Pallio-pallial tangential migrations and growth signaling: new scenario for cortical evolution?

Genoarchitectural Definition of the Adult Mouse Mesocortical Ring: A Contribution to Cortical Ring Theory

Data mining was performed at the databases of the Allen Institute for Brain Science (RRID:SCR_017001) searching for genes expressed selectively throughout the adult mouse mesocortex (transitional cortex ring predicted within the concentric ring theory of mammalian cortical structure, in contrast with central isocortex [ICx] and peripheral allocortex). We aimed to explore a shared molecular profile selective of all or most mesocortex areas. This approach checks and corroborates the precision of other previous definitory criteria, such as poor myelination and high kainate receptor level. Another aim was to examine which cortical areas properly belong to mesocortex. A total of 34 positive adult selective marker genes of mesocortex were identified, jointly with 12 negative selective markers, making a total of 46 markers. All of them identify the same set of cortical areas surrounding the molecularly different ICx as well as excluding adjacent allocortex. Four representative mesocortex markers-Crym, Lypd1, Cdh13, and Smoc2-are amply illustrated, jointly with complementary material including myelin basic protein, to check myelination, and Rorb, to check layer 4 presence. The retrosplenial (ReSp) area, long held to be mesocortical, does not share any of the 46 markers of mesocortex and instead expresses Nr4a2 and Tshz2, selective parahippocampal allocortex markers. Moreover, it is not hypomyelinic and lacks a Rorb-positive layer 4, aspects generally present in mesocortex. Exclusion of the ReSp area from the mesocortex ring reveals the latter to be closed at this locus instead by two adjacent areas previously thought to be associative visual ICx (reidentified here molecularly as postsplenial and parasplenial mesocortex areas). The concepts of ICx, mesocortex, and parahippocampal allocortex are thus subtly modified by substantial molecular evidence.

Evolution of the nervous system by acquisition of retrovirus-derived genes in mammals

In the course of evolution, the most highly developed organ is likely the brain, which has become more complex over time and acquired diverse forms and functions in different species. In particular, mammals have developed complex and high-functioning brains, and it has been reported that several genes derived from retroviruses were involved in mammalian brain evolution, that is, generating the complexity of the nervous system. Especially, the sushi-ichi-related retrotransposon homolog (SIRH)/retrotransposon gag-like (RTL) genes have been suggested to play a role in the evolutionary processes shaping brain morphology and function in mammals. Genetic mutation and altered expression of genes are linked to neurological disorders, highlighting how the acquisition of virus-derived genes in mammals has both driven brain evolution and imposed a susceptibility to diseases. This review provides an overview of the functions, diversity, evolution and diseases associated with SIRH/RTL genes in the nervous system. The contribution of retroviruses to brain evolution is an important research topic in evolutionary biology and neuroscience, and further insights are expected to be gained through future studies.

From active affordance to active inference: vertical integration of cognition in the cerebral cortex through dual subcortical control systems

In previous papers, we proposed that the dorsal attention system's top-down control is regulated by the dorsal division of the limbic system, providing a feedforward or impulsive form of control generating expectancies during active inference. In contrast, we proposed that the ventral attention system is regulated by the ventral limbic division, regulating feedback constraints and error-correction for active inference within the neocortical hierarchy. Here, we propose that these forms of cognitive control reflect vertical integration of subcortical arousal control systems that evolved for specific forms of behavior control. The feedforward impetus to action is regulated by phasic arousal, mediated by lemnothalamic projections from the reticular activating system of the lower brainstem, and then elaborated by the hippocampus and dorsal limbic division. In contrast, feedback constraint-based on environmental requirements-is regulated by the tonic activation furnished by collothalamic projections from the midbrain arousal control centers, and then sustained and elaborated by the amygdala, basal ganglia, and ventral limbic division. In an evolutionary-developmental analysis, understanding these differing forms of active affordance-for arousal and motor control within the subcortical vertebrate neuraxis-may help explain the evolution of active inference regulating the cognition of expectancy and error-correction within the mammalian 6-layered neocortex.

Behavioral Training Related Neurotransmitter Receptor Expression Dynamics in the Nidopallium Caudolaterale and the Hippocampal Formation of Pigeons

Learning and memory are linked to dynamic changes at the level of synapses in brain areas that are involved in cognitive tasks. For example, changes in neurotransmitter receptors are prerequisite for tuning signals along local circuits and long-range networks. However, it is still unclear how a series of learning events promotes plasticity within the system of neurotransmitter receptors and their subunits to shape information processing at the neuronal level. Therefore, we investigated the expression of different glutamatergic NMDA (GRIN) and AMPA (GRIA) receptor subunits, the GABAergic GABARG2 subunit, dopaminergic DRD1, serotonergic 5HTR1A and noradrenergic ADRA1A receptors in the pigeon’s brain. We studied the nidopallium caudolaterale, the avian analogue of the prefrontal cortex, and the hippocampal formation, after training the birds in a rewarded stimulus-response association (SR) task and in a simultaneous-matching-to-sample (SMTS) task. The results show that receptor expression changed differentially after behavioral training compared to an untrained control group. In the nidopallium caudolaterale, GRIN2B, GRIA3, GRIA4, DRD1D, and ADRA1A receptor expression was altered after SR training and remained constantly decreased after the SMTS training protocol, while GRIA2 and DRD1A decreased only under the SR condition. In the hippocampal formation, GRIN2B decreased and GABARG2 receptor expression increased after SR training. After SMTS sessions, GRIN2B remained decreased, GABARG2 remained increased if compared to the control group. None of the investigated receptors differed directly between both conditions, although differentially altered. The changes in both regions mostly occur in favor of the stimulus response task. Thus, the present data provide evidence that neurotransmitter receptor expression dynamics play a role in the avian prefrontal cortex and the hippocampal formation for behavioral training and is uniquely, regionally and functionally associated to cognitive processes including learning and memory.

The Multiple Contexts of Brain Scaling: Phenotypic Integration in Brain and Behavioral Evolution

Understanding the adaptive functions of increasing brain size have occupied scientists for decades.  Here, taking the general perspective of the Extended Evolutionary Synthesis, the question of how brains change in size will be considered in two developmental frameworks. The first framework will consider the particular developmental mechanisms that control and generate brain mass, concentrating on neurogenesis in a comparative vertebrate context. The consequences of limited adult neurogenesis in mammals, and the dominating role of duration of neurogenesis for mammalian evolution will be discussed for the particular case of the teleost versus mammalian retina, and for paths of brain evolution more generally. The second framework examines brain mass in terms of life history, particularly the features of life history that correlate highly, if imperfectly, with brain mass, including duration of development to adolescence, duration of parental care, body and range size, and longevity. This covariation will be examined in light of current work on genetic causes and consequences of covariation in craniofacial bone groupings. The eventual development of a multivariate structure for understanding brain evolution which specifically integrates formerly separate layers of analysis is the ultimate goal.

