Search

Cortical neurogenesis follows a simple lineage: apical radial glia cells (RGCs) generate basal progenitors, and these produce neurons. How this occurs in species with expanded germinal zones and a folded cortex, such as human, remains unclear. We used single-cell RNA sequencing from individual cortical germinal zones in ferret and barcoded lineage tracking to determine the molecular diversity of progenitor cells and their lineages. We identified multiple RGC classes that initiate parallel lineages, converging onto a common class of newborn neuron. Parallel RGC classes and transcriptomic trajectories were repeated across germinal zones and conserved in ferret and human, but not in mouse. Neurons followed parallel differentiation trajectories in the gyrus and sulcus, with different expressions of human cortical malformation genes. Progenitor cell lineage multiplicity is conserved in the folded mammalian cerebral cortex.

Intrinsic coupling modes (ICMs) can be observed in ongoing brain activity at multiple spatial and temporal scales. Two families of ICMs can be distinguished: phase and envelope ICMs. The principles that shape these ICMs remain partly elusive, in particular their relation to the underlying brain structure. Here we explored structure-function relationships in the ferret brain between ICMs quantified from ongoing brain activity recorded with chronically implanted micro-ECoG arrays and structural connectivity (SC) obtained from high-resolution diffusion MRI tractography. Large-scale computational models were used to explore the ability to predict both types of ICMs. Importantly, all investigations were conducted with ICM measures that are sensitive or insensitive to volume conduction effects. The results show that both types of ICMs are significantly related to SC, except for phase ICMs when using measures removing zero-lag coupling. The correlation between SC and ICMs increases with increasing frequency which is accompanied by reduced delays. Computational models produced results that were highly dependent on the specific parameter settings. The most consistent predictions were derived from measures solely based on SC. Overall, the results demonstrate that patterns of cortical functional coupling as reflected in both phase and envelope ICMs are both related, albeit to different degrees, to the underlying structural connectivity in the cerebral cortex.

Folding of the cerebral cortex is a fundamental milestone of mammalian brain evolution associated with dramatic increases in size and complexity. Cortex folding takes place during embryonic and perinatal development and is important to optimize the functional organization and wiring of the brain, while allowing fitting a large cortex in a limited cranial volume. Cortex growth and folding are the result of complex cellular and mechanical processes that involve neural stem progenitor cells and their lineages, the migration and differentiation of neurons, and the genetic programs that regulate and fine-tune these processes. Here, we provide an updated overview of the most significant and recent advances in our understanding of developmental mechanisms regulating cortical gyrification.

The size of the adult cerebral cortex is highly diverse between different species inside the amniote clade. These differences are mainly due to differences in the total number of brain cells generated, these being neurons and glia cells. The final number of brain cells that a given brain will have is controlled by a process called neurogenesis that takes place during embryonic development. Changes in different aspects of neurogenesis produce different types of cortices. The evolutionary expansion of cortical neurogenesis that takes place across amniotes has been linked to changes in neural progenitor cells, which acquired increased capacity of self-amplification, formed secondary proliferative layers, and increased neuron production. Here, we provide an overview of cellular aspects of neurogenesis that are differently regulated across amniotes and thus that prompt the generation of different types of structures with similar functions but very different size and structural composition.

The extracellular protein Reelin, expressed by Cajal–Retzius (CR) cells at early stages of cortical development and at late stages by GABAergic interneurons, regulates radial migration and the “inside-out” pattern of positioning. Current models of Reelin functions in corticogenesis focus on early CR cell–derived Reelin in layer I. However, developmental disorders linked to Reelin deficits, such as schizophrenia and autism, are related to GABAergic interneuron–derived Reelin, although its role in migration has not been established. Here we selectively inactivated the Reln gene in CR cells or GABAergic interneurons. We show that CR cells have a major role in the inside-out order of migration, while CR and GABAergic cells sequentially cooperate to prevent invasion of cortical neurons into layer I. Furthermore, GABAergic cell–derived Reelin compensates some features of the reeler phenotype and is needed for the fine tuning of the layer-specific distribution of cortical neurons. In the hippocampus, the inactivation of Reelin in CR cells causes dramatic alterations in the dentate gyrus and mild defects in the hippocampus proper. These findings lead to a model in which both CR and GABAergic cell–derived Reelin cooperate to build the inside-out order of corticogenesis, which might provide a better understanding of the mechanisms involved in the pathogenesis of neuropsychiatric disorders linked to abnormal migration and Reelin deficits.

