A distinct preisthmic histogenetic domain is defined by overlap of Otx2 and Pax2 gene expression in the avian caudal midbrain

Human Adapted Prosomeric Model: A Future for Brainstem Tumor Classification

This study reevaluates the conventional understanding of midbrain anatomy and neuroanatomical nomenclature in the context of recent genetic and anatomical discoveries. The authors assert that the midbrain should be viewed as an integral part of the forebrain due to shared genetic determinants and evolutionary lineage. The isthmo-mesencephalic boundary is recognized as a significant organizer for both the caudal midbrain and the isthmo-cerebellar area. The article adopts the prosomeric model, redefining the whole brain as neuromeres, offering a more precise depiction of brain development, including processes like proliferation, neurogenesis, cell migration, and differentiation. This shift in understanding challenges traditional definitions of the midbrain based on external brain morphology. The study also delves into the historical context of neuroanatomical models, including the columnar model proposed by Herrick in 1910, which has influenced our understanding of brain structure. Furthermore, the study has clinical implications, affecting neuroanatomy, neurodevelopmental studies, and the diagnosis and treatment of brain disorders. It emphasizes the need to integrate molecular research into human neuroanatomical studies and advocates for updating neuroanatomical terminology to reflect modern genetic and molecular insights. The authors propose two key revisions. First, we suggest reclassifying the isthmo-cerebellar prepontine region as part of the hindbrain, due to its role in cerebellar development and distinct location caudal to the genetically-defined midbrain. Second, we recommend redefining the anterior boundary of the genetically-defined midbrain to align with genetic markers. In conclusion, the authors highlight the importance of harmonizing neuroanatomical nomenclature with current scientific knowledge, promoting a more precise and informed understanding of brain structure, which is crucial for both research and clinical applications related to the human brain.

Atypical Course of the Habenulo-Interpeduncular Tract in Chick Embryos

Classical studies of the avian diencephalon hardly mention the habenulo-interpeduncular tract (a.k.a. retroflex tract), although both the habenula (HB) (its origin) and the interpeduncular nuclear complex (its target) are present. Retroflex tract fibers were described at early embryonic stages but seem absent in the adult in routine stains. However, this tract is a salient diencephalic landmark in all other vertebrate lineages. It typically emerges out of the caudal HB, courses dorsoventrally across thalamic alar and basal plates just in front of the thalamo-pretectal boundary, and then sharply bends 90° caudalwards at paramedian basal plate levels (this is the "retroflexion"), to approach longitudinally via paramedian pretectum and midbrain the rostralmost hindbrain, specifically the prepontine median interpeduncular complex across isthmus and rhombomere 1. We systematize this habenulo-interpeduncular course into four parts named subhabenular, retrothalamic, tegmental, and interpeduncular. We reexamined the chicken habenulo-interpeduncular fibers at stages HH30 and HH35 (6.5- and 9-day incubation) by mapping them specifically with immunoreaction for BEN protein, a well-known marker. We found that only a small fraction of the stained retroflex tract fibers approaches the basal plate by coursing along the standard dorsoventral pathway in front of the thalamo-pretectal boundary. Many other habenular fibers instead diverge into atypical dispersed courses across the thalamic cell mass (implying alteration of the first subhabenular part of the standard course) before reaching the basal plate; this dispersion explains their invisibility. A significant number of such transthalamic habenular fibers cross orthogonally the zona limitans (ZLI) (the rostral thalamic boundary) and invade the caudal alar prethalamus. Here, they immediately descend dorsoventrally, just rostrally to the ZLI, until reaching the prethalamic basal plate, where they bend (retroflex) caudalwards, entering the thalamic basal paramedian area. These atypical fibers gradually fasciculate with the other groups of habenular efferent fibers in their final longitudinal approach to the hindbrain interpeduncular complex. We conclude that the poor visibility of this tract in birds is due to its dispersion into a diversity of atypical alternative routes, though all components eventually reach the interpeduncular complex. This case merits further analysis of the diverse permissive versus nonpermissive guidance mechanisms called into action, which partially correlate distinctly with successive diencephalic, mesencephalic, and hindbrain neuromeric fields and their boundaries.

The Midbrain Preisthmus: A Poorly Known Effect of the Isthmic Organizer

This essay reexamines molecular evidence supporting the existence of the 'preisthmus', a caudal midbrain domain present in vertebrates (studied here in the mouse). It is thought to derive from the embryonic m2 mesomere and appears intercalated between the isthmus (caudally) and the inferior colliculus (rostrally). Among a substantial list of gene expression mappings examined from the Allen Developing and Adult Brain Atlases, a number of quite consistent selective positive markers, plus some neatly negative markers, were followed across embryonic stages E11.5, E13.5, E15.5, E18.5, and several postnatal stages up to the adult brain. Both alar and basal subdomains of this transverse territory were explored and illustrated. It is argued that the peculiar molecular and structural profile of the preisthmus is due to its position as rostrally adjacent to the isthmic organizer, where high levels of both FGF8 and WNT1 morphogens must exist at early embryonic stages. Isthmic patterning of the midbrain is discussed in this context. Studies of the effects of the isthmic morphogens usually do not attend to the largely unknown preisthmic complex. The adult alar derivatives of the preisthmus were confirmed to comprise a specific preisthmic sector of the periaqueductal gray, an intermediate stratum represented by the classic cuneiform nucleus, and a superficial stratum containing the subbrachial nucleus. The basal derivatives, occupying a narrow retrorubral domain intercalated between the oculomotor and trochlear motor nuclei, include dopaminergic and serotonergic neurons, as well as a variety of peptidergic neuron types.