Current status of the hypothesis of a claustro-insular homolog in sauropsids

The author worked before on the wide problem of the evolution of the vertebrate pallium. He proposed various Bauplan models based in the definition of a set of pallial sectors with characteristic (topologically invariant) mutual relationships and distinct molecular profiles. Out of one of these models, known as the 'updated tetrapartite pallium model', a modified definition of the earlier lateral pallium sector (LPall) emerged, which characterized it in mammals as consisting of an unitary claustro-insular transitional (mesocortical) complex intercalated between neocortex or dorsal pallium (DPall) above and olfactory cortex or ventral pallium (VPall) underneath. A distinctive molecular marker of the early-born deep claustral component of the LPall was found to be the transcription factor Nr4a2, which is not expressed significantly in the overlying insular cortex or in adjoining cortical territories (Puelles 2014). Given that earlier comparative studies had identified molecularly and topologically comparable VPall, LPall and DPall sectors in the avian pallium, an avian Nr4a2 probe was applied aiming to identify the reportedly absent avian claustro-insular complex. An early-born superficial subpopulation of the avian LPall that expresses selectively this marker through development was indeed found. This was proposed to be a claustrum homolog, whereas the remaining Nr4a2-negative avian LPall cells were assumed to represent a possible insular homolog (Puelles et al. 2016a). This last notion was supported by comparable selective expression of the mouse insular marker Cyp26b, also found restricted to the avian LPall (Puelles 2017). Some published data suggested that similar molecular properties and structure apply at the reptilian LPall. This analysis was reviewed in Puelles et al. (2017). The present commentary discusses 3-4 years later some international publications accrued in the interval that touch on the claustro-insular homology hypothesis. Some of them are opposed to the hypothesis whereas others corroborate or support it. This raises a number of secondary issues of general interest.

Analysis of the Expression Pattern of Cajal-Retzius Cell Markers in the Xenopus laevis Forebrain

Cajal-Retzius cells are essential for cortical development in mammals, and their involvement in the evolution of this structure has been widely postulated, but very little is known about their progenitor domains in non-mammalian vertebrates. Using in situhybridization and immunofluorescence techniques we analyzed the expression of some of the main Cajal-Retzius cell markers such as Dbx1, Ebf3, ER81, Lhx1, Lhx5, p73, Reelin, Wnt3a, Zic1, and Zic2 in the forebrain of the anuran Xenopus laevis, because amphibians are the only class of anamniote tetrapods and show a tetrapartite evaginated pallium, but no layered or nuclear organization. Our results suggested that the Cajal-Retzius cell progenitor domains were comparable to those previously described in amniotes. Thus, at dorsomedial telencephalic portions a region comparable to the cortical hem was defined in Xenopus based on the expression of Wnt3a, p73, Reelin, Zic1, and Zic2. In the septum, two different domains were observed: a periventricular dorsal septum, at the limit between the pallium and the subpallium, expressing Reelin, Zic1, and Zic2, and a related septal domain, expressing Ebf3, Zic1, and Zic2. In the lateral telencephalon, the ventral pallium next to the pallio-subpallial boundary, the lack of Dbx1 and the unique expression of Reelin during development defined this territory as the most divergent with respect to mammals. Finally, we also analyzed the expression of these markers at the prethalamic eminence region, suggested as Cajal-Retzius progenitor domain in amniotes, observing there Zic1, Zic2, ER81, and Lhx1 expression. Our data show that in anurans there are different subtypes and progenitor domains of Cajal-Retzius cells, which probably contribute to the cortical regional specification and territory-specific properties. This supports the notion that the basic organization of pallial derivatives in vertebrates follows a comparable fundamental arrangement, even in those that do not have a sophisticated stratified cortical structure like the mammalian cerebral cortex.

The Independent Evolution of Dorsal Pallia in Multiple Vertebrate Lineages

Comparative neurobiologists have long wondered when and how the dorsal pallium (e.g., mammalian neocortex) evolved. For the last 50 years, the most widely accepted answer has been that this structure was already present in the earliest vertebrates and, therefore, homologous between the major vertebrate lineages. One challenge for this hypothesis is that the olfactory bulbs project throughout most of the pallium in the most basal vertebrate lineages (notably lampreys, hagfishes, and lungfishes) but do not project to the putative dorsal pallia in teleosts, cartilaginous fishes, and amniotes (i.e., reptiles, birds, and mammals). To make sense of these data, one may hypothesize that a dorsal pallium existed in the earliest vertebrates and received extensive olfactory input, which was subsequently lost in several lineages. However, the dorsal pallium is notoriously difficult to delineate in many vertebrates, and its homology between the various lineages is often based on little more than its topology. Therefore, we suspect that dorsal pallia evolved independently in teleosts, cartilaginous fishes, and amniotes. We further hypothesize that the emergence of these dorsal pallia was accompanied by the phylogenetic restriction of olfactory projections to the pallium and the expansion of inputs from other sensory modalities. We do not deny that the earliest vertebrates may have possessed nonolfactory sensory inputs to some parts of the pallium, but such projections alone do not define a dorsal pallium.

Quail-chick grafting experiments corroborate that Tbr1-positive eminential prethalamic neurons migrate along three streams into hypothalamus, subpallium and septocommissural areas

The prethalamic eminence (PThE), a diencephalic caudal neighbor of the telencephalon and alar hypothalamus, is frequently described in mammals and birds as a transient embryonic structure, undetectable in the adult brain. Based on descriptive developmental analysis of Tbr1 gene brain expression in chick embryos, we previously reported that three migratory cellular streams exit the PThE rostralward, targeting multiple sites in the hypothalamus, subpallium and septocommissural area, where eminential cells form distinct nuclei or disperse populations. These conclusions needed experimental corroboration. In this work, we used the homotopic quail-chick chimeric grafting procedure at stages HH10/HH11 to demonstrate by fate-mapping the three predicted tangential migration streams. Some chimeric brains were processed for Tbr1 in situ hybridization, for correlation with our previous approach. Evidence supporting all three postulated migration streams is presented. The results suggested a slight heterochrony among the juxtapeduncular (first), the peripeduncular (next), and the eminentio-septal (last) streams, each of which followed differential routes. A possible effect of such heterochrony on the differential selection of medial to lateral habenular hodologic targets by the migrated neurons is discussed.

