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FIG URE 12 Dorsal and lateral views of the brain of the lacertid lizard P. algirus at S40 and a colored schematic representation of it showing the pallial divisions, according to the six-part model proposed here. In the right hemisphere of the colored diagram (dorsal view), there is a representation of the six divisions seen from the top, while in the left hemisphere there is a representation of the pallial divisions seen more ventrally. Note the small size of the dorsal pallium (DP), and its position comparable to that of the avian hyperpallium. Also note the presence of two novel divisions, the dorsolateral pallium (DLP, which includes the lateral cortical superposition plus an area rostral to it) and the ventrocaudal pallium (VCP, including the dorsolateral amygdala and nucleus sphericus, among other nuclei). The VCP of the lizard is comparable to the avian arcopallium. The olfactory bulbs (main or MOB, and accessory or AOB) were left in gray, since they may have contributions from the several pallial divisions (perhaps including DLP), in addition to cells from other sources. See Tables 1 and 2 for additional comparisons and see text for more details. As a reference, the major brain divisions are labeled in the figure: telencephalon (Tel), diencephalon (Di), mesencephalon (Mes), and rhombencephalon (Rh) with the cerebellum (Cb).
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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...
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Specification of dorsoventral regional identity in progenitors of the developing telencephalon is a first pivotal step in the development of the cerebral cortex and basal ganglia. Previously, we demonstrated that the two zinc finger doublesex and mab-3 related (Dmrt) genes, Dmrt5 (Dmrta2) and Dmrt3, which are coexpressed in high caudomedial to low...
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... Interest in the DVR as a possible cortical homolog grew through repeated observations that it shared similar molecular, connective, and functional properties with the mammalian cortex 56,74 . Importantly, however, the mammalian cortex develops from the dorsal pallium, whereas the DVR develops from a distinct ventral pallial zone that exhibits divergent neurodevelopmental transcriptomic trajectories 10,75,76 . In contrast, the reptilian aDC arises from the dorsal pallium, and also bears striking molecular, connective, and functional similarities with the mammalian cortex 7 . ...
The telencephalon has undergone remarkable diversification and expansion throughout vertebrate evolution, exhibiting striking variations in structural and functional complexity. Nevertheless, fundamental features are shared across vertebrate taxa, such as the presence of distinct regions including the pallium, subpallium, and olfactory structures. Teleost fishes have a uniquely “everted” telencephalon, which has confounded comparisons of their brain regions to other vertebrates. Here we combine spatial transcriptomics and single nucleus RNA-sequencing to generate a spatially-resolved transcriptional atlas of the Mchenga conophorus cichlid fish telencephalon. We then compare cell-types and anatomical regions in the cichlid telencephalon with those in amphibians, reptiles, birds, and mammals. We uncover striking transcriptional similarities between cell-types in the fish telencephalon and subpallial, hippocampal, and cortical cell-types in tetrapods, and find support for partial eversion of the teleost telencephalon. Ultimately, our work lends new insights into the organization and evolution of conserved cell-types and regions in the vertebrate forebrain.
... Studies have been performed to elucidate homologous regions of vertebrate pallium by combining expression regions of regulatory genes conserved in vertebrates in morphogenesis at early embryonic stages and morphological landmarks (Fernandez et al., 1998;Puelles et al., 2000). Currently, the embryonic pallium has been divided into four (Puelles et al., 2017) or six (Desfilis et al., 2018) components, it has been clarified that pallium derivatives correspond to adult brain regions. Here, we refer to the proposed pallium divisions as the medial, dorsal, dorsolateral/lateral, and ventral/ventrocaudal pallial divisions (Pessoa et al., 2019). ...
... Here, we refer to the proposed pallium divisions as the medial, dorsal, dorsolateral/lateral, and ventral/ventrocaudal pallial divisions (Pessoa et al., 2019). The brain structures derived from each component of the pallium in birds and mammals have been proposed as follows: the medial pallium gives rise to the hippocampal formation in birds and mammals; the dorsal pallium gives rise to the hyperpallium in birds and the neocortex in mammals; the dorsolateral/ lateral pallium give rise to the mesopallium in birds and the claustroinsular region, orbitofrontal cortex rostrally, and perirhinal/lateral entorhinal cortex caudally in mammals; and the ventral/ventrocaudal pallium give rise to the arcopallium and nidopallium in birds and the olfactory cortex and pallial amygdala, which is part of amygdala, in mammals (Puelles, 2001;Moreno and González, 2006;Medina and Abellán, 2009;Puelles et al., 2017;Medina et al., 2017a;Desfilis et al., 2018). Depending on the species, pallium-derived brain structures may undergo different developmental trajectories after late development and may acquire a different cytoarchitecture, neurochemical features, and connectivity (Puelles and Medina, 2002;Striedter, 2005;Medina et al., 2017a). ...