Cortical expansion in primate brains relies on enlargement of germinal zones during a prolonged developmental period. Although most mammals have two cortical germinal zones, the ventricular zone (VZ) and subventricular zone (SVZ), gyrencephalic species display an additional germinal zone, the outer subventricular zone (oSVZ), which increases the number and diversity of neurons generated during corticogenesis. How the oSVZ emerged during evolution is poorly understood, but recent studies suggest a role for non-coding RNAs, which allow tight genetic program regulation during development. Here, using in vivo functional genetics, single-cell RNA sequencing, live imaging, and electrophysiology to assess progenitor and neuronal properties in mice, we identify two oSVZ-expressed microRNAs (miRNAs), miR-137 and miR-122, which regulate key cellular features of cortical expansion. miR-137 promotes basal progenitor self-replication and superficial layer neuron fate, whereas miR-122 decreases the pace of neuronal differentiation. These findings support a cell-type-specific role of miRNA-mediated gene expression in cortical expansion.

The human brain is characterized by the large size and intricate folding of its cerebral cortex, which are fundamental for our higher cognitive function and frequently altered in pathological dysfunction. Cortex folding is not unique to humans, nor even to primates, but is common across mammals. Cortical growth and folding are the result of complex developmental processes that involve neural stem and progenitor cells and their cellular lineages, the migration and differentiation of neurons, and the genetic programs that regulate and fine-tune these processes. All these factors combined generate mechanical stress and strain on the developing neural tissue, which ultimately drives orderly cortical deformation and folding. In this review we examine and summarize the current knowledge on the molecular, cellular, histogenic, and mechanical mechanisms that are involved in and influence folding of the cerebral cortex, and how they emerged and changed during mammalian evolution. We discuss the main types of pathological malformations of human cortex folding, their specific developmental origin, and how investigating their genetic causes has illuminated our understanding of key events involved. We close our review by presenting the animal and in vitro models of cortex folding that are currently used to study these devastating developmental brain disorders in children, and what are the main challenges that remain ahead of us to fully understand brain folding.

The size of the cerebral cortex increases dramatically across amniotes, from reptiles to great apes. This is primarily due to different numbers of neurons and glial cells produced during embryonic development. The evolutionary expansion of cortical neurogenesis was linked to changes in neural stem and progenitor cells, which acquired increased capacity of self-amplification and neuron production. Evolution works via changes in the genome, and recent studies have identified a small number of new genes that emerged in the recent human and primate lineages, promoting cortical progenitor proliferation and increased neurogenesis. However, most of the mammalian genome corresponds to noncoding DNA that contains gene-regulatory elements, and recent evidence precisely points at changes in expression levels of conserved genes as key in the evolution of cortical neurogenesis. Here, we provide an overview of basic cellular mechanisms involved in cortical neurogenesis across amniotes, and discuss recent progress on genetic mechanisms that may have changed during evolution, including gene expression regulation, leading to the expansion of the cerebral cortex.

Information processing in the brain depends on the dialog between different regions orchestrated by oscillatory activity, especially theta (4-12 Hz) and gamma rhythms (30-100Hz). Clinical and basic research has confirmed a strong relationship between cognitive impairment and distortion of these rhythms in Schizophrenia. It is hypothesized that these oscillopathies originate from alterations of the excitatoryinhibitory (E-I) balance. Therefore, unveiling the relationship between brain rhythms and cognition is crucial for understanding the cognitive manifestations of Schizophrenia. In this work, we aimed to investigate how changes in the E-I balance might reproduce rhythm and behavioral alterations. We used a mouse pharmacological model of schizophrenia, administering MK801 to adult wild-type mice, an NMDAr blocker known to alter interneuron and pyramidal activity. Mice were implanted with microdrives mounting tetrodes aimed to the dorsal hippocampus and medial prefrontal cortex (mPFC). Then brain activity was recorded while mice performed spontaneous exploration in an open field, and while testing working memory in the y-maze, after the administration of MK801 or vehicle. Our results proved that NMDAr blocking exerts differential effects in the oscillatory activity of the hippocampus and the mPFC. While in CA1 and subiculum MK801 produced an increase in gamma oscillations and a distortion of theta/gamma coupling, mPFC activity was characterized by an augmentation of theta, gamma and emergence of high frequency oscillations (HFO, 155-185 Hz). Interestingly HFO in the mPFC was strongly modulated by local theta activity. In addition, we observed an increase in CA1-mPFC coherence in the delta and alpha band, changes in locomotor behavior, defective place cell function and impaired working memory in the y-maze. We observed that CA1 theta/gamma modulation was enhanced during right alternation in the y-maze, but none of the oscillatory changes produced by MK801 could predic

