Of

these transcription factors, evidence supportsa patterning role for Sp8 and Emx2, whereasthe effects of Pax6 on area patterning havebeen recently suggested by both gain- and loss-of-function studies to be indirect (Fig. 3B)(Manuel et al. 2007; Pinon et al. 2008).

Sp8 is the vertebrate homolog of theDrosophila Buttonhead (Btd) gene and is azinc factor transcription factor expressed inthe developing nervous system. It is stronglyexpressed in the commissural ridge at E8/E8.5and at E9.5 is expressed in a rostral-high tocaudal-low gradient across the neocortex. Theremoval of Sp8 from the developing cortexresults in a significant expansion of caudalareas at the expense of rostral areas, as assessedby cortical plate marker expression and cortical-thalamic connectivity (Fig. 3B) (Zembrzyckiet al. 2007). Sp8 is induced by FGF8 and canreciprocally regulate the expression of FGF8(Sahara et al. 2007). Sp8 may act directly topromote a rostral identity in the neocorticalstem cells, or indirectly by regulating theexpression of Fgf8 in the commissural plate.Although Sp8 expression is also lost in thecommissural plate of the cortex-specific knock-out, no change in FGF8 expression in thisregion was detected, and this together with thegraded expression of Sp8 in neocortical stemcells supports a direct role for this genein the patterning of neocortical stem cells(Zembrzycki et al. 2007). Additionally, invitro, Sp8 interacts on at a protein–proteinlevel with the caudal transcription factorEmx2, and Emx2 can repress the induction ofFgf8 by Sp8, indicating that these two transcrip-tion factors might cross-regulate one another(Zembrzycki et al. 2007).

The vertebrate homolog of the Drosophilaempty spiracles gene, Emx2, encodes a homeo-domain protein that is expressed in the rostro-lateral neural plate from E8.5 (Simeone et al.1992; Gulisano et al. 1996; Mallamaci et al.1998). In the neocortical primordium, Emx2is expressed in a caudomedial high to rostrolat-eral low expression gradient that is, like theCOUP-TFI expression gradient, sensitive toFGF signaling (Fig. 3). Examination of micewith absent, reduced, normal, and elevated





levels of Emx2 have revealed that rostrocaudalarea patterning shows a predictable and mea-sured response to Emx2 dosage (Hamasakiet al. 2004). The loss of Emx2 results in a con-traction of caudal areas and the expansionof rostral areas based on regionalized geneexpression in the cortical plate, with heterozy-gous mice showing an intermediate phenotype(Fig. 3) (Bishop et al. 2002; Muzio et al. 2002).In contrast, in mice overexpressing Emx2under the control of the Nestin promoter(ne-Emx2), there is an expansion of the caudalvisual area 1 (V1) and contraction of rostralmotor cortex (Hamasaki et al. 2004).

The shifts in area patterning observed inEmx2 mutant mice are smaller than thoseobserved in COUP-TFI mutants, and theCOUP-TFI expression gradient is unchangedin the neocortex of Emx2 null mice. Theseobservations, together with a reduction inEmx2 expression observed in COUP-TFI nullmice, indicate that Emx2 functions downstreamof COUP-TFI to refine the position of areaboundaries.

Like COUP-TFI, the expression of Emx2 isrepressed by FGF8 signaling. The augmentationof the rostral FGF8 source by in vivo electro-poration steepens the Emx2 gradient, whereasreduction of the endogenous FGF8 source,either by the electroporation of sFGFR3 orgenetically in FGF8 hypomorphic mice, causesan increase in rostral Emx2 expression that flat-tens, and nearly abolishes, the Emx2 gradient(Fukuchi-Shimogori and Grove 2003).