... Tables summarizing the pallium (A) and HF subdivisions (B) in context of whether the 5-HTR family genes (and their orthologs) are present in chicken and mouse genomes. The brain structures derived from each pallium is as follows: the MP gives rise to the HF in birds and mammals; the DP gives rise to the hyperpallium in birds and the neocortex in mammals; the DLP/LP gives rise to the mesopallium in birds and the claustro-insular region, orbitofrontal cortex rostrally, and perirhinal/lateral entorhinal cortex caudally in mammals; and the VP/VCP gives rise to the arcopallium and nidopallium in birds and the olfactory cortex and pallial amygdala in mammals (Puelles, 2001;Moreno and González, 2006;Medina and Abellán, 2009;Puelles et al., 2017;Medina et al., 2017a;Desfilis et al., 2018). Please refer to the main text for more details on pallium homology. ...
Serotonin (5-hydroxytryptamine, 5-HT) is a phylogenetically conserved neurotransmitter and modulator. Neurons utilizing serotonin have been identified in the central nervous systems of all vertebrates. In the central serotonergic system of vertebrate species examined so far, serotonergic neurons have been confirmed to exist in clusters in the brainstem. Although many serotonin-regulated cognitive, behavioral, and emotional functions have been elucidated in mammals, equivalents remain poorly understood in non-mammalian vertebrates. The purpose of this review is to summarize current knowledge of the anatomical organization and molecular features of the avian central serotonergic system. In addition, selected key functions of serotonin are briefly reviewed. Gene association studies between serotonergic system related genes and behaviors in birds have elucidated that the serotonergic system is involved in the regulation of behavior in birds similar to that observed in mammals. The widespread distribution of serotonergic modulation in the central nervous system and the evolutionary conservation of the serotonergic system provide a strong foundation for understanding and comparing the evolutionary continuity of neural circuits controlling corresponding brain functions within vertebrates. The main focus of this review is the chicken brain, with this type of poultry used as a model bird. The chicken is widely used not only as a model for answering questions in developmental biology and as a model for agriculturally useful breeding, but also in research relating to cognitive, behavioral, and emotional processes. In addition to a wealth of prior research on the projection relationships of avian brain regions, detailed subdivision similarities between avian and mammalian brains have recently been identified. Therefore, identifying the neural circuits modulated by the serotonergic system in avian brains may provide an interesting opportunity for detailed comparative studies of the function of serotonergic systems in mammals.
... The MC is homologous to the hippocampal dentate gyrus of mammals (Olucha et al., 1988;Medina et al., 2017;Desfilis et al., 2018), and harbors most of adult neurogenesis in the brain of lizards (Font et al., 2001). We studied newly generated neurons in the MC by monitoring their maturation at different times after the administration of [ 3 H]-thymidine, from 1 to 12 months. ...
In adult lizards, new neurons are generated from neural stem cells in the ventricular zone of the lateral ventricles. These new neurons migrate and integrate into the main telencephalic subdivisions. In this work we have studied adult neurogenesis in the lizard Podarcis liolepis (formerly Podarcis hispanica) by administering [³H]-thymidine and bromodeoxyuridine as proliferation markers and euthanizing the animals at different survival times to determine the identity of progenitor cells and to study their lineage derivatives. After short survival times, only type B cells are labeled, suggesting that they are neural stem cells. Three days after administration, some type A cells are labeled, corresponding to recently formed neuroblasts. Type A cells migrate to their final destinations, where they differentiate into mature neurons and integrate into functional circuits. Our results after long survival periods suggest that, in addition to actively dividing type B cells, there is also a type B subpopulation with low proliferative activity. We also found that new neurons incorporated into the olfactory bulb are generated both in situ, in the walls of the anterior extension of the lateral ventricle of the olfactory bulbs, but also at more caudal levels, most likely in anterior levels of the sulcus ventralis/terminalis. These cells follow a tangential migration toward the olfactory bulbs where they integrate. We hypothesized that at least part of the newly generated neurons would undergo a specialization process over time. In support of this prediction, we found two neuronal populations in the cellular layer of the medial cortex, which we named type I and II neurons. At intermediate survival times (1 month) only type II neurons were labeled with [³H]-thymidine, while at longer survival times (3, 6, or 12 months) both type I and type II neurons were labeled. This study sheds light on the ultrastructural characteristics of the ventricular zone of P. liolepis as a neurogenic niche, and adds to our knowledge of the processes whereby newly generated neurons in the adult brain migrate and integrate into their final destinations.