Sim1-expressing cells illuminate the origin and course of migration of the nucleus of the lateral olfactory tract in the mouse amygdala

We focus this report on the nucleus of the lateral olfactory tract (NLOT), a superficial amygdalar nucleus receiving olfactory input. Mixed with its Tbr1-expressing layer 2 pyramidal cell population (NLOT2), there are Sim1-expressing cells whose embryonic origin and mode of arrival remain unclear. We examined this population with Sim1-ISH and a Sim1-tauLacZ mouse line. An alar hypothalamic origin is apparent at the paraventricular area, which expresses Sim1 precociously. This progenitor area shows at E10.5 a Sim1-expressing dorsal prolongation that crosses the telencephalic stalk and follows the terminal sulcus, reaching the caudomedial end of the pallial amygdala. We conceive this Sim1-expressing hypothalamo-amygdalar corridor (HyA) as an evaginated part of the hypothalamic paraventricular area, which participates in the production of Sim1-expressing cells. From E13.5 onwards, Sim1-expressing cells migrated via the HyA penetrate the posterior pallial amygdalar radial unit and associate therein to the incipient Tbr1-expressing migration stream which swings medially past the amygdalar anterior basolateral nucleus (E15.5), crosses the pallio-subpallial boundary (E16.5), and forms the NLOT2 within the anterior amygdala by E17.5. We conclude that the Tbr1-expressing NLOT2 cells arise strictly within the posterior pallial amygdalar unit, involving a variety of required gene functions we discuss. Our results are consistent with the experimental data on NLOT2 origin reported by Remedios et al. (Nat Neurosci 10:1141–1150, 2007), but we disagree on their implication in this process of the dorsal pallium, observed to be distant from the amygdala.

In search of common developmental and evolutionary origin of the claustrum and subplate

The human claustrum, a major hub of widespread neocortical connections, is a thin, bilateral sheet of gray matter located between the insular cortex and the striatum. The subplate is a largely transient cortical structure that contains some of the earliest generated neurons of the cerebral cortex and has important developmental functions to establish intra- and extracortical connections. In human and macaque some subplate cells undergo regulated cell death, but some remain as interstitial white matter cells. In mouse and rat brains a compact layer is formed, Layer 6b, and it remains underneath the cortex, adjacent to the white matter. Whether Layer 6b in rodents is homologous to primate subplate or interstitial white matter cells is still debated. Gene expression patterns, such as those of Nurr1/Nr4a2, have suggested that the rodent subplate and the persistent subplate cells in Layer 6b and the claustrum might have similar origins. Moreover, the birthdates of the claustrum and Layer 6b are similarly precocious in mice. These observations prompted our speculations on the common developmental and evolutionary origin of the claustrum and the subplate. Here we systematically compare the currently available data on cytoarchitecture, evolutionary origin, gene expression, cell types, birthdates, neurogenesis, lineage and migration, circuit connectivity, and cell death of the neurons that contribute to the claustrum and subplate. Based on their similarities and differences we propose a partially common early evolutionary origin of the cells that become claustrum and subplate, a likely scenario that is shared in these cell populations across all amniotes.

Topological atlas of the hypothalamus in adult rhesus monkey

The prosomeric model explains the embryological development of the central nervous system (CNS) shared by all vertebrates as a Bauplan. As a primary event, the early neural plate is patterned by intersecting longitudinal plates and transverse segments, forming a mosaic of progenitor units. The hypothalamus is specified by three prosomeres (hp1, hp2, and the acroterminal domain) of the secondary prosencephalon with corresponding alar and basal plate parts, which develop apart from the diencephalon. Mounting evidence suggests that progenitor units within alar and basal plate parts of hp1 and hp2 give rise to distinct hypothalamic nuclei, which preserve their relative invariant positioning (topology) in the adult brain. Nonetheless, the principles of the prosomeric model have not been applied so far to the hypothalamus of adult primates. We parcellated hypothalamic nuclei in adult rhesus monkeys (Macaca mulatta) using various stains to view architectonic boundaries. We then analyzed the topological relations of hypothalamic nuclei and adjacent hypothalamic landmarks with homology across rodent and primate species to trace the origin of adult hypothalamic nuclei to the alar or basal plate components of hp1 and hp2. We generated a novel atlas of the hypothalamus of the adult rhesus monkey with developmental ontologies for each hypothalamic nucleus. The result is a systematic reinterpretation of the adult hypothalamus whose prosomeric ontology can be used to study relationships between the hypothalamus and other regions of the CNS. Further, our atlas may serve as a tool to predict causal patterns in physiological and pathological pathways involving the hypothalamus.

Sequential pattern of sublayer formation in the paleocortex and neocortex

The piriform cortex (paleocortex) is the olfactory cortex or the primary cortex for the sense of smell. It receives the olfactory input from the mitral and tufted cells of the olfactory bulb and is involved in the processing of information pertaining to odors. The piriform cortex and the adjoining neocortex have different cytoarchitectures; while the former has a three-layered structure, the latter has a six-layered structure. The regulatory mechanisms underlying the building of the six-layered neocortex are well established; in contrast, less is known about of the regulatory mechanisms responsible for structure formation of the piriform cortex. The differences as well as similarities in the regulatory mechanisms between the neocortex and the piriform cortex remain unclear. Here, the expression of neocortical layer-specific genes in the piriform cortex was examined. Two sublayers were found to be distinguished in layer II of the piriform cortex using Ctip2/Bcl11b and Brn1/Pou3f3. The sequential expression pattern of Ctip2 and Brn1 in the piriform cortex was similar to that detected in the neocortex, although the laminar arrangement in the piriform cortex exhibited an outside-in arrangement, unlike that observed in the neocortex.

Molecular architecture of the zebra finch arcopallium

The arcopallium, a key avian forebrain region, receives inputs from numerous brain areas and is a major source of descending sensory and motor projections. While there is evidence of arcopallial subdivisions, the internal organization or the arcopallium is not well understood. The arcopallium is also considered the avian homologue of mammalian deep cortical layers and/or amygdalar subdivisions, but one-to-one correspondences are controversial. Here we present a molecular characterization of the arcopallium in the zebra finch, a passerine songbird species and a major model organism for vocal learning studies. Based on in situ hybridization for arcopallial-expressed transcripts (AQP1, C1QL3, CBLN2, CNTN4, CYP19A1, ESR1/2, FEZF2, MGP, NECAB2, PCP4, PVALB, SCN3B, SCUBE1, ZBTB20, and others) in comparison with cytoarchitectonic features, we have defined 20 distinct regions that can be grouped into six major domains (anterior, posterior, dorsal, ventral, medial, and intermediate arcopallium, respectively; AA, AP, AD, AV, AM, and AI). The data also help to establish the arcopallium as primarily pallial, support a unique topography of the arcopallium in passerines, highlight similarities between the vocal robust nucleus of the arcopallium (RA) and AI, and provide insights into the similarities and differences of cortical and amygdalar regions between birds and mammals. We also propose the use of AMV (instead of nucleus taenia/TnA), AMD, AD, and AI as initial steps toward a universal arcopallial nomenclature. Besides clarifying the internal organization of the arcopallium, the data provide a coherent basis for further functional and comparative studies of this complex avian brain region.