Spatial cognition relies on a complex circuitry in which the hippocampal formation seems to be crucial. The subiculum is a region located at the core of this circuit, it receives inputs from grid cells located in the medial entorhinal cortex (MEC) and place cells from the CA1 area of the hippocampus. It integrates input from these two relevant spatial information sources and mediates the output from the hippocampus to cortical and subcortical areas also involved in spatial coding. Despite the potential relevance of the subiculum, its role in memory and spatial coding is still poorly understood. Previous work described a very heterogeneous population of spatial neurons in the subiculum, with evidence of its role in coding the geometry of the environment and in spatial navigation in darkness. However, its role in spatial coding remains to be unveiled. With the aim of understanding further the properties of spatial coding in the subiculum, we implanted mice with microdrives mounting tetrodes and multisite electrodes aiming at the CA1 and subicular area, and recorded neuronal activity across different behavioral paradigms. Our results indicate that place cells in CA1 present higher spatial resolution and sharper firing fields than those of subicular neurons. Also, place cells in the CA1 area seem to be differentially modulated by the local field potential. Interestingly, we found pyramidal neurons in the subiculum with periodic firing, the first evidence of this type of regular firing in subicular pyramidal neurons.

How the patterns of cortex folding are implemented during embryonic development is poorly understood. In this issue of Neuron, Han et al. (2021) establish that a population of neural progenitor cells co-expressing Neurog2 and Ascl1 are key in this process.

In vivo electroporation has become a key technique to study genetic mechanisms of brain development. However, electroporation of the embryonic pallium in oviparous species, interesting for evolutionary studies but distinct from in utero electroporation, is quite infrequent. Here, we detail the in ovo electroporation of the developing pallium in chick and snake embryos. This protocol allows gene manipulation through introducing exogenous DNA into brain progenitor cells and can be adapted to any type of gene manipulation of the embryonic telencephalon.

One of the greatest challenges in the study of Human Evolution is to understand the social and biological adaptive processes that took place along our evolutionary history. This challenge has molecular, genetic, behavioural, social and anatomic morphological dimensions and requires new multidisciplinary and technological approaches.

El Consejo Europeo de Investigación (ERC, por sus siglas en inglés) cumplió 10 años el 24 de marzo de 2017. A lo largo de esta década de vida, se ha financiado a unos 7.000 investigadores, permitiendo la publicación de cerca de 100.000 artículos en revistas científicas internacionales. En España, cerca de 400 investigadores han recibido un total de 650 millones de euros y el Consejo Superior de Investigaciones Científicas (CSIC) destaca por ser la institución española con mayor número de ayudas, por delante de la Universidad Pompeu Fabra, el Instituto de Ciencias Fotónicas, la Universidad de Barcelona y el Centro de Regulación Genómica. Entre los más de 400 proyectos que han obtenido una ayuda del Consejo Europeo de Investigación en España, más de un centenar han recaído en el CSIC.

The size and organization of the brain are determined by the activity of progenitor cells early in development. Key mechanisms regulating progenitor cell biology involve miRNAs. These small noncoding RNA molecules bind mRNAs with high specificity, controlling their abundance and expression. The role of miRNAs in brain development has been studied extensively, but their involvement at early stages remained unknown until recently. Here, recent findings showing the important role of miRNAs in the earliest phases of brain development are reviewed, and it is discussed how loss of specific miRNAs leads to pathological conditions, particularly adult and pediatric brain tumors. Let-7 miRNA downregulation and the initiation of embryonal tumors with multilayered rosettes (ETMR), a novel link recently discovered by the laboratory, are focused upon. Finally, it is discussed how miRNAs may be used for the diagnosis and therapeutic treatment of pediatric brain tumors, with the hope of improving the prognosis of these devastating diseases.