In turn, Emx2 has been shown to negativelyregulate FGF signaling. In the commissuralplate of Emx2 mutants, the expression of Fgf8and Fgf17 is up-regulated and Fgf15 is moregenerally up-regulated in the telencephalon(Cholfin and Rubenstein 2008). Converselythe endogenous expression of FGFs from thecommissural plate can be repressed by the elec-troporation of an Emx2 expressing plasmid(Fukuchi-Shimogori and Grove 2003). Theseobservations suggest that the patterning shiftsobserved in Emx2 null mice might be causedby elevated levels of FGF signaling, and thisidea has been tested by reducing rostral FGFlevels in Emx2 mutant animals. It was found

that the shifts observed in the Emx2 null neo-cortex can be rescued by reducing endogenousFGF levels via electroporation of a solubleFGF receptor (Fukuchi-Shimogori and Grove2003). Furthermore, the shifts can also besubstantially rescued by crossing Emx2 nullmice to a FGF17 null background (Cholfinand Rubenstein 2008). The observations thatEmx2 can bind Sp8 and inhibit the Sp8mediated induction of Fgf8 expression providea potential mechanism by which the negativeregulation of FGF signaling by Emx2 may beaccomplished (Sahara et al. 2007).

Therefore, FGF signaling also controls thegraded expression of transcription factorsother than COUP-TFI, most notably Sp8and Emx2, across the developing neocortex,which function in turn to cross-regulate Fgfsignaling and fine-tune the position of areaboundaries (Fig. 4). It is likely that significantcomponents of the neocortical patterningnetwork remain to be discovered. In particular,it is probable that COUP-TFI regulates theexpression of unknown factors that are impor-tant for caudal area identities.

TRANSLATING TRANSCRIPTION FACTORGRADIENTS TO AREA-SPECIFICEXPRESSION PATTERNS

Gradients function to pattern tissues bydirecting different cellular fates at differentthresholds of the graded signal. During em-bryogenesis, developmental gradients are typi-cally translated into distinct domains of geneexpression. In the ventral spinal cord, stemcells are initially spatially patterned by gradientsof gene expression in response to Shh signaling.In a dynamic and progressive process involvingthe use of cross-regulatory interactions, thesegradients are then translated into molecularlydistinct domains or pools of stem cells that areclearly delineated from one another (Dessaudet al. 2008). These domains of stem cells thengive rise to spatially appropriate neuronalprogeny (Fig. 5). Similarly, in the earlyDrosophila embryo, the gap genes interpret gra-dients to define spatially discrete populations ofstem cells (Ephrussi and St. Johnston 2004).





In the cortical plate of the maturing neo-cortex, as discussed previously, complexdomains of gene expression foreshadow theemergence of area boundaries, and it might beexpected that these domains of gene expres-sion are, in turn, prefigured by domains ofexpression in neocortical stem cells, in a situ-ation comparable to the formation of stempools across the dorsoventral axis in the spinalcord. However, extensive genomics screens fordiscrete rostrocaudal domains of expression inneocortical stem cells found no evidence forsuch domains, but rather revealed the gradedexpression of many more genes along this axis(Sansom et al. 2005). The role of these gradientsin encoding spatial information is illustrated byexperiments perturbing the high-caudal tolow-rostral gradient of Emx2. On the measuredincrease or decrease of the Emx2 protein gra-dient, predictable rostral-caudal shifts in areaposition are observed (Hamasaki et al. 2004).

A key question then is how the gradients ofgene expression in stem cells are read out toproduce regionalized gene expression in thecortical plate. Recent studies on the transcrip-tion factor Bhlhb5 suggest that the gradientsof expression in stem cells are transferred intograded expression of transcription factors in

neurons along the rostrocaudal axis (Joshiet al. 2008). Bhlhb5 is initially expressed ina high caudomedial to low rostrolateral gra-dient in all cortical neurons of layers 2–5.Subsequently, this gradient retracts from therostral cortex and is tuned into discretedomains of sharply bordered expressionbetween the sensory and motor cortex as thecortical plate matures. In the absence ofBhlhb5, the molecular identity of somatosen-sory and caudal motor cortex is severelyperturbed, indicating that this gene is crucialfor specifying area fate (Joshi et al. 2008).Thus, a currently poorly understood patterningprocess occurs in postmitotic neurons to trans-late gradients of expression into discretedomains of gene expression that reflect theorganization of cortical areas (Fig. 5B).