The Structural Model: a theory linking connections, plasticity, pathology, development and evolution of the cerebral cortex

The classical theory of cortical systematic variation has been independently described in reptiles, monotremes, marsupials and placental mammals, including primates, suggesting a common bauplan in the evolution of the cortex. The Structural Model is based on the systematic variation of the cortex and is a platform for advancing testable hypotheses about cortical organization and function across species, including humans. The Structural Model captures the overall laminar structure of areas by dividing the cortical architectonic continuum into discrete categories (cortical types), which can be used to test hypotheses about cortical organization. By type, the phylogenetically ancient limbic cortices-which form a ring at the base of the cerebral hemisphere-are agranular if they lack layer IV, or dysgranular if they have an incipient granular layer IV. Beyond the dysgranular areas, eulaminate type cortices have six layers. The number and laminar elaboration of eulaminate areas differ depending on species or cortical system within a species. The construct of cortical type retains the topology of the systematic variation of the cortex and forms the basis for a predictive Structural Model, which has successfully linked cortical variation to the laminar pattern and strength of cortical connections, the continuum of plasticity and stability of areas, the regularities in the distribution of classical and novel markers, and the preferential vulnerability of limbic areas to neurodegenerative and psychiatric diseases. The origin of cortical types has been recently traced to cortical development, and helps explain the variability of diseases with an onset in ontogeny.

Absence of Tangentially Migrating Glutamatergic Neurons in the Developing Avian Brain

Several neuronal populations orchestrate neocortical development during mammalian embryogenesis. These include the glutamatergic subplate-, Cajal-Retzius-, and ventral pallium-derived populations, which coordinate cortical wiring, migration, and proliferation, respectively. These transient populations are primarily derived from other non-cortical pallial sources that migrate to the dorsal pallium. Are these migrations to the dorsal pallium conserved in amniotes or are they specific to mammals? Using in ovo electroporation, we traced the entire lineage of defined chick telencephalic progenitors. We found that several pallial sources that produce tangential migratory neurons in mammals only produced radially migrating neurons in the avian brain. Moreover, ectopic expression of VP-specific mammalian Dbx1 in avian brains altered neurogenesis but did not convert the migration into a mammal-like tangential movement. Together, these data indicate that tangential cellular contributions of glutamatergic neurons originate from outside the dorsal pallium and that pallial Dbx1 expression may underlie the generation of the mammalian neocortex during evolution.

A Re-evaluation of the Anatomy of the Claustrum in Rodents and Primates-Analyzing the Effect of Pallial Expansion

The components of the claustrum have been identified by gene expression in mice, but there is still uncertainty about the location of homologous components in primates. To aid interpretation of homologous elements between rodents and primates, we used a current understanding of pallial topology, species-specific telencephalic deformation, and gene expression data. In both rodents and primates, pallial areas maintain conserved topological relationships regardless of relative differences in pallial expansion. The components of the claustrum in primates can, therefore, be identified on the basis of their conserved topological relationships and patterns of gene expression. In rodents, a fairly straight telencephalic long axis runs between the early septopreoptic and amygdalar poles of the pallium. In primates, however, the remarkable dorsal pallial expansion causes this axis to be distorted to form a C shape. This has resulted in a number of errors in the interpretation of the location of claustral components. These errors are likely to have resulted from the unexpected topographical positioning of claustral components due to the bent telencephalic axis. We argue that, once the telencephalic distortion has been accounted for, both rodents and primates have homologous claustral components, and that the topological relationships of these components are conserved regardless of differences in the relative expansion of pallial areas.

Concentric ring topology of mammalian cortical sectors and relevance for patterning studies

Models aiming to explain causally the evolutionary or ontogenetic emergence of the pallial isocortex and its regional/areal heterogeneity in mammals use simple or complex assumptions about the pallial structure present in basal mammals and nonmammals. The question arises: how complex is the pattern that needs to be accounted for in causal models? This topic is also paramount for comparative purposes, since some topological relationships may be explained as being ancestral, rather than newly emerged. The mouse pallium is apt to be reexamined in this context, due to the breadth of available molecular markers and correlative experimental patterning results. We center the present essay on a recapitulative glance at the classic theory of concentric mammalian allo-, meso-, and neocortex domains. In its simplest terms, this theory postulates a central neocortical island (6 layers) separated by a surrounding mesocortical ring (4-5 layers) from a peripheral allocortical ring (3 layers). These territories show additional partition into regional or areal subdivisions. There are also borderline amygdalar, claustral, and septal areas of the pallium, nuclear in structure. There has been little effort so far to contemplate the full concentric ring model in current "cortex patterning" models. In this essay, we recapitulate the ring idea in mammals (mouse) and consider a potential causal patterning scenario using topologic models. Finally, we briefly explore how far this theory may apply to pallium models proposed recently for sauropsids.

Mapping the brain of the chicken (Gallus gallus), with emphasis on the septal-hypothalamic region

There has been remarkable progress in discoveries made in the avian brain, particularly over the past two decades. This review first highlights some of the discoveries made in the forebrain and credits the Avian Brain Nomenclature Forum, responsible for changing many of the terms found in the cerebrum and for stimulating collaborative research thereafter. The Forum facilitated communication among comparative neurobiologists by eliminating confusing and inaccurate names. The result over the past 15yearshas been a standardized use of avian forebrain terms. Nonetheless, additional changes are needed. The goal of the paper is to encourage a continuing effort to unify the nomenclature throughout the entire avian brain. To emphasize the need for consensus for a single name for each neural structure, I have selected specific structures in the septum and hypothalamus that our laboratory has been investigating, to demonstrate a lack of uniformity in names applied to conservative brain regions compared to the forebrain. The specific areas reviewed include the distributions of gonadotropin-releasing hormone neurons and their terminal fields in circumventricular organs, deep-brain photoreceptors, gonadotropin inhibitory neurons and a complex structure and function of the nucleus of the hippocampal commissure.