Interhemispheric competition during sleep

Our understanding of the functions and mechanisms of sleep remains incomplete, reflecting their increasingly evident complexity1-3. Likewise, studies of interhemispheric coordination during sleep4-6 are often hard to connect precisely to known sleep circuits and mechanisms. Here, by recording from the claustra of sleeping bearded dragons (Pogona vitticeps), we show that, although the onsets and offsets of Pogona rapid-eye-movement (REMP) and slow-wave sleep are coordinated bilaterally, these two sleep states differ markedly in their inter-claustral coordination. During slow-wave sleep, the claustra produce sharp-wave ripples independently of one another, showing no coordination. By contrast, during REMP sleep, the potentials produced by the two claustra are precisely coordinated in amplitude and time. These signals, however, are not synchronous: one side leads the other by about 20 ms, with the leading side switching typically once per REMP episode or in between successive episodes. The leading claustrum expresses the stronger activity, suggesting bilateral competition. This competition does not occur directly between the two claustra or telencephalic hemispheres. Rather, it occurs in the midbrain and depends on the integrity of a GABAergic (γ-aminobutyric-acid-producing) nucleus of the isthmic complex, which exists in all vertebrates and is known in birds to underlie bottom-up attention and gaze control. These results reveal that a winner-take-all-type competition exists between the two sides of the brain of Pogona, which originates in the midbrain and has precise consequences for claustrum activity and coordination during REMP sleep.

Prosomeric classification of retinorecipient centers: a new causal scenario

The retina is known to target many superficial areas in the brain. These have always been studied under the tenets of the classic columnar brain model, which was not designed to produce causal explanations, being functionally oriented. This has led over the years to a remarkable absence of understanding or even hypothetical thinking about why the optic tract takes its precise course, why there are so many retinal targets (some of them at surprising sites), what mechanism places each one of them exactly at its standard position, which processes specify spatial aspects of retinotopy and differential physiological properties within the visual system, and so on, including questions about conserved and changing evolutionary aspects of the visual structures. The author posits that the origin of the current causally uninformative state of the field is the columnar model, which worked as a subliminal or cryptic dogma that disregards the molecular developmental advances accruing during the last 40 years, and in general distracts the attention of neuroscientists from causal approaches. There is now an alternative brain model, known as the prosomeric model, that does not have these problems. The author aims to show that once the data on retinal projections are mapped and analyzed within the prosomeric model the scenario changes drastically and multiple opportunities for formulating hypotheses for causal explanation of any aspects about the visual projections become apparent (emphasis is made on mouse and rabbit data, but any set of data on retinal projections in vertebrates can be used, as shown in some examples).

Review of the Midbrain Ascending Arousal Network Nuclei and Implications for Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS), Gulf War Illness (GWI) and Postexertional Malaise (PEM)

Myalgic Encephalomyelitis/Chronic Fatigue Syndrome (ME/CFS and Gulf War Illness (GWI) share features of post-exertional malaise (PEM), exertional exhaustion, or postexertional symptom exacerbation. In a two-day model of PEM, submaximal exercise induced significant changes in activation of the dorsal midbrain during a high cognitive load working memory task (Washington 2020) (Baraniuk this issue). Controls had no net change. However, ME/CFS had increased activity after exercise, while GWI had significantly reduced activity indicating differential responses to exercise and pathological mechanisms. These data plus findings of the midbrain and brainstem atrophy in GWI inspired a review of the anatomy and physiology of the dorsal midbrain and isthmus nuclei in order to infer dysfunctional mechanisms that may contribute to disease pathogenesis and postexertional malaise. The nuclei of the ascending arousal network were addressed. Midbrain and isthmus nuclei participate in threat assessment, awareness, attention, mood, cognition, pain, tenderness, sleep, thermoregulation, light and sound sensitivity, orthostatic symptoms, and autonomic dysfunction and are likely to contribute to the symptoms of postexertional malaise in ME/CFS and GWI.

Recollections on the Origins and Development of the Prosomeric Model

The prosomeric model was postulated jointly by L. Puelles and J. L. R. Rubenstein in 1993 and has been developed since by means of minor changes and a major update in 2012. This article explains the progressive academic and scientific antecedents leading LP to this collaboration and its subsequent developments. Other antecedents due to earlier neuroembryologists that also proposed neuromeric brain models since the late 19th century, as well as those who defended the alternative columnar model, are presented and explained. The circumstances that apparently caused the differential success of the neuromeric models in the recent neurobiological field are also explored.