One striking feature of many of the knownarea-specific patterns of gene expression inneurons is the difference in spatial expressionin different layers. For example, the Id2 genehas markedly different spatial expression inlayer-5 neurons compared with its expressionin the neurons of layers 2 and 3 (Garel et al.2003). Therefore, it is not the case that eachstem cell has a spatial code that it passes on toall progeny equally. Rather, these observations

Neocortex

Motor VisualSomatosensory

Spinal cord A B

Neurons

Stem cells

Stem cells

IIlllllVVVl

Neocorticalareas

Neuronsof thecortical plate

Stem cells

Figure 5. The translation of a gradient-based neocortical map in stem cells into spatially discrete patterns ofneuronal gene expression in the neocortex. (A) In the developing spinal cord, an initial graded pattern oftranscription factor expression along the dorsoventral axis is progressively resolved into clearly delineated,molecularly distinct stem cell domains. Each spatially defined group of stem cells goes on to produce differentclasses of spinal cord neurons. (B) In contrast, in the developing neocortex, gradients of expression in theventricular zone do not resolve into discrete spatial domains. Instead, those gradients appear to be translatedinto discrete spatial expression in the neurons generated by those stem cells. Furthermore, the spacialexpression of specific genes often differs between different neuronal layers, as shown here for the gene Epha7.





suggest that neocortical stem cells interpretspatial information encoded by gradients ofgene expression together with temporal infor-mation to generate the appropriate spatialpattern in each neocortical layer.

THE CLONAL ANATOMY OF AREASPECIFICATION

Evidence is now emerging for a possible mech-anism by which the graded spatial informationpresent in neocortical stem cells may be pre-cisely mapped onto the fundamental functionalneuronal building blocks of the cortex, andthereby generates the columns and areas of themature cortex. Within areas of the adult neo-cortex, neurons are typically arranged incolumns that can be defined on the basis offunctional, molecular, or connectivity proper-ties, and typically consist of thousands ofneurons. For example, in the primary visualcortex alone, ocular dominance, orientation,hyper and color columns have been identified(Hubel et al. 1977).

These functional columns can be furthersubdivided. Early investigators, using lightmicroscopy and Nissl staining, discovered thatcortical neurons are arranged in fine radialcolumns (De Felipe and Jones 1988). Theseradial columns, now often referred to as minior microcolumns, consist of stereotypicallyvertically interconnected arrays of neuronsthat span the neocortical layers and shareextrinsic connectivity (Jones 2000; Rakic2008). Contrary to initial reports, microcol-umns have been shown to vary in neuronalnumber, composition, and size. In the humantemporal cortex, evidence has been foundfor repeating chains of about 11 neurons(Buldyrev et al. 2000), whereas the modularcolumns of pyramidal cells in the visual cortexof the monkey are typically comprised of 142neurons (Peters and Yilmaz 1993). In an attrac-tive hypothesis, microcolumns are viewed as thefundamental functional processing units of theneocortex underlying all broader divisions(Mountcastle 1998).

The studies that verified the radial unithypothesis found that the neuronal progeny of

neocortical stem cells form vertical arrays ofneurons, and these have been described as theontogenic units or columns of the neocortex(Rakic 1988a). Like functional microcolumns,these ontogenic columns, or radial clones, arenot uniform but differ in structure betweenareas (Rakic 1988a). An attractive theory hasbeen that the mini or microcolumns of highlyconnected neurons in the adult are groups ofclonally related neurons. In support of this, ithas recently been reported that radiallyaligned, clonally related neurons preferentiallyconnect to one another, suggesting that onto-genic units/radial clones may be a componentof functional microcolumns (Yu et al. 2009).If this observation generalizes to differentparts of the cortex, it would provide a poten-tially powerful framework for integratingspatial patterning of the cortex with the for-mation of vertically arranged, area-specific cir-cuits within cortical columns. Effectively, eachstem cell would produce a vertical chain ofhighly connected neurons specific to a regionof the cortex, and this simple functional unitwould be the most basic component of a cor-tical column and area: The spatial identity ofthe stem cell would then be read out as the pro-duction of a region-appropriate unit of neurons.