... Although there is currently an intense debate about the subdivisions that constitute the pallial region, their derivatives and boundaries (Abellán et al., 2014;Puelles, 2017Puelles, , 2021Desfilis et al., 2018;Medina et al., 2021), numerous studies have demonstrated similarities between the mammalian hippocampal formation (HF) and derivatives of the medial pallium (MP) in the rest of vertebrates [for review see Butler (2017)] speaking for the homology of these structures. Topologically, the mammalian HF is located in the mediodorsal area of the telencephalon, bordered rostrally by the indusium griseum and the dorsal tenia tecta (Wyss and Sripanidkulchai, 1983;Künzle, 2004;Laplante et al., 2013). ...
... Furthermore, we aimed to have a direct comparison to amniotes and chose the turtle, Trachemys scripta elegans, the most abundant of three subspecies of this new world turtle species. In addition to available data in lizards (Desfilis et al., 2018), there are cell type data analyzed by RNAseq (Tosches et al., 2018). Therefore, the main objective of the present study was to comparatively analyze the development and adult organization of this region, by analyzing its cytoarchitecture and studying the expression pattern of medial pallial markers in both selected models. ...
... It is widely described in the literature that the LIMhomeodomain transcription factor Lhx2 is expressed in mouse (Bulchand et al., 2003;Roy et al., 2014), chicken (Abellán et al., 2014), lizard (Desfilis et al., 2018), and amphibian (Moreno and González, 2017) MP progenitors and in postmitotic cells in the adult. ...
In all vertebrates, the most dorsal region of the telencephalon gives rise to the pallium, which in turn, is formed by at least four evolutionarily conserved histogenetic domains. Particularly in mammals, the medial pallium generates the hippocampal formation. Although this region is structurally different among amniotes, its functions, attributed to spatial memory and social behavior, as well as the specification of the histogenetic domain, appears to be conserved. Thus, the aim of the present study was to analyze this region by comparative analysis of the expression patterns of conserved markers in two vertebrate models: one anamniote, the amphibian Xenopus laevis ; and the other amniote, the turtle Trachemys scripta elegans , during development and in adulthood. Our results show that, the histogenetic specification of both models is comparable, despite significant cytoarchitectonic differences, in particular the layered cortical arrangement present in the turtle, not found in anurans. Two subdivisions were observed in the medial pallium of these species: a Prox1 + and another Er81/Lmo4 +, comparable to the dentate gyrus and the mammalian cornu ammonis region, respectively. The expression pattern of additional markers supports this subdivision, which together with its functional involvement in spatial memory tasks, provides evidence supporting the existence of a basic program in the specification and functionality of the medial pallium at the base of tetrapods. These results further suggest that the anatomical differences found in different vertebrates may be due to divergences and adaptations during evolution.
... However, some authors have further suggested that the lateral region should be subdivided into lateral and ventral regions, with the lateral region corresponding to the claustrum and insula, and the ventral region corresponding to the pyriform region and the pallial amygdala (Puelles et al. 2019). Other authors have gone beyond this and suggested that the pallium can be divided into 6 regions, based on developmental gene expression data (Desfilis et al. 2018). The additional areas, however, are subdivisions of the original three areas. ...
The primate forebrain is a complex structure. Thousands of connections have been identified between cortical areas, and between cortical and sub-cortical areas. Previous work, however, has suggested that a number of principles can be used to reduce this complexity. Here, we integrate four principles that have been put forth previously, including a nested model of neocortical connectivity, gradients of connectivity between frontal cortical areas and the striatum and thalamus, shared patterns of sub-cortical connectivity between connected posterior and frontal cortical areas, and topographic organization of cortical–striatal–pallidal–thalamocortical circuits. We integrate these principles into a single model that accounts for a substantial amount of connectivity in the forebrain. We then suggest that studies in evolution and development can account for these four principles, by assuming that the ancestral vertebrate pallium was dominated by medial, hippocampal and ventral–lateral, pyriform areas, and at most a small dorsal pallium. The small dorsal pallium expanded massively in the lineage leading to primates. During this expansion, topological, adjacency relationships were maintained between pallial and sub-pallial areas. This maintained topology led to the connectivity gradients seen between cortex, striatum, pallidum, and thalamus.