Transmitter receptors reveal segregation of the arcopallium/amygdala complex in pigeons (Columba livia)

At the beginning of the 20th century it was suggested that a complex group of nuclei in the avian posterior ventral telencephalon is comparable to the mammalian amygdala. Subsequent findings, however, revealed that most of these structures share premotor characteristics, while some indeed constitute the avian amygdala. These developments resulted in 2004 in a change of nomenclature of these nuclei, which from then on were named arcopallial or amygdala nuclei and referred to as the arcopallium/amygdala complex. The structural basis for the similarities between avian and mammalian arcopallial and amygdala subregions is poorly understood. Therefore, we analyzed binding site densities for glutamatergic AMPA, NMDA and kainate, GABAergic GABA_A , muscarinic M₁ , M₂ and nicotinic acetylcholine (nACh; α₄ β₂ subtype), noradrenergic α₁ and α₂ , serotonergic 5-HT_1A and dopaminergic D_1/5 receptors using quantitative in vitro receptor autoradiography combined with a detailed analysis of the cyto- and myelo-architecture. Our approach supports a segregation of the pigeon's arcopallium/amygdala complex into the following subregions: the arcopallium anterius (AA), the arcopallium ventrale (AV), the arcopallium dorsale (AD), the arcopallium intermedium (AI), the arcopallium mediale (AM), the arcopallium posterius (AP), the nucleus posterioris amygdalopallii pars basalis (PoAb) and pars compacta (PoAc), the nucleus taeniae amgygdalae (TnA) and the area subpallialis amygdalae (SpA). Some of these subregions showed further subnuclei and each region of the arcopallium/amygdala complex are characterized by a distinct multi-receptor density expression. Here we provide a new detailed map of the pigeon's arcopallium/amygdala complex and compare the receptor architecture of the subregions to their possible mammalian counterparts.

Pattern of Neurogenesis and Identification of Neuronal Progenitor Subtypes during Pallial Development in Xenopus laevis

The complexity of the pallium during evolution has increased dramatically in many different respects. The highest level of complexity is found in mammals, where most of the pallium (cortex) shows a layered organization and neurons are generated during development following an inside-out order, a sequence not observed in other amniotes (birds and reptiles). Species-differences may be related to major neurogenetic events, from the neural progenitors that divide and produce all pallial cells. In mammals, two main types of precursors have been described, primary precursor cells in the ventricular zone (vz; also called radial glial cells or apical progenitors) and secondary precursor cells (called basal or intermediate progenitors) separated from the ventricle surface. Previous studies suggested that pallial neurogenetic cells, and especially the intermediate progenitors, evolved independently in mammalian and sauropsid lineages. In the present study, we examined pallial neurogenesis in the amphibian Xenopus laevis, a representative species of the only group of tetrapods that are anamniotes. The pattern of pallial proliferation during embryonic and larval development was studied, together with a multiple immunohistochemical analysis of putative progenitor cells. We found that there are two phases of progenitor divisions in the developing pallium that, following the radial unit concept from the ventricle to the mantle, finally result in an outside-in order of mature neurons, what seems to be the primitive condition of vertebrates. Gene expressions of key transcription factors that characterize radial glial cells in the vz were demonstrated in Xenopus. In addition, although mitotic cells were corroborated outside the vz, the expression pattern of markers for intermediate progenitors differed from mammals.

Developmental gene expression in the mouse clarifies the organization of the claustrum and related endopiriform nuclei

Studies on gene expression in the developing claustrum of the mouse have clarified the relationships and identity of the claustrum proper and related endopiriform nuclei. The cells of the claustrum primordium express Nr4a2; they are formed in combination with the Nr4a2-labeled subplate cells in the lateral pallium at the site of the future insular cortex. The insular cortex cells, which are born later and which are Nr4a2-negative, migrate through the claustrum toward the pial surface to form layers (2-6a) of the insular cortex. The claustrum is made up of distinct deep (subplate-like) and superficial (principal) parts. The cells of the dorsal endopiriform nucleus (which are also Nr4a2-positive) are formed in the deep part of the claustrum primordium in the lateral pallium, but they migrate ventrally to reach the ventral pallium deep to the piriform cortex at E14.5 in the mouse. On the other hand, the ventral endopiriform nucleus is formed by radially migrating Nr4a2-negative cells in the ventral pallium; it is therefore developmentally distinct from the Nr4a2-postive dorsal endopiriform nucleus, which is a lateral pallial derivative. J. Comp. Neurol. 525:1499-1508, 2017. © 2016 Wiley Periodicals, Inc.

Levels of homology and the problem of neocortex

The neocortex is found only in mammals, and the fossil record is silent on how this soft tissue evolved. Understanding neocortex evolution thus devolves to a search for candidate homologous neocortex traits in the extant nonmammalian amniotes. The difficulty is that homology is based on similarity, and the six-layered neocortex structure could hardly be more dissimilar in appearance from the nuclear organization that is so conspicuous in the dorsal telencephalon of birds and other reptiles. Recent molecular data have, however, provided new support for one prominent hypothesis, based on neuronal circuits, that proposes the principal neocortical input and output cell types are a conserved feature of amniote dorsal telencephalon. Many puzzles remain, the greatest being understanding the selective pressures and molecular mechanisms that underlie such tremendous morphological variation in telencephalon structure.

Radial derivatives of the mouse ventral pallium traced with Dbx1-LacZ reporters

The progeny of Dbx1-expressing progenitors was studied in the developing mouse pallium, using two transgenic mouse lines: (1) Dbx1(nlslacZ) mice, in which the gene of the β-galactosidase reporter (LacZ) is inserted directly under the control of the Dbx1 promoter, allowing short-term lineage tracing of Dbx1-derived cells; and (2) Dbx1(CRE) mice crossed with a Cre-dependent reporter strain (ROSA26(loxP-stop-loxP-LacZ)), in which the Dbx1-derived cells result permanently labeled (Bielle et al., 2005). We thus examined in detail the derivatives of the postulated longitudinal ventral pallium (VPall) sector, which has been defined among other features by its selective ventricular zone expression of Dbx1 (the recent ascription by Puelles, 2014 of the whole olfactory cortex primordium to the VPall was tested). Earlier notions about a gradiental caudorostral reduction of Dbx1 signal were corroborated, so that virtually no signal was found at the olfactory bulb and the anterior olfactory area. The piriform cortex was increasingly labeled caudalwards. The only endopiriform grisea labeled were the ventral endopiriform nucleus and the bed nucleus of the external capsule. Anterior and basolateral parts of the whole pallial amygdala also were densely marked, in contrast to the negative posterior parts of these pallial amygdalar nuclei (leaving apart medial amygdalar parts ascribed to subpallial or extratelencephalic sources of Dbx1-derived GABAergic and non-GABAergic neurons). Alternative tentative interpretations are discussed to explain the partial labeling obtained of both olfactory and amygdaloid structures. This includes the hypothesis of an as yet undefined part of the pallium, potentially responsible for the posterior amygdala, or the hypothesis that the VPall may not be wholly characterized by Dbx1 expression (this gene not being necessary for VPall molecular distinctness and histogenetic potency), which would leave a dorsal Dbx1-negative VPall subdomain of variable size that might contribute partially to olfactory and posterior amygdalar structures.