The mammalian cerebral cortex is the pinnacle of brain evolution, reaching its maximum complexity in terms of neuron number, diversity and functional circuitry. The emergence of this outstanding complexity begins during embryonic development, when a limited number of neural stem and progenitor cells manage to generate myriads of neurons in the appropriate numbers, types and proportions, in a process called neurogenesis. Here we review the current knowledge on the regulation of cortical neurogenesis, beginning with a description of the types of progenitor cells and their lineage relationships. This is followed by a review of the determinants of neuron fate, the molecular and genetic regulatory mechanisms, and considerations on the evolution of cortical neurogenesis in vertebrates leading to humans. We finish with an overview on how dysregulation of neurogenesis is a leading cause of human brain malformations and functional disabilities.

En esta memoria se describen los pasos a seguir para implementar un sistema de medición de la contaminación lumínica mediante drones. Las metodologías empleadas habitualmente para medir y valorar las prestaciones de las instalaciones mediante parámetros lumínicos, implican procesos de medición relativamente lentos y laboriosos, de manera que su uso se limita al estudio de áreas pequeñas y/o con un interés particular. Con la intención de extender dicha clase de estudios a grandes zonas, como municipios enteros, se han proyectado en el sistema desarrollado el uso de un dron en movimiento para la toma de medidas y de un SIG (Sistema de Información Geográfica) para gestionar y automatizar la obtención, manipulación y representación del gran número de medidas implicadas. En la primera fase del trabajo se explica una introducción sobre la contaminación lumínica con el fin de entender todos los conceptos. Posteriormente se trata el análisis del alumbrado artificial y los diferentes parámetros que intervienen. Además, se describen las normativas a seguir para dicho proyecto, ya que el uso de drones está muy restringido en distintas zonas urbanas. Por otro lado, se realizará un estudio de mercado con el fin de obtener el mejor dron para la causa. Finalmente, se describirá los procesos que se deberían desarrollar para realizar las mediciones y para posteriormente tratar los datos. A continuación, se hace un análisis de los parámetros clave que pueden afectar en el proyecto, y finalmente se plantea una solución-producto con el fin de estudiar la problemática

The evolution of the mammalian cerebral cortex leading to humans involved a remarkable sophistication of developmental mechanisms. Specific adaptations of progenitor cell proliferation and neuronal migration mechanisms have been proposed to play major roles in this evolution of neocortical development. One of the central elements influencing neocortex development is the extracellular matrix (ECM). The ECM provides both a structural framework during tissue formation and to present signaling molecules to cells, which directly influences cell behavior and movement. Here we review recent advances in the understanding of the role of ECM molecules on progenitor cell proliferation and neuronal migration, and how these contribute to cerebral cortex expansion and folding. We discuss how transcriptomic studies in human, ferret and mouse identify components of ECM as being candidate key players in cortex expansion during development and evolution. Then we focus on recent functional studies showing that ECM components regulate cortical progenitor cell proliferation, neuron migration and the mechanical properties of the developing cortex. Finally, we discuss how these features differ between lissencephalic and gyrencephalic species, and how the molecular evolution of ECM components and their expression profiles may have been fundamental in the emergence and evolution of cortex folding across mammalian phylogeny.

Structural integrity and cellular homeostasis of the embryonic stem cell niche are critical for normal tissue development. In the telencephalic neuroepithelium, this is controlled in part by cell adhesion molecules and regulators of progenitor cell lineage, but the specific orchestration of these processes remains unknown. Here, we studied the role of microRNAs in the embryonic telencephalon as key regulators of gene expression. By using the early recombiner Rx‐Cre mouse, we identify novel and critical roles of miRNAs in early brain development, demonstrating they are essential to preserve the cellular homeostasis and structural integrity of the telencephalic neuroepithelium. We show that Rx‐Cre;DicerF/F mouse embryos have a severe disruption of the telencephalic apical junction belt, followed by invagination of the ventricular surface and formation of hyperproliferative rosettes. Transcriptome analyses and functional experiments in vivo show that these defects result from upregulation of Irs2 upon loss of let‐7 miRNAs in an apoptosis‐independent manner. Our results reveal an unprecedented relevance of miRNAs in early forebrain development, with potential mechanistic implications in pediatric brain cancer.

