Changes in the regulation of cortical neurogenesis contribute to encephalization during amniote brain evolution

The microcephaly-associated transcriptional regulator AUTS2 cooperates with Polycomb complex PRC2 to produce upper-layer neurons in mice

AUTS2 syndrome is characterized by intellectual disability and microcephaly, and is often associated with autism spectrum disorder, but the underlying mechanisms, particularly concerning microcephaly, remain incompletely understood. Here, we analyze mice mutated for the transcriptional regulator AUTS2, which recapitulate microcephaly. Their brains exhibit reduced division of intermediate progenitor cells (IPCs), leading to fewer neurons and decreased thickness in the upper-layer cortex. Increased expression of the AUTS2 transcriptional target Robo1 in the mutant animals suppresses IPC division, and transcriptomic and chromatin profiling shows that AUTS2 primarily represses transcription of genes like Robo1 in IPCs. Regions around the transcriptional start sites of AUTS2 target genes are enriched for the repressive histone modification H3K27me3, which is reduced in Auts2 mutants. Furthermore, we find that AUTS2 interacts with Polycomb complex PRC2, with which it cooperates to promote IPC division. These findings shed light on the microcephaly phenotype observed in the AUTS2 syndrome.

Experimental Neurogenesis in the Embryos of the Gecko Paroedura picta

This research introduces innovative tools for studying embryonic neurogenesis in the gecko Paroedura picta. Traditional research methods have been adapted for the gecko's unique biology, including variations of birthdating techniques and the implementation of EdU for tracking neuron generation. We also employ hybridization chain reaction (HCR) to detect specific mRNAs without the need for species-specific antibodies, providing a powerful and flexible tool for studying gene expression patterns in the gecko embryonic brain. However, HCR can also be combined with immunohistochemistry to visualize both RNA and protein distribution. Our work proves the importance of this innovative toolset to understand the evolutionary aspects of nervous system development and the gecko as a valuable model for understanding the ancestral stem amniote.

Indirect neurogenesis in space and time

During central nervous system (CNS) development, neural progenitor cells (NPCs) generate neurons and glia in two different ways. In direct neurogenesis, daughter cells differentiate directly into neurons or glia, whereas in indirect neurogenesis, neurons or glia are generated after one or more daughter cell divisions. Intriguingly, indirect neurogenesis is not stochastically deployed and plays instructive roles during CNS development: increased generation of cells from specific lineages; increased generation of early or late-born cell types within a lineage; and increased cell diversification. Increased indirect neurogenesis might contribute to the anterior CNS expansion evident throughout the Bilateria and help to modify brain-region size without requiring increased NPC numbers or extended neurogenesis. Increased indirect neurogenesis could be an evolutionary driver of the gyrencephalic (that is, folded) cortex that emerged during mammalian evolution and might even have increased during hominid evolution. Thus, selection of indirect versus direct neurogenesis provides a powerful developmental and evolutionary instrument that drives not only the evolution of CNS complexity but also brain expansion and modulation of brain-region size, and thereby the evolution of increasingly advanced cognitive abilities. This Review describes indirect neurogenesis in several model species and humans, and highlights some of the molecular genetic mechanisms that control this important process.

Evolution of neural circuitry and cognition

Neural circuits govern the interface between the external environment, internal cues and outwardly directed behaviours. To process multiple environmental stimuli and integrate these with internal state requires considerable neural computation. Expansion in neural network size, most readily represented by whole brain size, has historically been linked to behavioural complexity, or the predominance of cognitive behaviours. Yet, it is largely unclear which aspects of circuit variation impact variation in performance. A key question in the field of evolutionary neurobiology is therefore how neural circuits evolve to allow improved behavioural performance or innovation. We discuss this question by first exploring how volumetric changes in brain areas reflect actual neural circuit change. We explore three major axes of neural circuit evolution-replication, restructuring and reconditioning of cells and circuits-and discuss how these could relate to broader phenotypes and behavioural variation. This discussion touches on the relevant uses and limitations of volumetrics, while advocating a more circuit-based view of cognition. We then use this framework to showcase an example from the insect brain, the multi-sensory integration and internal processing that is shared between the mushroom bodies and central complex. We end by identifying future trends in this research area, which promise to advance the field of evolutionary neurobiology.

The Phylotypic Brain of Vertebrates, from Neural Tube Closure to Brain Diversification

BACKGROUND

The phylotypic or intermediate stages are thought to be the most evolutionary conserved stages throughout embryonic development. The contrast with divergent early and later stages derived from the concept of the evo-devo hourglass model. Nonetheless, this developmental constraint has been studied as a whole embryo process, not at organ level. In this review, we explore brain development to assess the existence of an equivalent brain developmental hourglass. In the specific case of vertebrates, we propose to split the brain developmental stages into: (1) Early: Neurulation, when the neural tube arises after gastrulation. (2) Intermediate: Brain patterning and segmentation, when the neuromere identities are established. (3) Late: Neurogenesis and maturation, the stages when the neurons acquire their functionality. Moreover, we extend this analysis to other chordates brain development to unravel the evolutionary origin of this evo-devo constraint.

SUMMARY

Based on the existing literature, we hypothesise that a major conservation of the phylotypic brain might be due to the pleiotropy of the inductive regulatory networks, which are predominantly expressed at this stage. In turn, earlier stages such as neurulation are rather mechanical processes, whose regulatory networks seem to adapt to environment or maternal geometries. The later stages are also controlled by inductive regulatory networks, but their effector genes are mostly tissue-specific and functional, allowing diverse developmental programs to generate current brain diversity. Nonetheless, all stages of the hourglass are highly interconnected: divergent neurulation must have a vertebrate shared end product to reproduce the vertebrate phylotypic brain, and the boundaries and transcription factor code established during the highly conserved patterning will set the bauplan for the specialised and diversified adult brain.