Subdivisions of the mesencephalon and isthmus in the lizard Gekko gecko as revealed by ChAT immunohistochemistry

The distribution of cholinergic cell bodies and fibers was examined in the mesencephalon and isthmus of Gekko gecko. Distinct groups with prominent labeled cells were observed in the cranial nerve motor nuclei and isthmic nuclei, and weak labeled cell bodies and fibers were observed in the mesencephalic nucleus of the trigeminal nerve and the central nucleus of the torus semicircularis. After discussing the topological relationships within the tectum and isthmus, we unify the nomenclature of the caudal deep mesencephalic nucleus in lizards and the rostral magnocellular nucleus isthmi in turtles that is similar in terms of the preisthmic position, nontopographic connections with the tectum, and the same midbrain origin to the magnocellular preisthmic nucleus in birds, and may be homologous to the superficial cuneiform nucleus in mammals. None of them belong to the cholinergic nucleus isthmi, as the latter has isthmus origin and topographic reciprocal connections with the tectum. We also discuss the origin and intrinsic function of the inner longitudinal tract of the thick ChAT-ir fibers that course through the mesencephalon and diencephalon. We review the subdivisions of the mesencephalon and isthmus of Gekko gecko as revealed by ChAT immunohistochemistry, as well as the limits of the diencephalo-mesencephalic, mesencephalic-isthmo, and isthmo-rhombocephalic by the ChAT-ir cell- and fiber-poor distribution, and discuss the caudal limit of the isthmus. Our research on the subdivisions of the mesencephalon and isthmus in G. gecko as revealed by ChAT immunohistochemistry will serve as the neuroanatomical basis for subsequent relevant studies of Gekko gecko.

The mouse dorsal raphe nucleus as understood by temporal Fgf8 lineage analysis

Fgf8 is expressed transiently during embryogenesis at the midbrain-hindbrain border, an area that gives rise to a variety of neuronal populations including the dorsal raphe (DR) nucleus. Using an inducible Fgf8-cre allele, we identified the populations of neurons defined by Fgf8 lineage at different stages of development. When Fgf8-cre expression is induced at embryonic day 7.5 (T-E7.5), in the adult the entire DR and part of the median raphe (MnR) have Fgf8 lineage. When induced at later timepoints, Fgf8 lineage progressively ebbs from the caudal and ventral aspect of this domain, particularly on the midline. Successively excluded from Fgf8- lineage at T-E9.5 are serotonin neurons in the MnR and caudal-intrafascicular DR, followed at T-E11.5 by ventral-middle and caudal-dorsal DR. The last to show Fgf8 lineage are those serotonin neurons in the lateral wings and those at the rostral-dorsal pole of DR nucleus. Thus, the temporal succession of Fgf8 lineage correlates with organizational features of serotonin neurons in these nuclei.

The Basic Helix-Loop-Helix Gene Nato3 Drives Expression of Dopaminergic Neuron Transcription Factors in Neural Progenitors

The floor plate of the developing midbrain gives rise to dopaminergic (DA) neurons, an important class of cells involved in Parkinson's disease (PD). Neural progenitors of the midbrain floor plate utilize key genes in transcriptional networks to drive dopamine neurogenesis. Identifying factors that promote dopaminergic neuron transcriptional networks can provide insight into strategies for therapies in PD. Using the chick embryo, we developed a quantitative PCR (qPCR) based method to assess the potential of a candidate factor to drive DA neuron gene expression, including the basic helix-loop-helix transcription factor Nato3 (Ferd3l). We then showed that overexpression of Nato3 in the developing chick mesencephalon produces a regionally dependent increase in genes associated with the DA neurogenesis, (such as Foxa2, Lmx1b and Shh) as well as DA neuron genes Nurr1 (an immature DA neuron marker) and mRNA expression of tyrosine hydroxylase (TH, a mature DA neuron marker). Interestingly, our data also showed that Nato3 is a potent regulator of Lmx1b by its broad induction of Lmx1b expression in neural progenitors of multiple regions of the CNS, including the midbrain and spinal cord. These data introduce a new, in vivo approach to identifying a gene that can drive DA transcriptional networks and provide the new insight that Nato3 can drive expression of key DA neuron genes, including Lmx1b, in neural progenitors.

Survey of Midbrain, Diencephalon, and Hypothalamus Neuroanatomic Terms Whose Prosomeric Definition Conflicts With Columnar Tradition

Recent neuroanatomic concepts and terms referring to the non-telencephalic forebrain are presented and discussed, in context with the present scenario in which the old columnar paradigm is being substituted by the prosomeric model, largely on the basis of novel molecular and experimental evidence.

Clinical Phenotypes Associated to Engrailed 2 Gene Alterations in a Series of Neuropediatric Patients

The engrailed homeobox protein (EN) plays an important role in the regionalization of the neural tube. EN distribution regulates the cerebellum and midbrain morphogenesis, as well as retinotectal synaptogenesis. In humans, the EN1 and EN2 genes code for the EN family of transcription factors. Genetic alterations in the expression of EN2 have been related to different neurologic conditions and more particularly to autism spectrum disorders (ASD). We aimed to study and compare the phenotypes of three series of patients: (1) patients with encephalic structural anomalies (ESA) and abnormalities in the genomic (DNA) and/or transcriptomic (RNAm) of EN2 (EN2-g), (2) ESA patients having other gene mutations (OG-g), and (3) ESA patients free of these mutations (NM-g).