EVOLUTIONARY IMPLICATIONS OF AGRADIENT-BASED NEOCORTICALPATTERNING SYSTEM

The neocortex has rapidly evolved both in sizeand complexity during the evolution of themammalian phyla. Although the simplestmammals have only 10–20 neocortical areas,modern studies indicate that humans possessas many as 100 distinct neocortical areas(Mountcastle 1998). However, across evolution,the mammalian neocortex retains the samebasic areal organization, and it is clear that thearea pattern is progressively elaborated overevolutionary time (Striedter 2005).

The gradient based neocortical patterningmechanism described here provides the meansby which area pattern may have been conservedand elaborated as the neocortex increased insurface area and complexity. The expansion of





neocortical surface area throughout evolutionis thought to be because of an increase in thenumber of divisions undertaken by neocorticalstem cells, which has in turn resulted in anincreased number of ontogenic columns(Mountcastle 1998).

In line with this theory, a gradient-basedpatterning mechanism could readily scale asthe number of ontogenic units or clones in-creased, facilitating the observed increase incortical surface area while maintaining relativearea sizes and positions. If clones do comprisesome form of basic functional unit of thecortex, expanding stem cell number wouldsimply increase the number of processingunits available to form areas. Furthermore, thegradient-based patterning system describedhere, together with the gain of ontogenic unitsobserved across evolution, provides a meansfor novel area acquisition. As the neocortexincreased in size, new area identities along therostro-caudal axis may have been acquired bythe refinement of the mechanisms that interpretthe gradients patterning neocortical stem cells,and/or by the elaboration of the gradient-basedpattern in neocortical stem cells.

CONCLUSIONS

In the developing neocortex, area identity isspecified by gradients of transcription factorsestablished by morphogen gradient(s). Animportant feature of this system is that it doesnot operate in a linear manner but rather func-tions as a network (Fig. 4). At the core of thenetwork, Fgf signaling establishes a gradient ofthe transcription factor COUP-TFI, a processmodulated by the concomitant actions ofEmx2, which acts to repress Fgf signaling andfine-tune area position.

Although this patterning system shares fea-tures with the patterning of other tissues bysimilar gradients, it also has several strikingdifferences. In the spinal cord, a single mor-phogen, Shh, is responsible for patterning theventral half of the dorsoventral axis. In contrast,in the neocortex, different varieties of the mor-phogen, including FGF8, FGF17, and FGF15,act together to specify rostrocaudal fate.

Intriguingly, evidence is now emerging thatsuggests FGF8 acts to induce the expression ofFGF17 and FGF15 and that the different FGFligands specify distinct aspects of the neocorti-cal protomap. Recent evidence argues thatFGF8 acts to repress COUP-TFI signaling,whereas FGF17 has a cross-regulatory inter-action with Emx2 (Cholfin and Rubenstein2008).

Similar to the situation in the spinal cord,Fgf signaling from the rostral pole of the neo-cortex sets up gradients of transcriptionfactors by both activating and repressing geneexpression. However, in contrast to the spinalcord and the early Drosophila embryo, thesegradients are not then refined into distinctstem cell domains. In the neocortex, stem cellspatterned by gradients give rise to spatially pat-terned progeny. Strikingly, neocortical stemcells give rise to differently spatially patternedneuronal progeny at different times. Theseobservations suggest that neocortical stemcells interpret spatial information encoded bygradients of gene expression together withtemporal information to generate the appropri-ate spatial pattern in each neocortical layer.Therefore, in a mechanism unique to the neo-cortex, graded information is resolved intoregion-specific gene expression in the postmito-tic progeny of the stem cells.