KEY MESSAGES

The vertebrate brain is conserved at phylotypic stages, but the highly conserved mechanisms that occur during these brain mid-development stages (Inducing Regulatory Networks) are also present during other stages. Oppositely, other processes as cell interactions and functional neuronal genes are more diverse and majoritarian in early and late stages of development, respectively. These phenomena create an hourglass of transcriptomic diversity during embryonic development and evolution, with a really conserved bottleneck that set the bauplan for the adult brain around the phylotypic stage.

Non-uniform temporal scaling of developmental processes in the mammalian cortex

The time that it takes the brain to develop is highly variable across animals. Although staging systems equate major developmental milestones between mammalian species, it remains unclear how distinct processes of cortical development scale within these timeframes. Here, we compare the timing of cortical development in two mammals of similar size but different developmental pace: eutherian mice and marsupial fat-tailed dunnarts. Our results reveal that the temporal relationship between cell birth and laminar specification aligns to equivalent stages between these species, but that migration and axon extension do not scale uniformly according to the developmental stages, and are relatively more advanced in dunnarts. We identify a lack of basal intermediate progenitor cells in dunnarts that likely contributes in part to this timing difference. These findings demonstrate temporal limitations and differential plasticity of cortical developmental processes between similarly sized Therians and provide insight into subtle temporal changes that may have contributed to the early diversification of the mammalian brain.

Advanced Techniques Using In Vivo Electroporation to Study the Molecular Mechanisms of Cerebral Development Disorders

The mammalian cerebral cortex undergoes a strictly regulated developmental process. Detailed in situ visualizations, imaging of these dynamic processes, and in vivo functional gene studies significantly enhance our understanding of brain development and related disorders. This review introduces basic techniques and recent advancements in in vivo electroporation for investigating the molecular mechanisms underlying cerebral diseases. In utero electroporation (IUE) is extensively used to visualize and modify these processes, including the forced expression of pathological mutants in human diseases; thus, this method can be used to establish animal disease models. The advent of advanced techniques, such as genome editing, including de novo knockout, knock-in, epigenetic editing, and spatiotemporal gene regulation, has further expanded our list of investigative tools. These tools include the iON expression switch for the precise control of timing and copy numbers of exogenous genes and TEMPO for investigating the temporal effects of genes. We also introduce the iGONAD method, an improved genome editing via oviductal nucleic acid delivery approach, as a novel genome-editing technique that has accelerated brain development exploration. These advanced in vivo electroporation methods are expected to provide valuable insights into pathological conditions associated with human brain disorders.

Evolution of genetic mechanisms regulating cortical neurogenesis

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.

Visualizing Cortical Development and Evolution: A Toolkit Update

Visualizing the process of neural circuit formation during neurogenesis, using genetically modified animals or somatic transgenesis of exogenous plasmids, has become a key to decipher cortical development and evolution. In contrast to the establishment of transgenic animals, the designing and preparation of genes of interest into plasmids are simple and easy, dispensing with time-consuming germline modifications. These advantages have led to neuron labeling based on somatic transgenesis. In particular, mammalian expression plasmid, CRISPR-Cas9, and DNA transposon systems, have become widely used for neuronal visualization and functional analysis related to lineage labeling during cortical development. In this review, we discuss the advantages and limitations of these recently developed techniques.

Cell Type-Specific Transcriptional Control of Gsk3β in the Developing Mammalian Neocortex

Temporal control of neurogenesis is central for the development and evolution of species-specific brain architectures. The balance between progenitor expansion and neuronal differentiation is tightly coordinated by cell-intrinsic and cell-extrinsic cues. Wnt signaling plays pivotal roles in the proliferation and differentiation of neural progenitors in a temporal manner. However, regulatory mechanisms that adjust intracellular signaling amplitudes according to cell fate progression remain to be elucidated. Here, we report the transcriptional controls of Gsk3β, a critical regulator of Wnt signaling, in the developing mouse neocortex. Gsk3β expression was higher in ventricular neural progenitors, while it gradually declined in differentiated neurons. We identified active cis-regulatory module (CRM) of Gsk3β that responded to cell type-specific transcription factors, such as Sox2, Sox9, and Neurogenin2. Furthermore, we found extensive conservation of the CRM among mammals but not in non-mammalian amniotes. Our data suggest that a mammalian-specific CRM drives the cell type-specific activity of Gsk3β to fine tune Wnt signaling, which contributes to the tight control of neurogenesis during neocortical development.

Evolving Roles of Notch Signaling in Cortical Development

Expansion of the neocortex is thought to pave the way toward acquisition of higher cognitive functions in mammals. The highly conserved Notch signaling pathway plays a crucial role in this process by regulating the size of the cortical progenitor pool, in part by controlling the balance between self-renewal and differentiation. In this review, we introduce the components of Notch signaling pathway as well as the different mode of molecular mechanisms, including trans- and cis-regulatory processes. We focused on the recent findings with regard to the expression pattern and levels in regulating neocortical formation in mammals and its interactions with other known signaling pathways, including Slit–Robo signaling and Shh signaling. Finally, we review the functions of Notch signaling pathway in different species as well as other developmental process, mainly somitogenesis, to discuss how modifications to the Notch signaling pathway can drive the evolution of the neocortex.

Temperature sensitivity of Notch signaling underlies species-specific developmental plasticity and robustness in amniote brains

Ambient temperature significantly affects developmental timing in animals. The temperature sensitivity of embryogenesis is generally believed to be a consequence of the thermal dependency of cellular metabolism. However, the adaptive molecular mechanisms that respond to variations in temperature remain unclear. Here, we report species-specific thermal sensitivity of Notch signaling in the developing amniote brain. Transient hypothermic conditions increase canonical Notch activity and reduce neurogenesis in chick neural progenitors. Increased biosynthesis of phosphatidylethanolamine, a major glycerophospholipid components of the plasma membrane, mediates hypothermia-induced Notch activation. Furthermore, the species-specific thermal dependency of Notch signaling is associated with developmental robustness to altered Notch signaling. Our results reveal unique regulatory mechanisms for temperature-dependent neurogenic potentials that underlie developmental and evolutionary adaptations to a range of ambient temperatures in amniotes.