Subjects and Methods: We have performed a descriptive study on 109 patients who suffer from mental retardation (MR), cerebral palsy (CP), epilepsy (EP), and behavioral disorders (BD), showing also ESA in their encephalic MRI. We studied genomic DNA and transcriptional analysis (cDNA) on EN2 gene (EN2), and in other genes (OG): LIS1, PTAFR, PAFAH1B2, PAFAH1B3, FGF8, PAX2, D17S379, D17S1866, and SMG6 (D17S5), as a routine genetic diagnosis in ESA patients.

Results: From 109 patients, fifteen meet the exclusion criteria. From the remaining 94 patients, 12 (12.8%) showed mutations in EN2 (EN2-g), 20 showed mutations in other studied genes (OG-g), and 62 did not showed any mutation (NM-g). All EN2-g patients, suffered from MR, nine EP, seven BD and four CP. The proportions of these phenotypes in EN2-g did not differ from those in the OG-g, but it was significantly higher when comparing EN2-g with NM-g (MR: p = 0.013; EP: p = 0.001; BD: p = 0.0001; CP: p = 0.07, ns). Groups EN2-g and OG-g showed a 100 and a 70% of comorbidity, respectively, being significantly (p = 0.04) greater than NM-group (62.9%).

Conclusion: Our series reflects a significant effect of EN2 gene alterations in neurodevelopmental abnormalities associated to ESA. Conversely, although these EN2 related anomalies might represent a predisposition to develop brain diseases, our results did not support direct relationship between EN2 mutations and specific clinical phenotypes.

Mosaic Expression of Thyroid Hormone Regulatory Genes Defines Cell Type-Specific Dependency in the Developing Chicken Cerebellum

The cerebellum is a morphologically unique brain structure that requires thyroid hormones (THs) for the correct coordination of key cellular events driving its development. Unravelling the interplay between the multiple factors that can regulate intracellular TH levels is a key step to understanding their role in the regulation of these cellular processes. We therefore investigated the regional/cell-specific expression pattern of TH transporters and deiodinases in the cerebellum using the chicken embryo as a model. In situ hybridisation revealed expression of the TH transporters monocarboxylate transporter 8 (MCT8) and 10 (MCT10), L-type amino acid transporter 1 (LAT1) and organic anion transporting polypeptide 1C1 (OATP1C1) as well as the inactivating type 3 deiodinase (D3) in the fourth ventricle choroid plexus, suggesting a possible contribution of the resulting proteins to TH exchange and subsequent inactivation of excess hormone at the blood-cerebrospinal fluid barrier. Exclusive expression of LAT1 and the activating type 2 deiodinase (D2) mRNA was found at the level of the blood-brain barrier, suggesting a concerted function for LAT1 and D2 in the direct access of active T₃ to the developing cerebellum via the capillary endothelial cells. The presence of MCT8 mRNA in Purkinje cells and cerebellar nuclei during the first 2 weeks of embryonic development points to a potential role of this transporter in the uptake of T₃ in central neurons. At later stages, together with MCT10, detection of MCT8 signal in close association with the Purkinje cell dendritic tree suggests a role of both transporters in TH signalling during Purkinje cell synaptogenesis. MCT10 was also expressed in late-born cells in the rhombic lip lineage with a clear hybridisation signal in the outer external granular layer, indicating a potential role for MCT10 in the proliferation of granule cell precursors. By contrast, expression of D3 in the first-born rhombic lip-derived population may serve as a buffering mechanism against high T₃ levels during early embryonic development, a hypothesis supported by the pattern of expression of a fluorescent TH reporter in this lineage. Overall, this study builds a picture of the TH dependency in multiple cerebellar cell types starting from early embryonic development.

Mouse Fgf8-Cre-LacZ lineage analysis defines the territory of the postnatal mammalian isthmus

The isthmus is recognized as the most rostral segment of the hindbrain in non-mammalian vertebrates. In mammalian embryos, transient Fgf8 expression defines the developing isthmic region, lying between the midbrain and the first rhombomere, but there has been uncertainty about the existence of a distinct isthmic segment in postnatal mammals. We attempted to find if the region of early embryonic Fgf8 expression (which is considered to involve the entire extent of the prospective isthmus initially) might help to identify the boundaries of the isthmus in postnatal animals. By creating an Fgf8-Cre-LacZ lineage in mice, we were able to show that Fgf8-Cre reporter expression in postnatal mice is present in the same nuclei that characterize the isthmic region in birds. The 'signature' isthmic structures in birds include the trochlear nucleus, the dorsal raphe nucleus, the microcellular tegmental nuclei, the pedunculotegmental nucleus, the vermis of the cerebellum, rostral parts of the parabrachial complex and locus coeruleus, and the caudal parts of the substantia nigra and VTA. We found that all of these structures were labeled with the Fgf8-Cre reporter in the mouse brain, and we conclude that the isthmus is a distinct segment of the mammalian brain lying caudal to the midbrain and rostral to rhombomere 1 of the hindbrain.