REFERENCES

Anderson SA, Eisenstat DD, Shi L, Rubenstein JL. 1997.Interneuron migration from basal forebrain to neocortex:Dependence on Dlx genes. Science 278: 474–476.

Armentano M, Chou SJ, Tomassy GS, Leingartner A,O’Leary DD, Studer M. 2007. COUP-TFI regulates thebalance of cortical patterning between frontal/motorand sensory areas. Nat Neurosci 10: 1277–1286.

Bachler M, Neubuser A. 2001. Expression of members of theFgf family and their receptors during midfacial develop-ment. Mech Dev 100: 313–316.

Barbe MF, Levitt P. 1991. The early commitment of fetalneurons to the limbic cortex. J Neurosci 11: 519–533.

Bishop KM, Rubenstein JLR, O’Leary DDM. 2002. Distinctactions of Emx1, Emx2, and Pax6 in regulating the speci-fication of areas in the developing neocortex. J Neurosci22: 7627–7638.

Briscoe J, Ericson J. 2001. Specification of neuronal fatesin the ventral neural tube. Curr Opin Neurobiol 11:43–49.





Brodmann K. 2006. Brodmann’s localisation in the cerebralcortex. Springer, New York.

Buchwalter G, Gross C, Wasylyk B. 2004. Ets ternarycomplex transcription factors. Gene 324: 1–14.

Buldyrev SV, Cruz L, Gomez-Isla T, Gomez-Tortosa E,Havlin S, Le R, Stanley HE, Urbanc B, Hyman BT.2000. Description of microcolumnar ensembles inassociation cortex and their disruption in Alzheimerand Lewy body dementias. Proc Natl Acad Sci 97:5039–5043.

Cholfin JA, Rubenstein JL. 2008. Frontal cortex subdivisionpatterning is coordinately regulated by Fgf8, Fgf17, andEmx2. J Comp Neurol 509: 144–155.

Cohen-Tannoudji M, Babinet C, Wassef M. 1994. Earlydetermination of a mouse somatosensory cortexmarker. Nature 368: 460–463.

Creutzfeldt OD. 1977. Generality of the functional structureof the neocortex. Naturwissenschaften 64: 507–517.

De Felipe J, Jones EG. 1988. Cajal on the cerbral cortex.In An annotated translation of the complete writings.Oxford University Press, New York.

de Launoit Y, Baert JL, Chotteau-Lelievre A, Monte D,Coutte L, Mauen S, Firlej V, Degerny C, Verreman K.2006. The Ets transcription factors of the PEA3 group:Transcriptional regulators in metastasis. BiochimBiophys Acta 1766: 79–87.

Dessaud E, McMahon AP, Briscoe J. 2008. Pattern formationin the vertebrate neural tube: A sonic hedgehogmorphogen-regulated transcriptional network. Develop-ment 135: 2489–2503.

Ephrussi A, St. Johnston D. 2004. Seeing is believing:The bicoid morphogen gradient matures. Cell 116:143–152.

Ericson J, Briscoe J, Rashbass P, van Heyningen V, Jessell TM.1997. Graded sonic hedgehog signaling and the specifica-tion of cell fate in the ventral neural tube. Cold SpringHarb Symp Quant Biol 62: 451–466.

Faedo A, Tomassy GS, Ruan Y, Teichmann H, Krauss S,Pleasure SJ, Tsai SY, Tsai MJ, Studer M, Rubenstein JL.2008. COUP-TFI coordinates cortical patterning, neuro-genesis, and laminar fate and modulates MAPK/ERK,AKT, and beta-catenin signaling. Cereb Cortex 18:2117–2131.

Frost DO, Schneider GE. 1979. Plasticity of retinofugal pro-jections after partial lesions of the retina in newbornSyrian hamsters. J Comp Neurol 185: 517–567.

Fukuchi-Shimogori T, Grove EA. 2001. Neocortex pattern-ing by the secreted signaling molecule FGF8. Science294: 1071–1074.