Ambient temperature significantly affects embryogenesis, but adaptive molecular mechanisms that respond to temperature remain unclear. Here, the authors identified species-specific thermal sensitivity of Notch signaling in developing amniote brains.

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.

From Cell Types to an Integrated Understanding of Brain Evolution: The Case of the Cerebral Cortex

With the discovery of the incredible diversity of neurons, Cajal and coworkers laid the foundation of modern neuroscience. Neuron types are not only structural units of nervous systems but also evolutionary units, because their identities are encoded in the genome. With the advent of high-throughput cellular transcriptomics, neuronal identities can be characterized and compared systematically across species. The comparison of neurons in mammals, reptiles, and birds indicates that the mammalian cerebral cortex is a mosaic of deeply conserved and recently evolved neuron types. Using the cerebral cortex as a case study, this review illustrates how comparing neuron types across species is key to reconciling observations on neural development, neuroanatomy, circuit wiring, and physiology for an integrated understanding of brain evolution.

Time in Neurogenesis: Conservation of the Developmental Formation of the Cerebellar Circuitry

The cerebellum is a conserved structure of vertebrate brains that develops at the most anterior region of the alar rhombencephalon. All vertebrates display a cerebellum, making it one of the most highly conserved structures of the brain. Although it greatly varies at the morphological level, several lines of research point to strong conservation of its internal neural circuitry. To test the conservation of the cerebellar circuit, we compared the developmental history of the neurons comprising this circuit in three amniote species: mouse, chick, and gecko. We specifically researched the developmental time of generation of the main neuronal types of the cerebellar cortex. This developmental trajectory is known for the mammalian cell types but barely understood for sauropsid species. We show that the neurogenesis of the GABAergic lineage proceeds following the same chronological sequence in the three species compared: Purkinje cells are the first ones generated in the cerebellar cortex, followed by Golgi interneurons of the granule cell layer, and lately by the interneurons of the molecular layer. In the cerebellar glutamatergic lineage, we observed the same conservation of neurogenesis throughout amniotes, and the same vastly prolonged neurogenesis of granule cells, extending much further than for any other brain region. Together these data show that the cerebellar circuitry develops following a tightly conserved chronological sequence of neurogenesis, which is responsible for the preservation of the cerebellum and its function. Our data reinforce the developmental perspective of homology, whereby similarities in neurons and circuits are likely due to similarities in developmental sequence.

A protocol for in ovo electroporation of chicken and snake embryos to study forebrain development

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.

For complete information on the use and execution of this protocol, please refer to Cárdenas et al. (2018).

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In ovo electroporation of dorsal telencephalon in chick and snake embryos

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Maximal viability of the embryos makes the protocol highly efficient

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The simplicity of the procedure makes it accessible to non-expert researchers

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Adaptable to any type of gene manipulation of the embryonic telencephalon

Changes in Wnt-Dependent Neuronal Morphology Underlie the Anatomical Diversification of Neocortical Homologs in Amniotes

The six-layered neocortex is a shared characteristic of all mammals, but not of non-mammalian species, and its formation requires an inside-out pattern of neuronal migration. The extant reptilian dorsal cortex is thought to represent an ancestral form of the neocortex, although how the reptilian three-layered cortex is formed is poorly understood. Here, we show unique patterns of lamination and neuronal migration in the developing reptilian cortex. While the multipolar-to-bipolar transition of migrating neurons is essential for mammalian cortical development, the reptilian cortex lacks bipolar-shaped migrating neurons, resulting in an outside-in pattern of cortical development. Furthermore, dynamic regulation of Wnt signal strengths contributes to neuronal morphological changes, which is conserved across species. Our data preclude the idea that the six-layered mammalian neocortex emerged by simple addition to the reptilian dorsal cortex but suggest that the acquisition of a novel neuronal morphology based on conserved developmental programs contributed to neocortical evolution.

Igf signaling couples retina growth with body growth by modulating progenitor cell division

How the body and organs balance their relative growth is of key importance for coordinating size and function. This is of particular relevance in organisms, which continue to grow over their entire life span. We addressed this issue in the neuroretina of medaka fish (Oryzias latipes), a well-studied system with which to address vertebrate organ growth. We reveal that a central growth regulator, Igf1 receptor (Igf1r), is necessary and sufficient for proliferation control in the postembryonic retinal stem cell niche: the ciliary marginal zone (CMZ). Targeted activation of Igf1r signaling in the CMZ uncouples neuroretina growth from body size control, and we demonstrate that Igf1r operates on progenitor cells, stimulating their proliferation. Activation of Igf1r signaling increases retinal size while preserving its structural integrity, revealing a modular organization in which progenitor differentiation and neurogenesis are self-organized and highly regulated. Our findings position Igf signaling as a key module for controlling retinal size and composition, with important evolutionary implications.

Highlighted Article: Targeted activation of Igf1r signaling in the retinal stem cell niche increases retina size through expanding the progenitor but not stem cell population.