MCT8 deficiency in Purkinje cells disrupts embryonic chicken cerebellar development

Inactivating mutations in the human SLC16A2 gene encoding the thyroid hormone transporter monocarboxylate transporter 8 (MCT8) result in the Allan-Herndon-Dudley syndrome accompanied by severe locomotor deficits. The underlying mechanisms of the associated cerebellar maldevelopment were studied using the chicken as a model. Electroporation of an MCT8-RNAi vector into the cerebellar anlage of a 3-day-old embryo allowed knockdown of MCT8 in Purkinje cell precursors. This resulted in the downregulation of the thyroid hormone-responsive gene RORα and the Purkinje cell-specific differentiation marker LHX1/5 at day 6. MCT8 knockdown also results in a smaller and less complex dendritic tree at day 18 suggesting a pivotal role of MCT8 for cell-autonomous Purkinje cell maturation. Early administration of the thyroid hormone analogue 3,5,3'-triiodothyroacetic acid partially rescued early Purkinje cell differentiation. MCT8-deficient Purkinje cells also induced non-autonomous effects as they led to a reduced granule cell precursor proliferation, a thinner external germinal layer and a loss of PAX6 expression. By contrast, at day 18, the external germinal layer thickness was increased, with an increase in presence of Axonin-1-positive post-mitotic granule cells in the initial stage of radial migration. The concomitant accumulation of presumptive migrating granule cells in the molecular layer, suggests that inward radial migration to the internal granular layer is stalled. In conclusion, early MCT8 deficiency in Purkinje cells results in both cell-autonomous and non-autonomous effects on cerebellar development and indicates that MCT8 expression is essential from very early stages of development, providing a novel insight into the ontogenesis of the Allan-Herndon-Dudley syndrome.

Expression patterns of Irx genes in the developing chick inner ear

The vertebrate inner ear is a complex three-dimensional sensorial structure with auditory and vestibular functions. The molecular patterning of the developing otic epithelium creates various positional identities, consequently leading to the stereotyped specification of each neurosensory and non-sensory element of the membranous labyrinth. The Iroquois (Iro/Irx) genes, clustered in two groups (A: Irx1, Irx2, and Irx4; and B: Irx3, Irx5, and Irx6), encode for transcriptional factors involved directly in numerous patterning processes of embryonic tissues in many phyla. This work presents a detailed study of the expression patterns of these six Irx genes during chick inner ear development, paying particular attention to the axial specification of the otic anlagen. The Irx genes seem to play different roles at different embryonic periods. At the otic vesicle stage (HH18), all the genes of each cluster are expressed identically. Both clusters A and B seem involved in the specification of the lateral and posterior portions of the otic anlagen. Cluster B seems to regulate a larger area than cluster A, including the presumptive territory of the endolymphatic apparatus. Both clusters seem also to be involved in neurogenic events. At stages HH24/25-HH27, combinations of IrxA and IrxB genes participate in the specification of most sensory patches and some non-sensory components of the otic epithelium. At stage HH34, the six Irx genes show divergent patterns of expression, leading to the final specification of the membranous labyrinth, as well as to cell differentiation.

Deiodinase knockdown affects zebrafish eye development at the level of gene expression, morphology and function

Retinal development in vertebrates relies extensively on thyroid hormones. Their local availability is tightly controlled by several regulators, including deiodinases (Ds). Here we used morpholino technology to explore the roles of Ds during eye development in zebrafish. Transcriptome analysis at 3 days post fertilization (dpf) revealed a pronounced effect of knockdown of both T4-activating Ds (D1D2MO) or knockdown of T3-inactivating D3 (D3bMO) on phototransduction and retinoid recycling. This was accompanied by morphological defects (studied from 1 to 7 dpf) including reduced eye size, disturbed retinal lamination and strong reduction in rods and all four cone types. Defects were more prominent and persistent in D3-deficient fish. Finally, D3-deficient zebrafish larvae had disrupted visual function at 4 dpf and were less sensitive to a light stimulus at 5 dpf. These data demonstrate the importance of TH-activating and -inactivating Ds for correct zebrafish eye development, and point to D3b as a central player.

Expression of thyroid hormone transporters and deiodinases at the brain barriers in the embryonic chicken: Insights into the regulation of thyroid hormone availability during neurodevelopment

Thyroid hormones (THs) are key regulators in the development of the vertebrate brain. Therefore, TH access to the developing brain needs to be strictly regulated. The brain barriers separate the central nervous system from the rest of the body and impose specific transport mechanisms on the exchange of molecules between the general circulation and the nervous system. As such they form ideal structures for regulating TH exchange between the blood and the brain. To investigate the mechanism by which the developing brain regulates TH availability, we investigated the ontogenetic expression profiles of TH transporters, deiodinases and the TH distributor protein transthyretin (TTR) at the brain barriers during embryonic and early postnatal development using the chicken as a model. In situ hybridisation revealed expression of the TH transporters monocarboxylate transporter 8 (MCT8) and 10 (MCT10), organic anion transporting polypeptide 1C1 (OATP1C1) and L-type amino acid transporter 1 (LAT1) and the inactivating type 3 deiodinase (D3) in the choroid plexus which forms the blood-cerebrospinal fluid barrier. This was confirmed by quantitative PCR which additionally indicated strongly increasing expression of TTR as well as detectable expression of the activating type 2 deiodinase (D2) and the (in)activating type 1 deiodinase (D1). In the brain capillaries forming the blood-brain barrier in situ hybridisation showed exclusive expression of LAT1 and D2. The combined presence of LAT1 and D2 in brain capillaries suggests that the blood-brain barrier forms the main route for receptor-active T3 uptake into the embryonic chicken brain. Expression of multiple transporters, deiodinases and TTR in the choroid plexus indicates that the blood-cerebrospinal fluid barrier is also important in regulating early TH availability. The impact of these barrier systems can be deduced from the clear difference in T3 and T4 levels as well as the T3/T4 ratio between the developing brain and the general circulation. We conclude that the tight regulation of TH exchange at the brain barriers from early embryonic stages is one of the factors needed to allow the brain to develop within a relative microenvironment.