Evolution of the mammalian dentate gyrus

The dentate gyrus (DG), a part of the hippocampal formation, has important functions in learning, memory, and adult neurogenesis. Compared with homologous areas in sauropsids (birds and reptiles), the mammalian DG is larger and exhibits qualitatively different phenotypes: 1) folded (C- or V-shaped) granule neuron layer, concave toward the hilus and delimited by a hippocampal fissure; 2) nonperiventricular adult neurogenesis; and 3) prolonged ontogeny, involving extensive abventricular (basal) migration and proliferation of neural stem and progenitor cells (NSPCs). Although gaps remain, available data indicate that these DG traits are present in all orders of mammals, including monotremes and marsupials. The exception is Cetacea (whales, dolphins, and porpoises), in which DG size, convolution, and adult neurogenesis have undergone evolutionary regression. Parsimony suggests that increased growth and convolution of the DG arose in stem mammals concurrently with nonperiventricular adult hippocampal neurogenesis and basal migration of NSPCs during development. These traits could all result from an evolutionary change that enhanced radial migration of NSPCs out of the periventricular zones, possibly by epithelial-mesenchymal transition, to colonize and maintain nonperiventricular proliferative niches. In turn, increased NSPC migration and clonal expansion might be a consequence of growth in the cortical hem (medial patterning center), which produces morphogens such as Wnt3a, generates Cajal-Retzius neurons, and is regulated by Lhx2. Finally, correlations between DG convolution and neocortical gyrification (or capacity for gyrification) suggest that enhanced abventricular migration and proliferation of NSPCs played a transformative role in growth and folding of neocortex as well as archicortex.

A hypothesis for the evolution of the upper layers of the neocortex through co-option of the olfactory cortex developmental program

The neocortex is unique to mammals and its evolutionary origin is still highly debated. The neocortex is generated by the dorsal pallium ventricular zone, a germinative domain that in reptiles give rise to the dorsal cortex. Whether this latter allocortical structure contains homologs of all neocortical cell types it is unclear. Recently we described a population of DCX+/Tbr1+ cells that is specifically associated with the layer II of higher order areas of both the neocortex and of the more evolutionary conserved piriform cortex. In a reptile similar cells are present in the layer II of the olfactory cortex and the DVR but not in the dorsal cortex. These data are consistent with the proposal that the reptilian dorsal cortex is homologous only to the deep layers of the neocortex while the upper layers are a mammalian innovation. Based on our observations we extended these ideas by hypothesizing that this innovation was obtained by co-opting a lateral and/or ventral pallium developmental program. Interestingly, an analysis in the Allen brain atlas revealed a striking similarity in gene expression between neocortical layers II/III and piriform cortex. We thus propose a model in which the early neocortical column originated by the superposition of the lateral olfactory and dorsal cortex. This model is consistent with the fossil record and may account not only for the topological position of the neocortex, but also for its basic cytoarchitectural and hodological features. This idea is also consistent with previous hypotheses that the peri-allocortex represents the more ancient neocortical part. The great advances in deciphering the molecular logic of the amniote pallium developmental programs will hopefully enable to directly test our hypotheses in the next future.

Neuronal subtype specification in establishing mammalian neocortical circuits

The functional integrity of the neocortical circuit relies on the precise production of diverse neuron populations and their assembly during development. In recent years, extensive progress has been made in the understanding of the mechanisms that control differentiation of each neuronal type within the neocortex. In this review, we address how the elaborate neocortical cytoarchitecture is established from a simple neuroepithelium based on recent studies examining the spatiotemporal mechanisms of neuronal subtype specification. We further discuss the critical events that underlie the conversion of the stem amniotes cerebrum to a mammalian-type neocortex, and extend these key findings in the light of mammalian evolution to understand how the neocortex in humans evolved from common ancestral mammals.

Molecular profiling of the developing avian telencephalon: regional timing and brain subdivision continuities

In our companion study (Jarvis et al. [2013] J Comp Neurol. doi: 10.1002/cne.23404) we used quantitative brain molecular profiling to discover that distinct subdivisions in the avian pallium above and below the ventricle and the associated mesopallium lamina have similar molecular profiles, leading to a hypothesis that they may form as continuous subdivisions around the lateral ventricle. To explore this hypothesis, here we profiled the expression of 16 genes at eight developmental stages. The genes included those that define brain subdivisions in the adult and some that are also involved in brain development. We found that phyletic hierarchical cluster and linear regression network analyses of gene expression profiles implicated single and mixed ancestry of these brain regions at early embryonic stages. Most gene expression-defined pallial subdivisions began as one ventral or dorsal domain that later formed specific folds around the lateral ventricle. Subsequently a clear ventricle boundary formed, partitioning them into dorsal and ventral pallial subdivisions surrounding the mesopallium lamina. These subdivisions each included two parts of the mesopallium, the nidopallium and hyperpallium, and the arcopallium and hippocampus, respectively. Each subdivision expression profile had a different temporal order of appearance, similar in timing to the order of analogous cell types of the mammalian cortex. Furthermore, like the mammalian pallium, expression in the ventral pallial subdivisions became distinct during prehatch development, whereas the dorsal portions did so during posthatch development. These findings support the continuum hypothesis of avian brain subdivision development around the ventricle and influence hypotheses on homologies of the avian pallium with other vertebrates.

Global view of the functional molecular organization of the avian cerebrum: mirror images and functional columns

Based on quantitative cluster analyses of 52 constitutively expressed or behaviorally regulated genes in 23 brain regions, we present a global view of telencephalic organization of birds. The patterns of constitutively expressed genes revealed a partial mirror image organization of three major cell populations that wrap above, around, and below the ventricle and adjacent lamina through the mesopallium. The patterns of behaviorally regulated genes revealed functional columns of activation across boundaries of these cell populations, reminiscent of columns through layers of the mammalian cortex. The avian functionally regulated columns were of two types: those above the ventricle and associated mesopallial lamina, formed by our revised dorsal mesopallium, hyperpallium, and intercalated hyperpallium; and those below the ventricle, formed by our revised ventral mesopallium, nidopallium, and intercalated nidopallium. Based on these findings and known connectivity, we propose that the avian pallium has four major cell populations similar to those in mammalian cortex and some parts of the amygdala: 1) a primary sensory input population (intercalated pallium); 2) a secondary intrapallial population (nidopallium/hyperpallium); 3) a tertiary intrapallial population (mesopallium); and 4) a quaternary output population (the arcopallium). Each population contributes portions to columns that control different sensory or motor systems. We suggest that this organization of cell groups forms by expansion of contiguous developmental cell domains that wrap around the lateral ventricle and its extension through the middle of the mesopallium. We believe that the position of the lateral ventricle and its associated mesopallium lamina has resulted in a conceptual barrier to recognizing related cell groups across its border, thereby confounding our understanding of homologies with mammals.