Genetic Mechanisms Underlying Cortical Evolution in Mammals

The remarkable sensory, motor, and cognitive abilities of mammals mainly depend on the neocortex. Thus, the emergence of the six-layered neocortex in reptilian ancestors of mammals constitutes a fundamental evolutionary landmark. The mammalian cortex is a columnar epithelium of densely packed cells organized in layers where neurons are generated mainly in the subventricular zone in successive waves throughout development. Newborn cells move away from their site of neurogenesis through radial or tangential migration to reach their specific destination closer to the pial surface of the same or different cortical area. Interestingly, the genetic programs underlying neocortical development diversified in different mammalian lineages. In this work, I will review several recent studies that characterized how distinct transcriptional programs relate to the development and functional organization of the neocortex across diverse mammalian lineages. In some primates such as the anthropoids, the neocortex became extremely large, especially in humans where it comprises around 80% of the brain. It has been hypothesized that the massive expansion of the cortical surface and elaboration of its connections in the human lineage, has enabled our unique cognitive capacities including abstract thinking, long-term planning, verbal language and elaborated tool making capabilities. I will also analyze the lineage-specific genetic changes that could have led to the modification of key neurodevelopmental events, including regulation of cell number, neuronal migration, and differentiation into specific phenotypes, in order to shed light on the evolutionary mechanisms underlying the diversity of mammalian brains including the human brain.

The regulation of cortical neurogenesis

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.

Variations of telencephalic development that paved the way for neocortical evolution

Charles Darwin stated, "community in embryonic structure reveals community of descent". Thus, to understand how the neocortex emerged during mammalian evolution we need to understand the evolution of the development of the pallium, the source of the neocortex. In this article, we review the variations in the development of the pallium that enabled the production of the six-layered neocortex. We propose that an accumulation of subtle modifications from very early brain development accounted for the diversification of vertebrate pallia and the origin of the neocortex. Initially, faint differences of expression of secretable morphogens promote a wide variety in the proportions and organization of sectors of the early pallium in different vertebrates. It prompted different sectors to host varied progenitors and distinct germinative zones. These cells and germinative compartments generate diverse neuronal populations that migrate and mix with each other through radial and tangential migrations in a taxon-specific fashion. Together, these early variations had a profound influence on neurogenetic gradients, lamination, positioning, and connectivity. Gene expression, hodology, and physiological properties of pallial neurons are important features to suggest homologies, but the origin of cells and their developmental trajectory are fundamental to understand evolutionary changes. Our review compares the development of the homologous pallial sectors in sauropsids and mammals, with a particular focus on cell lineage, in search of the key changes that led to the appearance of the mammalian neocortex.

The Pace of Neurogenesis Is Regulated by the Transient Retention of the Apical Endfeet of Differentiating Cells

The development of the mammalian cerebral cortex involves a variety of temporally organized events such as successive waves of neuronal production and the transition of progenitor competence for each neuronal subtype generated. The number of neurons generated in a certain time period, that is, the rate of neuron production, varies across the regions of the brain and the specific developmental stage; however, the underlying mechanism of this process is poorly understood. We have recently found that nascent neurons communicate with undifferentiated progenitors and thereby regulate neurogenesis, through a transiently retained apical endfoot that signals via the Notch pathway. Here, we report that the retention time length of the neuronal apical endfoot correlates with the rate of neuronal production in the developing mouse cerebral cortex. We further demonstrate that a forced reduction or extension of the retention period through the disruption or stabilization of adherens junction, respectively, resulted in the acceleration or deceleration of neurogenesis, respectively. Our results suggest that the apical endfeet of differentiating cells serve as a pace controller for neurogenesis, thereby assuring the well-proportioned laminar organization of the neocortex.

Comparative Analysis of Brain Stiffness Among Amniotes Using Glyoxal Fixation and Atomic Force Microscopy

Brain structures are diverse among species despite the essential molecular machinery of neurogenesis being common. Recent studies have indicated that differences in the mechanical properties of tissue may result in the dynamic deformation of brain structure, such as folding. However, little is known about the correlation between mechanical properties and species-specific brain structures. To address this point, a comparative analysis of mechanical properties using several animals is required. For a systematic measurement of the brain stiffness of remotely maintained animals, we developed a novel strategy of tissue-stiffness measurement using glyoxal as a fixative combined with atomic force microscopy. A comparison of embryonic and juvenile mouse and songbird brain tissue revealed that glyoxal fixation can maintain brain structure as well as paraformaldehyde (PFA) fixation. Notably, brain tissue fixed by glyoxal remained much softer than PFA-fixed brains, and it can maintain the relative stiffness profiles of various brain regions. Based on this method, we found that the homologous brain regions between mice and songbirds exhibited different stiffness patterns. We also measured brain stiffness in other amniotes (chick, turtle, and ferret) following glyoxal fixation. We found stage-dependent and species-specific stiffness in pallia among amniotes. The embryonic chick and matured turtle pallia showed gradually increasing stiffness along the apico-basal tissue axis, the lowest region at the most apical region, while the ferret pallium exhibited a catenary pattern, that is, higher in the ventricular zone, the inner subventricular zone, and the cortical plate and the lowest in the outer subventricular zone. These results indicate that species-specific microenvironments with distinct mechanical properties emerging during development might contribute to the formation of brain structures with unique morphology.

Dual Role of Rbpj in the Maintenance of Neural Progenitor Cells and Neuronal Migration in Cortical Development

The development of the cerebral cortex is directed by a series of methodically precise events, including progenitor cell proliferation, neural differentiation, and cell positioning. Over the past decade, many studies have demonstrated the critical contributions of Notch signaling in neurogenesis, including that in the developing telencephalon. However, in vivo evidence for the role of Notch signaling in cortical development still remains limited partly due to the redundant functions of four mammalian Notch paralogues and embryonic lethality of the knockout mice. Here, we utilized the conditional deletion and in vivo gene manipulation of Rbpj, a transcription factor that mediates signaling by all four Notch receptors, to overcome these challenges and examined the specific roles of Rbpj in cortical development. We report severe structural abnormalities in the embryonic and postnatal cerebral cortex in Rbpj conditional knockout mice, which provide strong in vivo corroboration of previously reported functions of Notch signaling in neural development. Our results also provide evidence for a novel dual role of Rbpj in cell type-specific regulation of two key developmental events in the cerebral cortex: the maintenance of the undifferentiated state of neural progenitor cells, and the radial and tangential allocation of neurons, possibly through stage-dependent differential regulation of Ngn1.