Mesencephalic basolateral domain specification is dependent on Sonic Hedgehog

In the study of central nervous system morphogenesis, the identification of new molecular markers allows us to identify domains along the antero-posterior and dorso-ventral (DV) axes. In the past years, the alar and basal plates of the midbrain have been divided into different domains. The precise location of the alar-basal boundary is still under discussion. We have identified Barhl1, Nhlh1 and Six3 as appropriate molecular markers to the adjacent domains of this transition. The description of their expression patterns and the contribution to the different mesencephalic populations corroborated their role in the specification of these domains. We studied the influence of Sonic Hedgehog on these markers and therefore on the specification of these territories. The lack of this morphogen produced severe alterations in the expression pattern of Barhl1 and Nhlh1 with consequent misspecification of the basolateral (BL) domain. Six3 expression was apparently unaffected, however its distribution changed leading to altered basal domains. In this study we confirmed the localization of the alar-basal boundary dorsal to the BL domain and demonstrated that the development of the BL domain highly depends on Shh.

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

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

Role of Shh in the development of molecularly characterized tegmental nuclei in mouse rhombomere 1

Hindbrain rhombomeres in general are differentially specified molecularly by unique combinations of Hox genes with other developmental genes. Rhombomere 1 displays special features, including absence of Hox gene expression. It lies within the hindbrain range of the Engrailed genes (En1, En2), controlled by the isthmic organizer via diffusion of FGF8. It is limited rostrally by the isthmus territory, and caudally by rhombomere 2. It is double the normal size of any other rhombomere. Its dorsal part generates the cerebellar hemispheres and its ventral part gives rise to several populations, such as some raphe nuclei, the interpeduncular nucleus, the rhabdoid nucleus, anterior, dorsal, ventral and posterodorsal tegmental nuclei, the cholinergic pedunculopontine and laterodorsal tegmental nuclei, rostral parts of the hindbrain reticular formation, the locus coeruleus, and part of the lateral lemniscal and paralemniscal nuclei, among other formations. Some of these populations migrate tangentially before reaching their final positions. The morphogen Sonic Hedgehog (Shh) is normally released from the local floor plate and underlying notochord. In the present report we explore, first, whether Shh is required in the specification of these r1 populations, and, second, its possible role in the guidance of tangentially migrating neurons that approach the midline. Our results indicate that when Shh function is altered selectively in a conditional mutant mouse strain, most populations normally generated in the medial basal plate of r1 are completely absent. Moreover, the relocation of some neurons that normally originate in the alar plate and migrate tangentially into the medial basal plate is variously altered. In contrast, neurons that migrate radially (or first tangentially and then radially) into the lateral basal plate were not significantly affected.

The developmental brain gene NPAS3 contains the largest number of accelerated regulatory sequences in the human genome

To identify the evolutionary genetic novelties that contributed to shape human-specific traits such as the use of a complex language, long-term planning and exceptional learning abilities is one of the ultimate frontiers of modern biology. Evolutionary signatures of functional shifts could be detected by comparing noncoding regions that are highly conserved across mammals or primates and rapidly accumulated nucleotide substitutions only in the lineage leading to humans. As gene loci densely populated with human-accelerated elements (HAEs) are more likely to have contributed to human-specific novelties, we sought to identify the transcriptional units and genomic 1 Mb intervals of the entire human genome carrying the highest number of HAEs. To this end, we took advantage of four available data sets of human genomic accelerated regions obtained through different comparisons and algorithms and performed a meta-analysis of the combined data. We found that the brain developmental transcription factor neuronal PAS domain-containing protein 3 (NPAS3) contains the largest cluster of noncoding-accelerated regions in the human genome with up to 14 elements that are highly conserved in mammals, including primates, but carry human-specific nucleotide substitutions. We then tested the ability of the 14 HAEs identified at the NPAS3 locus to act as transcriptional regulatory sequences in a reporter expression assay performed in transgenic zebrafish. We found that 11 out of the 14 HAEs present in NPAS3 act as transcriptional enhancers during development, particularly within the nervous system. As NPAS3 is known to play a crucial role during mammalian brain development, our results indicate that the high density of HAEs present in the human NPAS3 locus could have modified the spatiotemporal expression pattern of NPAS3 in the developing human brain and, therefore, contributed to human brain evolution.