Fukuchi-Shimogori T, Grove EA. 2003. Emx2 patterns theneocortex by regulating FGF positional signaling. NatNeurosci 6: 825–831.

Gaillard A, Nasarre C, Roger M. 2003. Early (E12) corticalprogenitors can change their fate upon heterotopic trans-plantation. Eur J Neurosci 17: 1375–1383.

Garel S, Huffman KJ, Rubenstein JLR. 2003. Molecularregionalization of the neocortex is disrupted in Fgf8hypomorphic mutants. Development 130: 1903–1914.

Gaspard N, Bouschet T, Hourez R, Dimidschstein J, NaeijeG, van den Ameele J, Espuny-Camacho I, Herpoel A,Passante L, Schiffmann SN, et al. 2008. An intrinsic

mechanism of corticogenesis from embryonic stemcells. Nature 455: 351–357.

Gitton Y, Cohen-Tannoudji M, Wassef M. 1999a. Role ofthalamic axons in the expression of H-2Z1, a mousesomatosensory cortex specific marker. Cereb Cortex 9:611–620.

Gitton Y, Cohen-Tannoudji M, Wassef M. 1999b. Specifi-cation of somatosensory area identity in corticalexplants. J Neurosci 19: 4889–4898.

Gulisano M, Broccoli V, Pardini C, Boncinelli E. 1996.Emx1 and Emx2 show different patterns of expressionduring proliferation and differentiation of the developingcerebral cortex in the mouse. Eur J Neurosci 8:1037–1050.

Hamasaki T, Leingartner A, Ringstedt T, O’Leary DDM.2004. EMX2 regulates sizes and positioning of theprimary sensory and motor areas in neocortex by directspecification of cortical progenitors. Neuron 43:359–372.

Hebert JM, Lin M, Partanen J, Rossant J, McConnellSK. 2003. FGF signaling through FGFR1 is required forolfactory bulb morphogenesis. Development 130: 1101–1111.

Hubel DH, Wiesel TN, Stryker MP. 1977. Orientationcolumns in macaque monkey visual cortex demonstratedby the 2-deoxyglucose autoradiographic technique.Nature 269: 328–330.

Jones EG. 2000. Microcolumns in the cerebral cortex. ProcNatl Acad Sci 97: 5019–5021.

Joshi PS, Molyneaux BJ, Feng L, Xie X, Macklis JD, Gan L.2008. Bhlhb5 regulates the postmitotic acquisition ofarea identities in layers II-Vof the developing neocortex.Neuron 60: 258–272.

Kornack DR, Rakic P. 1995. Radial and horizontal deploy-ment of clonally related cells in the primate neocortex:Relationship to distinct mitotic lineages. Neuron 15:311–321.

Kuljis RO, Rakic P. 1990. Hypercolumns in primate visualcortex can develop in the absence of cues from photo-receptors. Proc Natl Acad Sci 87: 5303–5306.

Lein ES, Hawrylycz MJ, Ao N, Ayres M, Bensinger A,Bernard A, Boe AF, Boguski MS, Brockway KS, ByrnesEJ, et al. 2007. Genome-wide atlas of gene expression inthe adult mouse brain. Nature 445: 168–176.

Liu Q, Dwyer ND, O’Leary DD. 2000. Differentialexpression of COUP-TFI, CHL1, and two novel genesin developing neocortex identified by differentialdisplay PCR. J Neurosci 20: 7682–7690.

Loncar D, Singer SJ. 1995. Cell membrane formation duringthe cellularization of the syncytial blastoderm ofDrosophila. Proc Natl Acad Sci 92: 2199–2203.

Mackarehtschian K, Lau CK, Caras I, McConnell SK. 1999.Regional differences in the developing cerebral cortexrevealed by ephrin-A5 expression. Cereb Cortex 9:601–610.

Mallamaci A, Iannone R, Briata P, Pintonello L, Mercurio S,Boncinelli E, Corte G. 1998. EMX2 protein in the devel-oping mouse brain and olfactory area. Mech Dev 77:165–172.