A SMAD1/5-YAP signalling module drives radial glia self-amplification and growth of the developing cerebral cortex

The growth and evolutionary expansion of the cerebral cortex are defined by the spatial-temporal production of neurons, which itself depends on the decision of radial glial cells (RGCs) to self-amplify or to switch to neurogenic divisions. The mechanisms regulating these RGC fate decisions are still incompletely understood. Here, we describe a novel and evolutionarily conserved role of the canonical BMP transcription factors SMAD1/5 in controlling neurogenesis and growth during corticogenesis. Reducing the expression of both SMAD1 and SMAD5 in neural progenitors at early mouse cortical development caused microcephaly and an increased production of early-born cortical neurons at the expense of late-born ones, which correlated with the premature differentiation and depletion of the pool of cortical progenitors. Gain- and loss-of-function experiments performed during early cortical neurogenesis in the chick revealed that SMAD1/5 activity supports self-amplifying RGC divisions and restrains the neurogenic ones. Furthermore, we demonstrate that SMAD1/5 stimulate RGC self-amplification through the positive post-transcriptional regulation of the Hippo signalling effector YAP. We anticipate this SMAD1/5-YAP signalling module to be fundamental in controlling growth and evolution of the amniote cerebral cortex.

Molecular and cellular evolution of corticogenesis in amniotes

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 modular organization of the cerebral cortex: Evolutionary significance and possible links to neurodevelopmental conditions

The recognition of discernible anatomical regularities that appear to self-organize during development makes apparent the modular organization of the cerebral cortex. The metabolic cost engendered in sustaining interneuronal communications has emphasized the viability of short connections among neighboring neurons. This pattern of connectivity establishes a microcircuit which is repeated in parallel throughout the cerebral cortex. This canonical circuit is contained within the smallest module of information processing of the cerebral cortex; one which Vernon Mountcastle called the minicolumn. Plasticity within the brain is accounted, in part, by the presence of weak linkages that allow minicolumns to process information from a variety of sources and to quickly adapt to environmental exigencies without a need for genetic change. Recent research suggests that interlaminar correlated firing between minicolumns during the decision phase of target selection provides for the emergence of some executive functions. Bottlenecks of information processing within this modular minicolumnar organization may account for a variety of mental disorders observed in neurodevelopmental conditions.

Evolution of neuronal identity in the cerebral cortex

To understand neocortex evolution, we must define a theory for the elaboration of cell types, circuits, and architectonics from an ancestral structure that is consistent with developmental, molecular, and genetic data. To this end, cross-species comparison of cortical cell types emerges as a very informative approach. We review recent results that illustrate the contribution of molecular and transcriptomic data to the construction of plausible models of cortical cell-type evolution.

Differential potentials of neural progenitors for the generation of neurons and non-neuronal cells in the developing amniote brain

Mature mammalian brains consist of variety of neuronal and non-neuronal cell types, which are progressively generated from embryonic neural progenitors through the embryonic and postnatal periods. However, it remains unknown whether all embryonic progenitors equivalently contribute to multiple cell types, or individual neural progenitors have variable potentials to generate specific cell types in a stochastic manner. Here, we performed population-level tracing of mouse embryonic neural progenitors by using Tol2-mediated genome integration vectors. We identified that neural progenitors in early embryonic stages predominantly contribute to cortical or subcortical neurons than astrocytes, ependymal cells, and neuroblasts in the postnatal brain. Notably, neurons and astrocytes were cumulatively labeled by the increase of total labeled cells, suggesting constant neurogenic and gliogenic potentials of individual neural progenitors. On the contrary, numbers of labeled ependymal cell are more fluctuated, implicating intrinsic variability of progenitor potentials for ependymal cell generation. Differential progenitor potentials that contribute to neurons, astrocytes, and ependymal cells were also detected in the developing avian pallium. Our data suggest evolutionary conservations of coherent and variable potentials of neural progenitors that generate multiple cell types in the developing amniote brain.

Evidence for neurogenesis in the medial cortex of the leopard gecko, Eublepharis macularius

Although lizards are often described as having robust neurogenic abilities, only a handful of the more than 6300 species have been explored. Here, we provide the first evidence of homeostatic neurogenesis in the leopard gecko (Eublepharis macularius). We focused our study on the medial cortex, homologue of the mammalian hippocampal formation. Using immunostaining, we identified proliferating pools of neural stem/progenitor cells within the sulcus septomedialis, the pseudostratified ventricular zone adjacent to the medial cortex. Consistent with their identification as radial glia, these cells expressed SOX2, glial fibrillary acidic protein, and Vimentin, and demonstrated a radial morphology. Using a 5-bromo-2′-deoxyuridine cell tracking strategy, we determined that neuroblast migration from the ventricular zone to the medial cortex takes ~30-days, and that newly generated neuronal cells survived for at least 140-days. We also found that cell proliferation within the medial cortex was not significantly altered following rupture of the tail spinal cord (as a result of the naturally evolved process of caudal autotomy). We conclude that the sulcus septomedialis of the leopard gecko demonstrates all the hallmarks of a neurogenic niche.