Development of the serotonergic cells in murine raphe nuclei and their relations with rhombomeric domains

The raphe nuclei represent the origin of central serotonergic projections. The literature distinguishes seven nuclei grouped into rostral and caudal clusters relative to the pons. The boundaries of these nuclei have not been defined precisely enough, particularly with regard to developmental units, notably hindbrain rhombomeres. We hold that a developmental point of view considering rhombomeres may explain observed differences in connectivity and function. There are twelve rhombomeres characterized by particular genetic profiles, and each develops between one and four distinct serotonergic populations. We have studied the distribution of the conventional seven raphe nuclei among these twelve units. To this aim, we correlated 5-HT-immunoreacted neurons with rhombomeric boundary landmarks in sagittal mouse brain sections at different developmental stages. Furthermore, we performed a partial genoarchitectonic analysis of the developing raphe nuclei, mapping all known serotonergic differentiation markers, and compared these results, jointly with others found in the literature, with our map of serotonin-containing populations, in order to examine regional variations in correspondence. Examples of regionally selective gene patterns were identified. As a result, we produced a rhombomeric classification of some 45 serotonergic populations, and suggested a corresponding modified terminology. Only a minor rostral part of the dorsal raphe nucleus lies in the midbrain. Some serotonergic neurons were found in rhombomere 4, contrary to the conventional assumption that it lacks such neurons. We expect that our reclassification of raphe nuclei may be useful for causal analysis of their differential molecular specification, as well as for studies of differential connectivity and function.

Dynamic mRNA distribution pattern of thyroid hormone transporters and deiodinases during early embryonic chicken brain development

Maternal thyroid hormones (THs) are important in early brain development long before the onset of embryonic TH secretion, but information about the regulation of TH availability in the brain at these early stages is still limited. We therefore investigated in detail the mRNA distribution pattern of the TH activating type 2 and inactivating type 3 deiodinases (D2 and D3) and the TH transporters, organic anion transporting polypeptide 1c1 (Oatp1c1) and monocarboxylate transporter 8 (Mct8), in chicken embryonic brain as well as in retina and inner ear from day 3 to day 10 of development. Oatp1c1, Mct8 and D3 are expressed in the choroid plexus and its precursors allowing selective uptake of THs at the blood-cerebrospinal fluid-barrier with subsequent inactivation of excess hormone. In contrast, the developing blood-brain-barrier does not express Oatp1c1 or Mct8 but appears to be a site for TH activation by D2. Expression of D3 in several sensory brain centers may serve as protection against premature TH action. Expression of D2 and Mct8 but not D3 in the developing pituitary gland allows accumulation of active THs even at early stages. Mct8 is widely expressed in gray matter throughout the brain. This is the first comprehensive study on the dynamic distribution pattern of TH-transporters and deiodinases at stages of embryonic brain development when only maternal THs are available. It provides the essential background for further research aimed at understanding early developmental processes depending on maternal THs.

Sites of origin and developmental dynamics of the neurons in the core and shell regions of torus semicircularis in the Chinese softshell turtle (Pelodiscus sinensis)

To know the embryogenesis of the core and shell regions of the midbrain auditory nucleus, a single dose of [(3)H]-thymidine was injected into the turtle embryos at peak stages of neurogenesis in the shell and core of the torus semicircularis. Following sequential survival times, labeled neurons and the dynamics of cell proliferation were examined. The expression of vimentin (VM), reelin, calbindin, parvalbumin, and substance P were also studied. The results showed that: 1) progenitor cells for the core and shell regions were generated in different sites of the ventricular zone; 2) the length of the cell cycle or S-phase for the shell region were both longer than those for the core region (4.7 and 3.2 hours longer, respectively), suggesting that mitotic activity in the core region is higher than it is in the shell region; 3) the elongated cell bodies of the labeled core and shell cells had close apposition to VM fibers, suggesting that the migration of these cells is guided by VM fibers; 4) the germinal sites of the core and shell constructed by projecting the orientation of radial VM fibers back to the ventricular zone was consistent with those obtained by short and sequential survival [(3)H]-thymidine radiography; and 5) the beginning of positive staining for parvalbumin in the core region was interposed between those for calbindin and substance P in the shell regions. This study contributes to the understanding of how auditory nuclei are organized and how their components developed and evolved.

Cocaine supersensitivity and enhanced motivation for reward in mice lacking dopamine D2 autoreceptors

Dopamine (DA) D2 receptors expressed in DA neurons (D2 autoreceptors) exert a negative feedback regulation that reduces DA neuron firing, DA synthesis and DA release. As D2 receptors are mostly expressed in postsynaptic neurons, pharmacological and genetic approaches have been unable to definitively address the in vivo contribution of D2 autoreceptors to DA-mediated behaviors. We found that midbrain DA neurons from mice deficient in D2 autoreceptors (Drd2(loxP/loxP); Dat(+/IRES-cre), referred to as autoDrd2KO mice) lacked DA-mediated somatodendritic synaptic responses and inhibition of DA release. AutoDrd2KO mice displayed elevated DA synthesis and release, hyperlocomotion and supersensitivity to the psychomotor effects of cocaine. The mice also exhibited increased place preference for cocaine and enhanced motivation for food reward. Our results highlight the importance of D2 autoreceptors in the regulation of DA neurotransmission and demonstrate that D2 autoreceptors are important for normal motor function, food-seeking behavior, and sensitivity to the locomotor and rewarding properties of cocaine.