Transcriptional regulation of enhancers active in protodomains of the developing cerebral cortex

Elucidating the genetic control of cerebral cortical (pallial) development is essential for understanding function, evolution, and disorders of the brain. Transcription factors (TFs) that embryonically regulate pallial regionalization are expressed in gradients, raising the question of how discrete domains are generated. We provide evidence that small enhancer elements active in protodomains integrate broad transcriptional information. CreER(T2) and GFP expression from 14 different enhancer elements in stable transgenic mice allowed us to define a comprehensive regional fate map of the pallium. We explored transcriptional mechanisms that control the activity of the enhancers using informatics, in vivo occupancy by TFs that regulate cortical patterning (CoupTFI, Pax6, and Pbx1), and analysis of enhancer activity in Pax6 mutants. Overall, the results provide insights into how broadly expressed patterning TFs regulate the activity of small enhancer elements that drive gene expression in pallial protodomains that fate map to distinct cortical regions.

A common developmental plan for neocortical gene-expressing neurons in the pallium of the domestic chicken Gallus gallus domesticus and the Chinese softshell turtle Pelodiscus sinensis

The six-layered neocortex is a unique characteristic of mammals and likely provides the neural basis of their sophisticated cognitive abilities. Although all mammalian species share the layered structure of the neocortex, the sauropsids exhibit an entirely different cytoarchitecture of the corresponding pallial region. Our previous gene expression study revealed that the chicken pallium possesses neural subtypes that express orthologs of layer-specific genes of the mammalian neocortex. To understand the evolutionary steps leading toward animal group-specific neuronal arrangements in the pallium in the course of amniote diversification, we examined expression patterns of the same orthologs and a few additional genes in the pallial development of the Chinese softshell turtle Pelodiscus sinensis, and compared these patterns to those of the chicken. Our analyses highlighted similarities in neuronal arrangements between the two species; the mammalian layer 5 marker orthologs are expressed in the medial domain and the layer 2/3 marker orthologs are expressed in the lateral domain in the pallia of both species. We hypothesize that the mediolateral arrangement of the neocortical layer-specific gene-expressing neurons originated in their common ancestor and is conserved among all sauropsid groups, whereas the neuronal arrangement within the pallium could have highly diversified independently in the mammalian lineage.

The nuclear receptors COUP-TF: a long-lasting experience in forebrain assembly

Chicken ovalbumin upstream promoter transcription factors (COUP-TFs) are nuclear receptors belonging to the superfamily of the steroid/thyroid hormone receptors. Members of this family are internalized to the nucleus both in a ligand-dependent or -independent manner and act as strong transcriptional regulators by binding to the DNA of their target genes. COUP-TFs are defined as orphan receptors, since ligands regulating their activity have not so far been identified. From the very beginning of metazoan evolution, these molecules have been involved in various key events during embryonic development and organogenesis. In this review, we will mainly focus on their function during development and maturation of the central nervous system, which has been well characterized in various animal classes ranging from ctenophores to mammals. We will start by introducing the current knowledge on COUP-TF mechanisms of action and then focus our discussion on the crucial processes underlying forebrain ontogenesis, with special emphasis on mammalian development. Finally, the conserved roles of COUP-TFs along phylogenesis will be highlighted, and some hypotheses, worth exploring in future years to gain more insight into the mechanisms controlled by these factors, will be proposed.

How unique is the human neocortex?

The human cerebral cortex is generally considered the most complex organ, and is the structure that we hold responsible for the repertoire of behavior that distinguishes us from our closest living and extinct relatives. At a recent Company of Biologists Workshop, ‘Evolution of the Human Neocortex: How Unique Are We?’ held in September 2013, researchers considered new information from the fields of developmental biology, genetics, genomics, molecular biology and ethology to understand unique features of the human cerebral cortex and their developmental and evolutionary origin.

Neural progenitors, patterning and ecology in neocortical origins

The anatomical organization of the mammalian neocortex stands out among vertebrates for its laminar and columnar arrangement, featuring vertically oriented, excitatory pyramidal neurons. The evolutionary origin of this structure is discussed here in relation to the brain organization of other amniotes, i.e., the sauropsids (reptiles and birds). Specifically, we address the developmental modifications that had to take place to generate the neocortex, and to what extent these modifications were shared by other amniote lineages or can be considered unique to mammals. In this article, we propose a hypothesis that combines the control of proliferation in neural progenitor pools with the specification of regional morphogenetic gradients, yielding different anatomical results by virtue of the differential modulation of these processes in each lineage. Thus, there is a highly conserved genetic and developmental battery that becomes modulated in different directions according to specific selective pressures. In the case of early mammals, ecological conditions like nocturnal habits and reproductive strategies are considered to have played a key role in the selection of the particular brain patterning mechanisms that led to the origin of the neocortex.

Adult pallium transcriptomes surprise in not reflecting predicted homologies across diverse chicken and mouse pallial sectors

The thorniest problem in comparative neurobiology is the identification of the particular brain region of birds and reptiles that corresponds to the mammalian neocortex [Butler AB, Reiner A, Karten HJ (2011) Ann N Y Acad Sci 1225:14-27; Wang Y, Brzozowska-Prechtl A, Karten HJ (2010) Proc Natl Acad Sci USA 107(28):12676-12681]. We explored which genes are actively transcribed in the regions of controversial ancestry in a representative bird (chicken) and mammal (mouse) at adult stages. We conducted four analyses comparing the expression patterns of their 5,130 most highly expressed one-to-one orthologous genes that considered global patterns of expression specificity, strong gene markers, and coexpression networks. Our study demonstrates transcriptomic divergence, plausible convergence, and, in two exceptional cases, conservation between specialized avian and mammalian telencephalic regions. This large-scale study potentially resolves the complex relationship between developmental homology and functional characteristics on the molecular level and settles long-standing evolutionary debates.

A developmental ontology for the mammalian brain based on the prosomeric model

In the past, attempts to create a hierarchical classification of brain structures (an ontology) have been limited by the lack of adequate data on developmental processes. Recent studies on gene expression during brain development have demonstrated the true morphologic interrelations of different parts of the brain. A developmental ontology takes into account the progressive rostrocaudal and dorsoventral differentiation of the neural tube, and the radial migration of derivatives from progenitor areas, using fate mapping and other experimental techniques. In this review, we used the prosomeric model of brain development to build a hierarchical classification of brain structures based chiefly on gene expression. Because genomic control of neural morphogenesis is remarkably conservative, this ontology should prove essentially valid for all vertebrates, aiding terminological unification.