The cerebral cortex varies dramatically in size and complexity between amniotes due to differences in neuron number and composition. These differences emerge during embryonic development as a result of variations in neurogenesis, which are thought to recapitulate modifications occurred during evolution that culminated in the human neocortex. Here, we review work from the last few decades leading to our current understanding of the evolution of neurogenesis and size of the cerebral cortex. Focused on specific examples across vertebrate and amniote phylogeny, we discuss developmental mechanisms regulating the emergence, lineage, complexification and fate of cortical germinal layers and progenitor cell types. At the cellular level, we discuss the fundamental impact of basal progenitor cells and the advent of indirect neurogenesis on the increased number and diversity of cortical neurons and layers in mammals, and on cortex folding. Finally, we discuss recent work that unveils genetic and molecular mechanisms underlying this progressive expansion and increased complexity of the amniote cerebral cortex during evolution, with a particular focus on those leading to human-specific features. Whereas new genes important in human brain development emerged the recent hominid lineage, regulation of the patterns and levels of activity of highly conserved signaling pathways are beginning to emerge as mechanisms of central importance in the evolutionary increase in cortical size and complexity across amniotes.

The Genetic and molecular mechanisms that control development and increased size of the cerebral cortex along amniote evolution are largely unknown. Cortex size depends on neurons produced during embryonic development. Neurons are produced directly from Radial Glia Cells (RGCs), or indirectly from RGCs via generating Intermediate Progenitor Cells (IPCs). The balance between aRGC selfrenewal, production of IPCs or consumption toward neurogenesis, is under a complex set of regulatory mechanisms. Our previous studies in the mouse neocortex showed that very low levels of membrane receptors Robo1 and Robo2 are essential in controlling the balance between RGC self-renewal and production of IPCs. This is controlled partly by Robo driving Hes1 transcription, a component of Notch pathway. Recently, our lab showed high levels of Robo signalling in RGCs, concomitant with low Dll1 levels (ligand of Notch), promote direct neurogenesis with aRGC self-consumption, whereas inversion of these levels lead to indirect neurogenesis via production of IPCs. We further showed attenuation of Robo signalling was responsible for increasing neuron production and cortical expansion in vertebrate evolution. Here we investigate the intracellular interactome of Robo to characterize the molecular signalling pathway downstream of Robo activation that leads to direct neurogenesis. To this aim we are performing structurefunction analyses of the intracellular domain of Robo to identify the functional domains, and predicted interacting proteins, that are key in promoting direct neurogenesis upon Robo activation. Our findings should illuminate the mode of interaction between Robo and Notch signalling, two extensively studied signalling pathways not previously related.

Cerebral cortex size and complexity varies greatly across amniote phylogeny. In species with a small and simple cortex, like reptiles and small birds, the vast majority of neurons are born directly from apical Radial Glia Cells (aRGCs), whereas in mammals most neocortical neurons are born indirectly via the production of Intermediate Progenitors Cells (IPCs), which greatly amplify the neurogenic output of aRGCs. Thus, the balance between direct and indirect neurogenesis is crucial for cortical growth. A previous study recently carried out in our lab demonstrated that high levels of Robo1 and Robo2 expression in aRGCs lead to low levels of Dll1 and promote direct neurogenesis, while low Robo1/2 and high Dll1 lead to production of IPCs and indirect neurogenesis. Most importantly, this mechanism is conserved in amniote phylogeny, from reptiles to humans, indicating that attenuation of Robo signaling during evolution led to the emergence of IPCs and expansion of the cerebral cortex. We hypothesize that these differences are due to changes in the mechanisms that regulate Robo transcription. Here we use BrdU labeling in vivo, and two-photon videomicroscopy of brain slices, to precisely define the frequency of occurrence of direct and indirect neurogenesis along embryonic development in both mouse and chick. This will lead us to identify the most relevant stages to search for variations in the regulation of Robo transcription across developmental time within and between species.