Conserved and divergent functions of Pax6 underlie species-specific neurogenic patterns in the developing amniote brain

The evolution of unique organ structures is associated with changes in conserved developmental programs. However, characterizing the functional conservation and variation of homologous transcription factors (TFs) that dictate species-specific cellular dynamics has remained elusive. Here, we dissect shared and divergent functions of Pax6 during amniote brain development. Comparative functional analyses revealed that the neurogenic function of Pax6 is highly conserved in the developing mouse and chick pallium, whereas stage-specific binary functions of Pax6 in neurogenesis are unique to mouse neuronal progenitors, consistent with Pax6-dependent temporal regulation of Notch signaling. Furthermore, we identified that Pax6-dependent enhancer activity of Dbx1 is extensively conserved between mammals and chick, although Dbx1 expression in the developing pallium is highly divergent in these species. Our results suggest that spatiotemporal changes in Pax6-dependent regulatory programs contributed to species-specific neurogenic patterns in mammalian and avian lineages, which underlie the morphological divergence of the amniote pallial architectures.

Highlighted Article: Pax6 promotes neuronal differentiation in the developing chick and mouse telencephalon via Notch inhibition, whereas its stage-specific function in RGC maintenance in the VZ is unique to mammalian neocortical progenitors.

Madagascar ground gecko genome analysis characterizes asymmetric fates of duplicated genes

Background

Conventionally, comparison among amniotes – birds, mammals, and reptiles – has often been approached through analyses of mammals and, for comparison, birds. However, birds are morphologically and physiologically derived and, moreover, some parts of their genomes are recognized as difficult to sequence and/or assemble and are thus missing in genome assemblies. Therefore, sequencing the genomes of reptiles would aid comparative studies on amniotes by providing more comprehensive coverage to help understand the molecular mechanisms underpinning evolutionary changes.

Results

Herein, we present the whole genome sequences of the Madagascar ground gecko (Paroedura picta), a promising study system especially in developmental biology, and used it to identify changes in gene repertoire across amniotes. The genome-wide analysis of the Madagascar ground gecko allowed us to reconstruct a comprehensive set of gene phylogenies comprising 13,043 ortholog groups from diverse amniotes. Our study revealed 469 genes retained by some reptiles but absent from available genome-wide sequence data of both mammals and birds. Importantly, these genes, herein collectively designated as ‘elusive’ genes, exhibited high nucleotide substitution rates and uneven intra-genomic distribution. Furthermore, the genomic regions flanking these elusive genes exhibited distinct characteristics that tended to be associated with increased gene density, repeat element density, and GC content.

Conclusion

This highly continuous and nearly complete genome assembly of the Madagascar ground gecko will facilitate the use of this species as an experimental animal in diverse fields of biology. Gene repertoire comparisons across amniotes further demonstrated that the fate of a duplicated gene can be affected by the intrinsic properties of its genomic location, which can persist for hundreds of millions of years.

Electronic supplementary material

The online version of this article (10.1186/s12915-018-0509-4) contains supplementary material, which is available to authorized users.

Evolutionarily conserved anterior expansion of the central nervous system promoted by a common PcG-Hox program

A conserved feature of the central nervous system (CNS) is the prominent expansion of anterior regions (brain) when compared to posterior (nerve cord). The cellular and regulatory processes driving anterior CNS expansion are not well understood in any bilaterian species. Here, we address this expansion in Drosophila and mouse. We find that when compared to the nerve cord the brain, in both Drosophila and mouse, displays extended progenitor proliferation, more elaborate daughter cell proliferation and more rapid cell cycle speed. These features contribute to anterior CNS expansion in both species. With respect to genetic control, enhanced brain proliferation is severely reduced by ectopic Hox gene expression, by either Hox misexpression or by loss of Polycomb Group (PcG) function. Strikingly, in PcG mutants, early CNS proliferation appears unaffected, whereas subsequently, brain proliferation is severely reduced. Hence, a conserved PcG-Hox program promotes the anterior expansion of the CNS. The profound differences in proliferation and in the underlying genetic mechanisms between brain and nerve cord lend support to the emerging concept of separate evolutionary origins of these two CNS regions.

The evolution of cortical development: the synapsid-diapsid divergence

The cerebral cortex covers the rostral part of the brain and, in higher mammals and particularly humans, plays a key role in cognition and consciousness. It is populated with neuronal cell bodies distributed in radially organized layers. Understanding the common and lineage-specific molecular mechanisms that orchestrate cortical development and evolution are key issues in neurobiology. During evolution, the cortex appeared in stem amniotes and evolved divergently in two main branches of the phylogenetic tree: the synapsids (which led to present day mammals) and the diapsids (reptiles and birds). Comparative studies in organisms that belong to those two branches have identified some common principles of cortical development and organization that are possibly inherited from stem amniotes and regulated by similar molecular mechanisms. These comparisons have also highlighted certain essential features of mammalian cortices that are absent or different in diapsids and that probably evolved after the synapsid-diapsid divergence. Chief among these is the size and multi-laminar organization of the mammalian cortex, and the propensity to increase its area by folding. Here, I review recent data on cortical neurogenesis, neuronal migration and cortical layer formation and folding in this evolutionary perspective, and highlight important unanswered questions for future investigation.

Life-Long Neurogenic Activity of Individual Neural Stem Cells and Continuous Growth Establish an Outside-In Architecture in the Teleost Pallium

Spatiotemporal variations of neurogenesis are thought to account for the evolution of brain shape. In the dorsal telencephalon (pallium) of vertebrates, it remains unresolved which ancestral neurogenesis mode prefigures the highly divergent cytoarchitectures that are seen in extant species. To gain insight into this question, we developed genetic tools to generate here the first 4-dimensional (3D + birthdating time) map of pallium construction in the adult teleost zebrafish. Using a Tet-On-based genetic birthdating strategy, we identify a “sequential stacking” construction mode where neurons derived from the zebrafish pallial germinal zone arrange in outside-in, age-related layers from a central core generated during embryogenesis. We obtained no evidence for overt radial or tangential neuronal migrations. Cre-lox-mediated tracing, which included following Brainbow clones, further demonstrates that this process is sustained by the persistent neurogenic activity of individual pallial neural stem cells (NSCs) from embryo to adult. Together, these data demonstrate that the spatiotemporal control of NSC activity is an important driver of the macroarchitecture of the zebrafish adult pallium. This simple mode of pallium construction shares distinct traits with pallial genesis in mammals and non-mammalian amniotes such as birds or reptiles, suggesting that it may exemplify the basal layout from which vertebrate pallial architectures were elaborated.