Manuel M, Georgala PA, Carr CB, Chanas S, KleinjanDA, Martynoga B, Mason JO, Molinek M, Pinson J,





Pratt T, et al. 2007. Controlled overexpression of Pax6 invivo negatively autoregulates the Pax6 locus, causingcell-autonomous defects of late cortical progenitor pro-liferation with little effect on cortical arealization.Development 134: 545–555.

Mason JM, Morrison DJ, Basson MA, Licht JD. 2006.Sprouty proteins: Multifaceted negative-feedback regula-tors of receptor tyrosine kinase signaling. Trends Cell Biol16: 45–54.

McConnell SK. 1988. Fates of visual cortical neurons in theferret after isochronic and heterochronic transplantation.J Neurosci 8: 945–974.

Minowada G, Jarvis LA, Chi CL, Neubuser A, Sun X,Hacohen N, Krasnow MA, Martin GR. 1999. VertebrateSprouty genes are induced by FGF signaling and cancause chondrodysplasia when overexpressed. Develop-ment 126: 4465–4475.

Miyashita-Lin EM, Hevner R, Wassarman KM, Martinez S,Rubenstein JL. 1999. Early neocortical regionalizationin the absence of thalamic innervation. Science 285:906–909.

Mountcastle VB. 1998. The cerebral cortex. HarvardUniversity Press, Cambridge, Massachusetts.

Muzio L, DiBenedetto B, Stoykova A, Boncinelli E, Gruss P,Mallamaci A. 2002. Emx2 and Pax6 control regionaliza-tion of the pre-neuronogenic cortical primordium.Cereb Cortex 12: 129–139.

Nakagawa Y, Johnson JE, O’Leary DD. 1999. Graded andareal expression patterns of regulatory genes and cadher-ins in embryonic neocortex independent of thalamocor-tical input. J Neurosci 19: 10877–10885.

Nothias F, Fishell G, Ruiz i Altaba A. 1998. Cooperation ofintrinsic and extrinsic signals in the elaboration ofregional identity in the posterior cerebral cortex. CurrBiol 8: 459–462.

O’Leary DD. 1989. Do cortical areas emerge from a proto-cortex? Trends Neurosci 12: 400–406.

O’Rourke NA, Sullivan DP, Kaznowski CE, Jacobs AA,McConnell SK. 1995. Tangential migration of neuronsin the developing cerebral cortex. Development 121:2165–2176.

Peters A, Yilmaz E. 1993. Neuronal organization in area 17of cat visual cortex. Cereb Cortex 3: 49–68.

Pinon MC, Tuoc TC, Ashery-Padan R, Molnar Z, StoykovaA. 2008. Altered molecular regionalization and normalthalamocortical connections in cortex-specific Pax6knock-out mice. J Neurosci 28: 8724–8734.

Price J, Thurlow L. 1988. Cell lineage in the rat cerebralcortex: A study using retroviral-mediated gene transfer.Development 104: 473–482.

Rakic P. 1988a. Specification of cerebral cortical areas.Science 241: 170–176.

Rakic P. 1990. Radial unit hypothesis of cerebral corticalevolution. In Principles and design and operation of thebrain (ed. J.C. Eccles and O. Creutzfeldt), pp. 25–48.Vatican City: Pontificae Academiae Scripta Varia.

Rakic P. 1991. Experimental manipulation of cerebralcortical areas in primates. Philos Trans R Soc Lond BBiol Sci 331: 291–294.

Rakic P. 2008. Confusing cortical columns. Proc Natl AcadSci 105: 12099–12100.

Rakic PSW. 1988b. Intrinsic and extrinsic determinantsof neocortical parcelation: A radial unit model.In Neurobiology of the neocortex, pp. 2–28. New York,Wiley.

Rubenstein JL, Anderson S, Shi L, Miyashita-Lin E, BulfoneA, Hevner R. 1999. Genetic control of corticalregionalization and connectivity. Cereb Cortex 9:524–532.