Folding of the cerebral cortex is a fundamental milestone of mammalian brain evolution and is associated with dramatic increases in size and complexity. New animal models, genetic tools and bioengineering materials have moved the study of cortical folding from simple phenomenological observation to sophisticated experimental testing. Here, we provide an overview of how genetics, cell biology and biomechanics shape this complex and multifaceted process and affect each other. We discuss the evolution of cortical folding and the genomic changes in the primate lineage that seem to be responsible for the advent of larger brains and cortical folding. Emerging technologies now provide unprecedented tools to analyse and manipulate cortical folding, with the promise of elucidating the mechanisms underlying the stereotyped folding of the cerebral cortex in its full complexity.

The existence of axons extending from one retina to the other has been reported during perinatal development in different vertebrates. However, it has been thought that these axons are either a labelling artefact or misprojections. Here, we unequivocally show that a small subset of retinal ganglion cells (RGCs) project to the opposite retina and that the guidance receptor Unc5c, expressed in the retinal region where the R-R neurons are located, is necessary and sufficient to guide axons to the opposite retina. In addition, Netrin1, an Unc5c ligand, is expressed in the ventral diencephalon in a pattern that is consistent with impeding the growth of Unc5c retinal axons into the brain. We have also generated a mathematical model to explore the formation of retinotopic maps in the presence and absence of a functional connection between both eyes. This model predicts that an R-R connection is required for the bilateral coordination of axonal refinement in species where refinement depends upon spontaneous retinal waves. Consistent with this idea, the retinal expression of Unc5c correlates with the existence and size of a R-R projection in different species and with the extent of axonal refinement in visual targets. These findings demonstrate that active guidance drives the formation of the R-R projection and suggest an important role for these projections in visual mapping to ensure congruent bilateral refinement.

The neocortex is the largest part of the mammalian brain and is the seat of our higher cognitive functions. This outstanding neural structure increased massively in size and complexity during evolution in a process recapitulated today during the development of extant mammals. Accordingly, defects in neocortical development commonly result in severe intellectual and social deficits. Thus, understanding the development of the neocortex benefits from understanding its evolution and disease and also informs about their underlying mechanisms. Here, I briefly summarize the most recent and outstanding advances in our understanding of neocortical development and focus particularly on dorsal progenitors and excitatory neurons. I place special emphasis on the specification of neural stem cells in distinct classes and their proliferation and production of neurons and then discuss recent findings on neuronal migration. Recent discoveries on the genetic evolution of neocortical development are presented with a particular focus on primates. Progress on all these fronts is being accelerated by high-throughput gene expression analyses and particularly single-cell transcriptomics. I end with novel insights into the involvement of microglia in embryonic brain development and how improvements in cultured cerebral organoids are gradually consolidating them as faithful models of neocortex development in humans.

The anatomical wiring of the brain is a central focus in network neuroscience. Diffusion MRI tractography offers the unique opportunity to investigate the brain fiber architecture in vivo and noninvasively. However, its reliability is still highly debated. Here, we explored the ability of diffusion MRI tractography to match invasive anatomical tract-tracing connectivity data of the ferret brain. We also investigated the influence of several state-of-the-art tractography algorithms on this match to ground truth connectivity data. Tract-tracing connectivity data were obtained from retrograde tracer injections into the occipital, parietal, and temporal cortices of adult ferrets. We found that the relative densities of projections identified from the anatomical experiments were highly correlated with the estimates from all the studied diffusion tractography algorithms (Spearman’s rho ranging from 0.67 to 0.91), while only small, nonsignificant variations appeared across the tractography algorithms. These results are comparable to findings reported in mouse and monkey, increasing the confidence in diffusion MRI tractography results. Moreover, our results provide insights into the variations of sensitivity and specificity of the tractography algorithms, and hence into the influence of choosing one algorithm over another., [Author Summary]: In this article we used tract-tracing data as a gold standard to validate the use of diffusion MRI tractography for inferring structural connectivity in the ferret brain as well as for assessing the influence of several state-of-the-art tractography algorithms on the inferred connections. We found high correspondence between diffusion MRI tractography and tract-tracing with little differences between the explored algorithms. We conclude that diffusion MRI tractography provides a worthwhile whole-brain estimate of structural connectivity that can be employed in further anatomical, developmental, and computational studies of the ferret brain.