... This was based on data on combinatorial expression patterns of genes encoding transcription factor (such as Emx1, Emx2, Pax6) known to play key roles in early regionalization and specification of the pallium, which together with topology led to a divisional scheme of four pallial divisions (medial, dorsal, lateral, and ventral) comparable across amniotes and to a new understanding of the areas derived from each division [Puelles et al., 2000]. In addition, all of these pallial divisions express transcription factors involved in production and differentiation of glutamatergic neurons, such as Tbr1 and Tbr2 [Bulfone et al., 1999;Puelles et al., 2000;Medina and Abellán, 2009;Moreno et al., 2010;Desfilis et al., 2018]. According to this scheme, the neocortex was found to derive from the dorsal pallium and as such was compared to the avian Wulst [Puelles et al., 2000]. ...
... In contrast, the pallial claustroamygdaloid region was proposed to derive from the lateral and ventral pallium [Puelles et al., 2000;Medina et al., 2004] and was compared to the DVR of sauropsids [Puelles et al., 2000;Puelles, 2001;. More recently, these proposals have been refined based on new data and updated interpretations of the pallial divisions and their derivatives Puelles, 2017;Puelles et al., 2019;Desfilis et al., 2018; see also Moreau et al., 2021]. One of the most important aspects to remark is that the lateral pallium does not participate in the formation of the amygdala, implying that the claustrum and the pallial amygdala have origins in different pallial sectors and that it makes no sense to talk about the claustroamygdaloid region as done before [Puelles, 2017; see more below]. ...
... In general, the sectors producing the pallial amygdala are now recognized to locate caudal to those producing the olfactory bulb [see discussion by Desfilis et al., 2018] and piriform/endopiriform region [García-Calero et al., 2020]. ...
The amygdala is a central node in functional networks regulating emotions, social behavior and social cognition. It develops in the telencephalon and includes pallial and subpallial parts, but these are extremely complex with multiple subdivisions, cell types and connections. The homology of the amygdala in non-mammals is highly controversial, especially for the pallial part, and we are still far from understanding general principles on its organization that are common to different groups. Here we review data on the adult functional architecture and developmental genoarchitecture of the amygdala in different amniotes (mammals and sauropsids), which are helping to disentangle and to better understand this complex structure. The use of an evolutionary developmental biology (evodevo) approach has helped to distinguish three major divisions in the amygdala, derived from the pallium, the subpallium and from a newly identified division called telencephalon-opto-hypothalamic domain (TOH). This approach has also helped to identify homologous cell populations with identical embryonic origins and molecular profiles in the amygdala of different amniotes. While subpallial cells produce different subtypes of GABAergic neurons, the pallium and TOH are major sources of glutamatergic cells. Available data point to a development-based molecular code that contributes to shape distinct functional subsystems in the amygdala, and comparative genoarchitecture is helping to delineate the cells involved in same subsystems in non-mammals. Thus, the evodevo approach can provide crucial information to understand common organizing principles of the amygdalar cells and networks that control behavior, emotions and cognition in amniotes.
... Brain regions and individual nuclei within them (spatially validated by the criteria listed above) are thus identifiable by the combinatorial expression of sets of transcription factors. For example, telencephalic glutamatergic neurons could be distinguished by the coexpression of foxg1 and zbtb18 (18); thalamic glutamatergic neurons by tcf7l2 and lhx9 (19,20); habenular neurons by tcf7l2, zic1, lhx9, and pou4f1 (19); cerebellar granule cells and interneurons, but not Purkinje cells, by zic1, zbtb18, and nfix (21); medial ganglionic eminence-derived g-aminobutyric acidreleasing (GABAergic) neurons by foxg1, arx, lhx6, dlx5, and zeb2 (22); lateral ganglionic eminence-derived GABAergic neurons by foxg1, meis2, and dlx5 (23); and GABAergic neurons of the tegmentum and tectum express tal1 and gata3, but mostly tegmental neurons express nr2f2 (24) (Fig. 1F). Using in situ hybridization and data available in the literature (data S2), we identified individual nuclei within these brain regions, such as the claustrum and dorsolateral amygdala in the pallium (25); septum in the subpallium; dorsomedial thalamic nucleus in the thalamus; and paraventricular, ventromedial, and mammillary nuclei in the hypothalamus ( S5A). ...