Sahara S, Kawakami Y, Izpisua Belmonte JC, O’Leary DD.2007. Sp8 exhibits reciprocal induction with Fgf8 buthas an opposing effect on anterior-posterior corticalarea patterning. Neural Develop 2: 10.

Salas R, Petit FG, Pipaon C, Tsai MJ, Tsai SY. 2002.Induction of chicken ovalbumin upstream promoter-transcription factor I (COUP-TFI) gene expression ismediated by ETS factor binding sites. Eur J Biochem269: 317–325.

Sansom SN, Hebert JM, Thammongkol U, Smith J, NisbetG, Surani MA, McConnell SK, Livesey FJ. 2005.Genomic characterisation of a Fgf-regulated gradient-based neocortical protomap. Development 132: 3947–3961.

Sato T, Joyner AL, Nakamura H. 2004. How does Fgfsignaling from the isthmic organizer induce midbrainand cerebellum development? Dev Growth Differ 46:487–494.

Schlaggar BL, O’Leary DD. 1991. Potential of visual cortexto develop an array of functional units unique to somato-sensory cortex. Science 252: 1556–1560.

Simeone A, Gulisano M, Acampora D, Stornaiuolo A,Rambaldi M, Boncinelli E. 1992. Two vertebrate homeo-box genes related to the Drosophila empty spiracles geneare expressed in the embryonic cerebral cortex. Embo J 11:2541–2550.

Soriano E, Dumesnil N, Auladell C, Cohen-Tannoudji M,Sotelo C. 1995. Molecular heterogeneity of progenitorsand radial migration in the developing cerebral cortexrevealed by transgene expression. PNAS 92: 11676–11680.

Stoykova A, Gruss P. 1994. Roles of Pax-genes in developingand adult brain as suggested by expression patterns.J Neurosci 14: 1395–1412.

Striedter G. 2005. Principles of brain evolution. Sinauer,Sunderland, Massachusetts.

Sur M, Garraghty PE, Roe AW. 1988. Experimentallyinduced visual projections into auditory thalamus andcortex. Science 242: 1437–1441.

Tan SS, Breen S. 1993. Radial mosaicism and tangential celldispersion both contribute to mouse neocortical devel-opment. Nature 362: 638–640.

Thomson AM, Lamy C. 2007. Functional maps of neocorti-cal local circuitry. Front Neurosci 1: 19–42.

van der Loos H, Woolsey TA. 1973. Somatosensory cortex:Structural alterations following early injury to senseorgans. Science 179: 395–398.

Walsh C, Cepko CL. 1992. Widespread dispersion of neur-onal clones across functional regions of the cerebralcortex. Science 255: 434–440.

Walsh C, Cepko CL. 1993. Clonal dispersion in proliferativelayers of developing cerebral cortex. Nature 362:632–635.





Walther C, Gruss P. 1991. Pax-6, a murine paired box gene, isexpressed in the developing CNS. Development 113:1435–1449.

Wonders CP, Anderson SA. 2006. The origin and specifica-tion of cortical interneurons. Nat Rev Neurosci 7:687–696.

Wree A, Zilles K, Schleicher A. 1983. A quantitativeapproach to cytoarchitectonics. VIII. The areal patternof the cortex of the albino mouse. Anat Embryol (Berl)166: 333–353.

Yu YC, Bultje RS, Wang X, Shi SH. 2009. Specific synapsesdevelop preferentially among sister excitatory neuronsin the neocortex. Nature 458: 501–504.

Zembrzycki A, Griesel G, Stoykova A, Mansouri A. 2007.Genetic interplay between the transcription factors Sp8and Emx2 in the patterning of the forebrain. NeuralDevelop 2: 8.

Zhou C, Tsai SY, Tsai MJ. 2001. COUP-TFI: An intrinsicfactor for early regionalization of the neocortex. Genesand Dev 15: 2054–2059.





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