The expansion of brain size is accompanied by a relative enlargement of the subventricular zone during development. Epithelial-like neural stem cells divide in the ventricular zone at the ventricles of the embryonic brain, self-renew and generate basal progenitors1 that delaminate and settle in the subventricular zone in enlarged brain regions2. The length of time that cells stay in the subventricular zone is essential for controlling further amplification and fate determination. Here we show that the interphase centrosome protein AKNA has a key role in this process. AKNA localizes at the subdistal appendages of the mother centriole in specific subtypes of neural stem cells, and in almost all basal progenitors. This protein is necessary and sufficient to organize centrosomal microtubules, and promote their nucleation and growth. These features of AKNA are important for mediating the delamination process in the formation of the subventricular zone. Moreover, AKNA regulates the exit from the subventricular zone, which reveals the pivotal role of centrosomal microtubule organization in enabling cells to both enter and remain in the subventricular zone. The epithelial-to-mesenchymal transition is also regulated by AKNA in other epithelial cells, demonstrating its general importance for the control of cell delamination.

The existence of axons extending from one retina to the other has been reported during perinatal development in different vertebrates. However, it has been thought that these axons are either a labeling artifact or misprojections. Here, we show unequivocally that a small subset of retinal ganglion cells (RGCs) project to the opposite retina and that the guidance receptor Unc5c, expressed in the retinal region where the retinal-retinal (R-R) RGCs are located, is necessary and sufficient to guide axons to the opposite retina. In addition, Netrin1, an Unc5c ligand, is expressed in the ventral diencephalon in a pattern that is consistent with impeding the growth of Unc5c-positive retinal axons into the brain. We also have generated a mathematical model to explore the formation of retinotopic maps in the presence and absence of a functional connection between both eyes. This model predicts that an R-R connection is required for the bilateral coordination of axonal refinement in species where refinement depends upon spontaneous retinal waves. Consistent with this idea, the retinal expression of Unc5c correlates with the existence and size of an R-R projection in different species and with the extent of axonal refinement in visual targets. These findings demonstrate that active guidance drives the formation of the R-R projection and suggest an important role for these projections in visual mapping to ensure congruent bilateral refinement.

Excitatory neurons of the cerebral cortex migrate radially from their place of birth to their final position in the cortical plate during development. Radially‐migrating neurons display a single leading process that establishes the direction of movement. This leading process has been described as being unbranched, and the occurrence of branches proposed to impair radial migration. Here we have analyzed the detailed morphology of leading process in radially‐migrating pyramidal neurons and its impact on radial migration. We have compared ferret and mouse to identify differences between cortices that undergo folding or not. In mouse, we find that half of radially‐migrating neurons exhibit a branched leading process, this being even more frequent in ferret. Branched leading processes are less parallel to radial glia fibers than those unbranched, suggesting some independence from radial glia fibers. Two‐photon videomicroscopy revealed that a vast majority of neurons branch their leading process at some point during radial migration, but this does not reduce their migration speed. We have tested the functional impact of exuberant leading process branching by expressing a dominant negative Cdk5. We confirm that loss of Cdk5 function significantly impairs radial migration, but this is independent from increased branching of the leading process. We propose that excitatory neurons may branch their leading process as an evolutionary mechanism to allow cells changing their trajectory of migration to disperse laterally, such that increased branching in gyrencephalic species favors the tangential dispersion of radially‐migrating neurons, and cortical folding.

Folding of the cerebral cortex is as highly intriguing as poorly understood. At first sight, this may appear as simple tissue crumpling inside an excessively small cranium, but the process is clearly much more complex and developmentally predetermined. Whereas theoretical modeling supports a critical role for biomechanics, experimental evidence demonstrates the fundamental role of specific progenitor cell types, cellular processes, and genetic programs on cortical folding.