The existence of evolutionarily conserved regions in the vertebrate brain is well established. The rules and constraints underlying the evolution of neuron types, however, remain poorly understood. To compare neuron types across brain regions and species, we generated a cell type atlas of the brain of a bearded dragon and compared it with mouse datasets. Conserved classes of neurons could be identified from the expression of hundreds of genes, including homeodomain-type transcription factors and genes involved in connectivity. Within these classes, however, there are both conserved and divergent neuron types, precluding a simple categorization of the brain into ancestral and novel areas. In the thalamus, neuronal diversification correlates with the evolution of the cortex, suggesting that developmental origin and circuit allocation are drivers of neuronal identity and evolution.
... In order to produce intermediate stellate cells from human iPSCs, we applied a forward programming approach in combination with culture conditions to pattern the progenitors at an early stage into a medial pallial fate. Evidence suggests the MEC is derived from the medial pallium (Bruce and Neary, 1995;Abellan et al., 2014;Desfilis et al., 2018) and a combination of BMP4 and CHIR 99021 (GSK3 inhibitor and Wnt agonist) can induce human embryonic stem cells into dorsomedial telencephalic tissue (Sakaguchi et al., 2015). As a positive control, we applied a standard forward programming protocol which creates iNeurons (using overexpression of Ngn2). ...
Stellate cells are principal neurons in the entorhinal cortex that contribute to spatial processing. They also play a role in the context of Alzheimer’s disease as they accumulate Amyloid beta early in the disease. Producing human stellate cells from pluripotent stem cells would allow researchers to study early mechanisms of Alzheimer’s disease, however, no protocols currently exist for producing such cells. In order to develop novel stem cell protocols, we characterize at high resolution the development of the porcine medial entorhinal cortex by tracing neuronal and glial subtypes from mid-gestation to the adult brain to identify the transcriptomic profile of progenitor and adult stellate cells. Importantly, we could confirm the robustness of our data by extracting developmental factors from the identified intermediate stellate cell cluster and implemented these factors to generate putative intermediate stellate cells from human induced pluripotent stem cells. Six transcription factors identified from the stellate cell cluster including RUNX1T1 , SOX5 , FOXP1 , MEF2C , TCF4 , EYA2 were overexpressed using a forward programming approach to produce neurons expressing a unique combination of RELN , SATB2 , LEF1 and BCL11B observed in stellate cells. Further analyses of the individual transcription factors led to the discovery that FOXP1 is critical in the reprogramming process and omission of RUNX1T1 and EYA2 enhances neuron conversion. Our findings contribute not only to the profiling of cell types within the developing and adult brain’s medial entorhinal cortex but also provides proof-of-concept for using scRNAseq data to produce entorhinal intermediate stellate cells from human pluripotent stem cells in-vitro .
... Our second goal is to compare the distribution of glutamatergic neurons in the pallium of reptiles, birds, and mammals. Developmental data on transcription factors that play critical roles in cortical development in mammals (Pax6, Emx1/2, and Tbr1) show a conserved expression pattern on the dorsal side of the amniote telencephalon, implying that all amniotes have a homologous brain part in the telencephalon, termed the pallium (Fernandez et al., 1998;Puelles et al., 2000Puelles et al., , 2017Dugas-Ford et al., 2012;Desfilis et al., 2017;Medina et al., 2021). Using mouse embryos, Emx1-and Tbr1-expressing neurons in the dorsal part of the telencephalon (pallium) were shown to use glutamate as a neurotransmitter (Gorski et al., 2002;Hevner et al., 2003Hevner et al., , 2006. ...
Glutamate acts as the main excitatory neurotransmitter in the brain and plays a vital role in physiological and pathological neuronal functions. In mammals, glutamate can cause detrimental excitotoxic effects under anoxic conditions. In contrast, Trachemys scripta, a freshwater turtle, is one of the most anoxia-tolerant animals, being able to survive up to months without oxygen. Therefore, turtles have been investigated to assess the molecular mechanisms of neuroprotective strategies used by them in anoxic conditions, such as maintaining low levels of glutamate, increasing adenosine and GABA, upregulating heat shock proteins, and downregulating KATP channels. These mechanisms of anoxia tolerance of the turtle brain may be applied to finding therapeutics for human glutamatergic neurological disorders such as brain injury or cerebral stroke due to ischemia. Despite the importance of glutamate as a neurotransmitter and of the turtle as an ideal research model, the glutamatergic circuits in the turtle brain remain less described whereas they have been well studied in mammalian and avian brains. In reptiles, particularly in the turtle brain, glutamatergic neurons have been identified by examining the expression of vesicular glutamate transporters (VGLUTs). In certain areas of the brain, some ionotropic glutamate receptors (GluRs) have been immunohistochemically studied, implying that there are glutamatergic target areas. Based on the expression patterns of these glutamate-related molecules and fiber connection data of the turtle brain that is available in the literature, many candidate glutamatergic circuits could be clarified, such as the olfactory circuit, hippocampal–septal pathway, corticostriatal pathway, visual pathway, auditory pathway, and granule cell–Purkinje cell pathway. This review summarizes the probable glutamatergic pathways and the distribution of glutamatergic neurons in the pallium of the turtle brain and compares them with those of avian and mammalian brains. The integrated knowledge of glutamatergic pathways serves as the fundamental basis for further functional studies in the turtle brain, which would provide insights on physiological and pathological mechanisms of glutamate regulation as well as neural circuits in different species.