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Neurons of the teleost pallium are arranged in concentric age-dependent layers

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Neurons of the central pallial domain, Dc, are born during embryogenesis

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Most pallial neurons are generated from ventricular her4-positive radial glia

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The majority of individual pallial radial glia are neurogenic throughout life

Neural stem/progenitor cells are activated during tail regeneration in the leopard gecko (Eublepharis macularius)

As for many lizards, the leopard gecko (Eublepharis macularius) can self-detach its tail to avoid predation and then regenerate a replacement. The replacement tail includes a regenerated spinal cord with a simple morphology: an ependymal layer surrounded by nerve tracts. We hypothesized that cells within the ependymal layer of the original spinal cord include populations of neural stem/progenitor cells (NSPCs) that contribute to the regenerated spinal cord. Prior to tail loss, we performed a bromodeoxyuridine pulse-chase experiment and found that a subset of ependymal layer cells (ELCs) were label-retaining after a 140-day chase period. Next, we conducted a detailed spatiotemporal characterization of these cells before, during, and after tail regeneration. Our findings show that SOX2, a hallmark protein of NSPCs, is constitutively expressed by virtually all ELCs before, during, and after regeneration. We also found that during regeneration, ELCs express an expanded panel of NSPC and lineage-restricted progenitor cell markers, including MSI-1, SOX9, and TUJ1. Using electron microscopy, we determined that multiciliated, uniciliated, and biciliated cells are present, although the latter was only observed in regenerated spinal cords. Our results demonstrate that cells within the ependymal layer of the original, regenerating and fully regenerate spinal cord represent a heterogeneous population. These include radial glia comparable to Type E and Type B cells, and a neuronal-like population of cerebrospinal fluid-contacting cells. We propose that spinal cord regeneration in geckos represents a truncation of the restorative trajectory observed in some urodeles and teleosts, resulting in the formation of a structurally distinct replacement.

Expression of regulatory genes in the embryonic brain of a lizard and implications for understanding pallial organization and evolution

The comparison of gene expression patterns in the embryonic brain of mouse and chicken is being essential for understanding pallial organization. However, the scarcity of gene expression data in reptiles, crucial for understanding evolution, makes it difficult to identify homologues of pallial divisions in different amniotes. We cloned and analyzed the expression of the genes Emx1, Lhx2, Lhx9, and Tbr1 in the embryonic telencephalon of the lacertid lizard Psammodromus algirus. The comparative expression patterns of these genes, critical for pallial development, are better understood when using a recently proposed six‐part model of pallial divisions. The lizard medial pallium, expressing all genes, includes the medial and dorsomedial cortices, and the majority of the dorsal cortex, except the region of the lateral cortical superposition. The latter is rich in Lhx9 expression, being excluded as a candidate of dorsal or lateral pallia, and may belong to a distinct dorsolateral pallium, which extends from rostral to caudal levels. Thus, the neocortex homolog cannot be found in the classical reptilian dorsal cortex, but perhaps in a small Emx1‐expressing/Lhx9‐negative area at the front of the telencephalon, resembling the avian hyperpallium. The ventral pallium, expressing Lhx9, but not Emx1, gives rise to the dorsal ventricular ridge and appears comparable to the avian nidopallium. We also identified a distinct ventrocaudal pallial sector comparable to the avian arcopallium and to part of the mammalian pallial amygdala. These data open new venues for understanding the organization and evolution of the pallium.

The Future Vocation of Neural Stem Cells: Lineage Commitment in Brain Development and Evolution

Understanding the fate commitment of neural stem cells is critical to identify the regulatory mechanisms in developing brains. Genetic lineage-tracing has provided a powerful strategy to unveil the heterogeneous nature of stem cells and their descendants. However, recent studies have reported controversial data regarding the heterogeneity of neural stem cells in the developing mouse neocortex, which prevents a decisive conclusion on this issue. Here, we review the progress that has been made using lineage-tracing analyses of the developing neocortex and discuss stem cell heterogeneity from the viewpoint of comparative and evolutionary biology.

Cell cycle dynamics and complement expression distinguishes mature haematopoietic subsets arising from hemogenic endothelium

The emergence of haematopoietic stem and progenitor cells (HSPCs) from hemogenic endothelium results in the formation of sizeable HSPC clusters attached to the vascular wall. We evaluate the cell cycle and proliferation of HSPCs involved in cluster formation, as well as the molecular signatures from their initial appearance to the point when cluster cells are capable of adult engraftment (definitive HSCs). We uncover a non-clonal origin of HSPC clusters with differing cell cycle, migration, and cell signaling attributes. In addition, we find that the complement cascade is highly enriched in mature HSPC clusters, possibly delineating a new role for this pathway in engraftment.

Testing hypotheses of developmental constraints on mammalian brain partition evolution, using marsupials

There is considerable debate about whether the partition volumes of the mammalian brain (e.g. cerebrum, cerebellum) evolve according to functional selection, or whether developmental constraints of conserved neurogenetic scheduling cause predictable partition scaling with brain size. Here we provide the first investigation of developmental constraints on partition volume growth, derived from contrast-enhanced micro-computed tomography of hydrogel-stabilized brains from three marsupial species. ANCOVAs of partition vs. brain volume scaling, as well as growth curve comparisons, do not support several hypotheses consistent with developmental constraints: brain partition growth significantly differs between species, or between developing vs. adult marsupials. Partition growth appears independent of adult brain volume, with no discernable growth spurts/lags relatable to internal structural change. Rather, adult proportion differences appear to arise through growth rate/duration heterochrony. Substantial phylogenetic signal in adult brain partitions scaling with brain volume also counters expectations of development-mediated partition scaling conservatism. However, the scaling of olfactory bulb growth is markedly irregular, consistent with suggestions that it is less constrained. The very regular partition growth curves suggest intraspecific developmental rigidity. We speculate that a rigid, possibly neuromer-model-like early molecular program might be responsible both for regular growth curves within species and impressions of a link between neurogenesis and partition evolution.