Cerebral cortex size differs dramatically between reptiles, birds, and mammals, owing to developmental differences in neuron production. In mammals, signaling pathways regulating neurogenesis have been identified, but genetic differences behind their evolution across amniotes remain unknown. We show that direct neurogenesis from radial glia cells, with limited neuron production, dominates the avian, reptilian, and mammalian paleocortex, whereas in the evolutionarily recent mammalian neocortex, most neurogenesis is indirect via basal progenitors. Gain- and loss-of-function experiments in mouse, chick, and snake embryos and in human cerebral organoids demonstrate that high Slit/Robo and low Dll1 signaling, via Jag1 and Jag2, are necessary and sufficient to drive direct neurogenesis. Attenuating Robo signaling and enhancing Dll1 in snakes and birds recapitulates the formation of basal progenitors and promotes indirect neurogenesis. Our study identifies modulation in activity levels of conserved signaling pathways as a primary mechanism driving the expansion and increased complexity of the mammalian neocortex during amniote evolution.

The folding of the mammalian cerebral cortex into sulci and gyri is thought to be favored by the amplification of basal progenitor cells and their tangential migration. Here, we provide a molecular mechanism for the role of migration in this process by showing that changes in intercellular adhesion of migrating cortical neurons result in cortical folding. Mice with deletions of FLRT1 and FLRT3 adhesion molecules develop macroscopic sulci with preserved layered organization and radial glial morphology. Cortex folding in these mutants does not require progenitor cell amplification but is dependent on changes in neuron migration. Analyses and simulations suggest that sulcus formation in the absence of FLRT1/3 results from reduced intercellular adhesion, increased neuron migration, and clustering in the cortical plate. Notably, FLRT1/3 expression is low in the human cortex and in future sulcus areas of ferrets, suggesting that intercellular adhesion is a key regulator of cortical folding across species.

Increasingly available high-resolution brain imaging data require specialized processing tools that can leverage their anatomical detail and handle their size. Here, we present user-friendly Python tools for cortical depth resolved analysis in such data. Our implementation is based on the CBS High-Res Brain Processing framework, and aims to make high-resolution data processing tools available to the broader community.

Folding of the cerebral cortex during brain development is a complex process that depends on the orchestrated action of a number of factors, including generation and proliferation of basal progenitor cells, and the radial migration of neurons. Patterns of primary cortical folding are stereotyped between individuals and across phylogeny, reflecting a strong genetic regulation of the underlying cellular mechanisms. Here we summarize recent findings on cellular and genetic mechanisms regulating this fascinating process that underlies expansion and functional complexification of the mammalian cerebral cortex.

Neocortex evolutionary expansion is primarily due to increased proliferative capacity of neural progenitor cells during cortical development. Exploiting insights into the cell biology of cortical progenitors gained during the past two decades, recent studies uncovered a variety of gene expression differences that underlie differential cortical progenitor behavior. These comprise both, differences between cortical areas that likely provide a molecular basis for cortical folding, and differences across species thought to be responsible for increases in neocortex size. Human-specific signatures have been identified for gene regulatory elements, non-coding gene products, and protein-encoding genes, and have been functionally examined in in vivo as well as novel in vitro model systems.

Subventricular zone (SVZ) progenitors are a hallmark of the developing neocortex. Recent studies described a novel type of SVZ progenitor that retains a basal process at mitosis, sustains expression of radial glial markers, and is capable of self-renewal. These progenitors, referred to here as basal radial glia (bRG), occur at high relative abundance in the SVZ of gyrencephalic primates (human) and nonprimates (ferret) but not lissencephalic rodents (mouse). Here, we analyzed the occurrence of bRG cells in the embryonic neocortex of the common marmoset Callithrix jacchus, a near-lissencephalic primate. bRG cells, expressing Pax6, Sox2 (but not Tbr2), glutamate aspartate transporter, and glial fibrillary acidic protein and retaining a basal process at mitosis, occur at similar relative abundance in the marmoset SVZ as in human and ferret. The proportion of progenitors in M-phase was lower in embryonic marmoset than developing ferret neocortex, raising the possibility of a longer cell cycle. Fitting the gyrification indices of 26 anthropoid species to an evolutionary model suggested that the marmoset evolved from a gyrencephalic ancestor. Our results suggest that a high relative abundance of bRG cells may be necessary, but is not sufficient, for gyrencephaly and that the marmoset's lissencephaly evolved secondarily by changing progenitor parameters other than progenitor type