Neocortical arealization: evolution, mechanisms, and open questions

The mammalian neocortex is a structure with no equals in the vertebrates and is the seat of the highest cerebral functions, such as thoughts and consciousness. It is radially organized into six layers and tangentially subdivided into functional areas deputed to the elaboration of sensory information, association between different stimuli, and selection and triggering of voluntary movements. The process subdividing the neocortical field into several functional areas is called "arealization". Each area has its own cytoarchitecture, connectivity, and peculiar functions. In the last century, several neuroscientists have investigated areal structure and the mechanisms that have led during evolution to the rising of the neocortex and its organization. The extreme conservation in the positioning and wiring of neocortical areas among different mammalian families suggests a conserved genetic program orchestrating neocortical patterning. However, the impressive plasticity of the neocortex, which is able to rewire and reorganize areal structures and connectivity after impairments of sensory pathways, argues for a more complex scenario. Indeed, even if genetics and molecular biology helped in identifying several genes involved in the arealization process, the logic underlying the neocortical bauplan is still beyond our comprehension. In this review, we will introduce the present knowledge and hypotheses on the ontogenesis and evolution of neocortical areas. Then, we will focus our attention on some open issues, which are still unresolved, and discuss some recent studies that might open new directions to be explored in the next few years.

Foxg1 coordinates the switch from nonradially to radially migrating glutamatergic subtypes in the neocortex through spatiotemporal repression

The specification of neuronal subtypes in the cerebral cortex proceeds in a temporal manner; however, the regulation of the transitions between the sequentially generated subtypes is poorly understood. Here, we report that the forkhead box transcription factor Foxg1 coordinates the production of neocortical projection neurons through the global repression of a default gene program. The delayed activation of Foxg1 was necessary and sufficient to induce deep-layer neurogenesis, followed by a sequential wave of upper-layer neurogenesis. A genome-wide analysis revealed that Foxg1 binds to mammalian-specific noncoding sequences to repress over 12 transcription factors expressed in early progenitors, including Ebf2/3, Dmrt3, Dmrta1, and Eya2. These findings reveal an unexpected prolonged competence of progenitors to initiate corticogenesis at a progressed stage during development and identify Foxg1 as a critical initiator of neocorticogenesis through spatiotemporal repression, a system that balances the production of nonradially and radially migrating glutamatergic subtypes during mammalian cortical expansion.

The evolution of brains from early mammals to humans

The large size and complex organization of the human brain makes it unique among primate brains. In particular, the neocortex constitutes about 80% of the brain, and this cortex is subdivided into a large number of functionally specialized regions, the cortical areas. Such a brain mediates accomplishments and abilities unmatched by any other species. How did such a brain evolve? Answers come from comparative studies of the brains of present-day mammals and other vertebrates in conjunction with information about brain sizes and shapes from the fossil record, studies of brain development, and principles derived from studies of scaling and optimal design. Early mammals were small, with small brains, an emphasis on olfaction, and little neocortex. Neocortex was transformed from the single layer of output pyramidal neurons of the dorsal cortex of earlier ancestors to the six layers of all present-day mammals. This small cap of neocortex was divided into 20-25 cortical areas, including primary and some of the secondary sensory areas that characterize neocortex in nearly all mammals today. Early placental mammals had a corpus callosum connecting the neocortex of the two hemispheres, a primary motor area, M1, and perhaps one or more premotor areas. One line of evolution, Euarchontoglires, led to present-day primates, tree shrews, flying lemurs, rodents, and rabbits. Early primates evolved from small-brained, nocturnal, insect-eating mammals with an expanded region of temporal visual cortex. These early nocturnal primates were adapted to the fine branch niche of the tropical rainforest by having an even more expanded visual system that mediated visually guided reaching and grasping of insects, small vertebrates, and fruits. Neocortex was greatly expanded and included an array of cortical areas that characterize neocortex of all living primates. Specializations of the visual system included new visual areas that contributed to a dorsal stream of visuomotor processing in a greatly enlarged region of posterior parietal cortex and an expanded motor system and the addition of a ventral premotor area. Higher visual areas in a large temporal lobe facilitated object recognition, and frontal cortex included granular prefrontal cortex. Auditory cortex included the primary and secondary auditory areas that characterize prosimian and anthropoid primates today. As anthropoids emerged as diurnal primates, the visual system specialized for detailed foveal vision. Other adaptations included an expansion of prefrontal cortex and insular cortex. The human and chimpanzee-bonobo lineages diverged some 6-8 million years ago with brains that were about one third the size of modern humans. Over the last 2 million years, the brains of our more recent ancestors increased greatly in size, especially in the prefrontal, posterior parietal, lateral temporal, and insular regions. Specialization of the two cerebral hemispheres for related, but different functions became pronounced, and language and other impressive cognitive abilities emerged. WIREs Cogn Sci 2013, 4:33-45. doi: 10.1002/wcs.1206 This article is categorized under: Neuroscience > Anatomy.

GABAergic interneuron migration and the evolution of the neocortex

A neocortex is present in all mammals but is not present in other classes of vertebrates, and the neocortex is extremely elaborate in humans. Changes in excitatory projection neurons and their progenitors within the developing dorsal pallium in the most recent common ancestor of mammals are thought to have been involved in the evolution of the neocortex. Our recent findings suggest that changes in the migratory ability of inhibitory interneurons derived from outside the neocortex may also have been involved in the evolution of the neocortex. In this article we review the literature on the migratory profile of inhibitory interneurons in several different species and the literature on comparisons between the intrinsic migratory ability of interneurons derived from different species. Finally, we propose a hypothesis about the mammalian-specific evolution of the migratory ability of interneurons and its potential contribution to the establishment of a functional neocortex.

Expression of the neocortical marker, RORβ, in the entopallium and field L2 of adult chicken

Two opposing hypotheses on the homology of the avian brain suggest that the dorsal ventricular ridge of birds is comparable in certain respects either to the neocortex or to the claustroamygdalar complex of mammals. To help resolve this issue, we examined in adult chicken brains the gene expression of RORβ mRNA, a selective marker for layer IV of mammalian neocortex. RORβ mRNA was expressed in neurons of the chicken's visual entopallium and auditory field L2, but not in other regions of the nidopallium, hyperpallium, mesopallium or arcopallium. Together with hodological evidence of direct thalamic projections conveying primary sensory information to the entopallium and field L2, our results support the contention that these two regions are composed of neurons comparable to those in layer IV of mammalian neocortex.