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.

Cellular basis of brain maturation and acquisition of complex behaviors in salamanders

The overall bauplan of the tetrapod brain is highly conserved, yet significant variations exist among species in terms of brain size, structural composition and cellular diversity. Understanding processes underlying neural and behavioral development in a wide range of species is important both from an evolutionary developmental perspective as well as for the identification of cell sources with post-developmental neurogenic potential. Here we characterize germinal processes in the brain of Notophthalmus viridescens and Pleurodeles waltl during both development and adulthood. Using a combination of cell tracking tools, including clonal analyses in new transgenic salamander lines we examine the origin of neural stem and progenitor cells found in the adult brain, determine regional variability in cell cycle length of progenitor cells, and show spatio-temporally orchestrated neurogenesis. We analyze how maturation of different brain regions and neuronal subpopulations are linked to the acquisition of complex behaviors, and how these behaviors are altered upon chemical ablation of dopamine neurons. Our data analyzed from an evolutionary perspective reveal both common and species-specific processes in tetrapod brain formation and function.

In Vitro and Ex Ovo Culture of Reptilian and Avian Neural Progenitor Cells

Reptiles and birds have been highlighted as excellent experimental models for the study of developmental biology; however, due to technical limitations in cellular analysis, dynamics of neural stem/progenitor cells of these animals remain unclear. In this chapter, we introduce the protocols for neurosphere culture and ex ovo embryonic culture of developing reptilian and avian embryos, which are modified from the method originally established for rodent embryos. Applications of these techniques provide powerful strategies for the study of comparative neural development of amniotes.

A Sensitive and Versatile In Situ Hybridization Protocol for Gene Expression Analysis in Developing Amniote Brains

The detection of specific RNA molecules in embryonic tissues has wide research applications including studying gene expression dynamics in brain development and evolution. Recent advances in sequencing technologies have introduced new animal models to explore the molecular principles underlying the assembly and diversification of brain circuits between different amniote species. Here, we provide a step-by-step protocol for a versatile in situ hybridization method that is immediately applicable to a range of amniote embryos including zebra finch and Madagascar ground gecko, two new model organisms that have rapidly emerged for comparative brain studies over recent years. The sensitive detection of transcripts from low to high abundance expression range using the same platform enables direct comparison of gene of interest among different amniotes, providing high-resolution spatiotemporal information of gene expression to dissect the molecular principles underlying brain evolution.

A hybrid agent-based model of the developing mammary terminal end bud

Mammary gland ductal elongation is spearheaded by terminal end buds (TEBs), where populations of highly proliferative cells are maintained throughout post-pubertal organogenesis in virgin mice until the mammary fat pad is filled by a mature ductal tree. We have developed a hybrid multiscale agent-based model to study how cellular differentiation pathways, cellular proliferation capacity, and endocrine and paracrine signaling play a role during development of the mammary gland. A simplified cellular phenotypic hierarchy that includes stem, progenitor, and fully differentiated cells within the TEB was implemented. Model analysis finds that mammary gland development was highly sensitive to proliferation events within the TEB, with progenitors likely undergoing 2-3 proliferation cycles before transitioning to a non-proliferative phenotype, and this result is in agreement with our previous experimental work. Endocrine and paracrine signaling were found to provide reliable ductal elongation rate regulation, while variations in the probability a new daughter cell will be of a proliferative phenotype were seen to have minimal effects on ductal elongation rates. Moreover, the distribution of cellular phenotypes within the TEB was highly heterogeneous, demonstrating significant allowable plasticity in possible phenotypic distributions while maintaining biologically relevant growth behavior. Finally, simulation results indicate ductal elongation rates due to cellular proliferation within the TEB may have a greater sensitivity to upstream endocrine signaling than endothelial to stromal paracrine signaling within the TEB. This model provides a useful tool to gain quantitative insights into cellular population dynamics and the effects of endocrine and paracrine signaling within the pubertal terminal end bud.

The evolution of basal progenitors in the developing non-mammalian brain

The amplification of distinct neural stem/progenitor cell subtypes during embryogenesis is essential for the intricate brain structures present in various vertebrate species. For example, in both mammals and birds, proliferative neuronal progenitors transiently appear on the basal side of the ventricular zone of the telencephalon (basal progenitors), where they contribute to the enlargement of the neocortex and its homologous structures. In placental mammals, this proliferative cell population can be subdivided into several groups that include Tbr2⁺ intermediate progenitors and basal radial glial cells (bRGs). Here, we report that basal progenitors in the developing avian pallium show unique morphological and molecular characteristics that resemble the characteristics of bRGs, a progenitor population that is abundant in gyrencephalic mammalian neocortex. Manipulation of LGN (Leu-Gly-Asn repeat-enriched protein) and Cdk4/cyclin D1, both essential regulators of neural progenitor dynamics, revealed that basal progenitors and Tbr2⁺ cells are distinct cell lineages in the developing avian telencephalon. Furthermore, we identified a small population of subapical mitotic cells in the developing brains of a wide variety of amniotes and amphibians. Our results suggest that unique progenitor subtypes are amplified in mammalian and avian lineages by modifying common mechanisms of neural stem/progenitor regulation during amniote brain evolution.

Highlighted article: In the developing chick pallium, a basal progenitor population resembles mammalian cortical basal radial glia, suggesting a more ancient evolutionary origin for this cell type.