... Although its identification in other vertebrates is a challenge due to differences in shape, size, or cellular composition, homologous territories have been identified in many vertebrates [Puelles et al., 2000;Martínez-García et al., 2002;Medina et al., 2005;Reiner et al., 2005;González, 2006, 2007b;García-López et al., 2008;Martínez-García et al., 2008;Bupesh et al., 2011a, b;Medina et al., 2011;Abellán et al., 2013;Maximino et al., 2013;Biechl et al., 2017;Medina et al., 2019;Puelles et al., 2019;Porter and Mueller, 2020;Gerlach and Wullimann, an update of the tetrapartite pallial model, making this a key moment in the re-evaluation of the vertebrate pallium [Puelles, 2017], especially affecting the ventropallial derivatives and the origin of the pallial amygdala (reviewed in ). An additional proposal of hexapartite pallial organization suggests that the ventrocaudal pallium gives rise to the posterior pole of the pallial amygdala [Desfilis et al., 2018] (reviewed in Medina et al. [2021]). In this sense, the structural model proposal divides the mammalian cortex into discrete categories (cortical types) based on their laminar structure, which can be used to test hypotheses about organization, cortical connections, and even their similar developmental origin [García-Cabezas et al., 2019]. ...
... Thus, the vomeronasal projection defines the MeA that is visualized after BDA injections in the accessory olfactory bulb (AOB) and allows the identification of the boundary with the adjacent pallial amygdala, which does not receive vomeronasal projections (Fig. 1g). Some of the main markers of the pallial amygdala in vertebrates are members of the LIM-hd family, particularly Lhx9 (analyzed in detail in mouse [Garcia-Calero and , in chicken [Abellán et al., 2009[Abellán et al., , 2014, in lizard [Desfilis et al., 2018], and in anurans ). At caudal telencephalic levels, a population of Lhx9-expressing cells is observed at the caudal end of the lateral ventricle in the pallial amygdala (Fig. 1h). ...
... The extended amygdala constitutes the subpallial component, comprised of the central nucleus of the amygdala, the main component of the autonomic amygdaloid subdivision, and the medial amygdala as an important secondary vomeronasal center [Swanson and Petrovich, 1998]. This classical system is being revisited in the last years with the reanalysis of the pallial derivatives [Medina et al., 2017;Puelles, 2017;Desfilis et al., 2018] (reviewed in Medina et al. [2019], Medina et al. [2021] ...
The amygdaloid complex plays a crucial role in socio-emotional conduct, learning, survival, and reproductive behaviors. It is constituted by a set of nuclei presenting a great cellular heterogeneity and embryonic origin diversity (pallial, subpallial and even extra-telencephalic). In the last two decades, the tetrapartite pallial paradigm defined the pallial portion of the amygdala as a derivative of the lateroventral pallium. However, the pallial conception is currently being reanalyzed and one of these new proposals is to consider the mouse pallial amygdala as a radial histogenetic domain independent from the rest of the pallial subdomains. In anamniotes, and particularly in amphibian anurans, the amygdaloid complex was described as a region with pallial and subpallial components similar to those described in amniotes. In the present study carried out in Xenopus laevis, after a detailed analysis of the orientation of the amygdalar radial glia, we propose an additional amygdala derived from the pallial region. It is independent of the vomeronasal/olfactory amygdaloid nuclei described in anurans, expresses markers such as Lhx9 present in the mammalian pallial amygdala, and lacks Otp-expressing cells, detected in the adjacent medial amygdala. Further studies are needed to clarify the functional involvement of this area, and whether it is a derivative of the adjacent ventral pallium or an independent pallial domain.