Comparison of Pretectal Genoarchitectonic Pattern between Quail and Chicken Embryos

Regionalization of the central nervous system is controlled by local networks of transcription factors that establish and maintain the identities of neuroepithelial progenitor areas and their neuronal derivatives. The conserved cerebral Bauplan of vertebrates must result essentially from conserved patterns of developmentally expressed transcription factors. We have previously produced detailed molecular maps for the alar plate of prosomere 1 (the pretectal region) in chicken (Ferran et al., 2007, 2008, 2009). Here we compare the early molecular signature of the pretectum of two closely related avian species of the family Phasianidae, Coturnix japonica (Japanese quail) and Gallus gallus (chicken), aiming to test conservation of the described pattern at a microevolutionary level. We studied the developmental pretectal expression of Bhlhb4, Dbx1, Ebf1, Gata3, Gbx2, Lim1, Meis1, Meis2, Pax3, Pax6, Six3, Tal2, and Tcf7l2 (Tcf4) mRNA, using in situ hybridization, and PAX7 immunohistochemistry. The genoarchitectonic profile of individual pretectal domains and strata was produced, using comparable section planes. Remarkable conservation of the combinatorial genoarchitectonic code was observed, fundamented in a tripartite anteroposterior subdivision. However, we found that at corresponding developmental stages the pretectal region of G. gallus was approximately 30% larger than that of C. japonica, but seemed relatively less mature. Altogether, our results on a conserved genoarchitectonic pattern highlight the importance of early developmental gene networks that causally underlie the production of homologous derivatives in these two evolutionarily closely related species. The shared patterns probably apply to sauropsids in general, as well as to more distantly related vertebrate species.

Topography of Somatostatin Gene Expression Relative to Molecular Progenitor Domains during Ontogeny of the Mouse Hypothalamus

The hypothalamus comprises alar, basal, and floor plate developmental compartments. Recent molecular data support a rostrocaudal subdivision into rostral (terminal) and caudal (peduncular) halves. In this context, the distribution of neuronal populations expressing somatostatin (Sst) mRNA was analyzed in the developing mouse hypothalamus, comparing with the expression pattern of the genes Orthopedia (Otp), Distal-less 5 (Dlx5), Sonic Hedgehog (Shh), and Nk2 homeobox 1 (Nkx2.1). At embryonic day 10.5 (E10.5), Sst mRNA was first detectable in the anterobasal nucleus, a Nkx2.1-, Shh-, and Otp-positive basal domain. By E13.5, nascent Sst expression was also related to two additional Otp-positive domains within the alar plate and one in the basal plate. In the alar plate, Sst-positive cells were observed in rostral and caudal ventral subdomains of the Otp-positive paraventricular complex. An additional basal Sst-expressing cell group was found within a longitudinal Otp-positive periretromamillary band that separates the retromamillary area from tuberal areas. Apart of subsequent growth of these initial populations, at E13.5 and E15.5 some Sst-positive derivatives migrate tangentially into neighboring regions. A subset of cells produced at the anterobasal nucleus disperses ventralward into the shell of the ventromedial hypothalamic nucleus and the arcuate nucleus. Cells from the rostroventral paraventricular subdomain reach the suboptic nucleus, whereas a caudal contingent migrates radially into lateral paraventricular, perifornical, and entopeduncular nuclei. Our data provide a topologic map of molecularly defined progenitor areas originating a specific neuron type during early hypothalamic development. Identification of four main separate sources helps to understand causally its complex adult organization.

Ontogenetic expression of sonic hedgehog in the chicken subpallium

Sonic hedgehog (SHH) is a secreted signaling factor that is implicated in the molecular patterning of the central nervous system (CNS), somites, and limbs in vertebrates. SHH has a crucial role in the generation of ventral cell types along the entire rostrocaudal axis of the neural tube. It is secreted early in development by the axial mesoderm (prechordal plate and notochord) and the overlying ventral neural tube. Recent studies clarified the impact of SHH signaling mechanisms on dorsoventral patterning of the spinal cord, but the corresponding phenomena in the rostral forebrain are slightly different and more complex. This notably involves separate Shh expression in the preoptic part of the forebrain alar plate, as well as in the hypothalamic floor and basal plates. The present work includes a detailed spatiotemporal description of the singular alar Shh expression pattern in the rostral preoptic forebrain of chick embryos, comparing it with FoxG1, Dlx5, Nkx2.1, and Nkx2.2 mRNA expression at diverse stages of development. As a result of this mapping, we report a subdivision of the preoptic region in dorsal and ventral zones; only the dorsal part shows Shh expression. The positive area impinges as well upon a median septocommissural preoptic domain. Our study strongly suggests tangential migration of Shh-positive cells from the preoptic region into other subpallial domains, particularly into the pallidal mantle and the intermediate septum.

A role for receptor protein tyrosine phosphatase lambda in midbrain development

The mid-hindbrain boundary (MHB) harbors an important organizing center for the adjacent brain regions. Here, we present evidence that the receptor protein tyrosine phosphatase lambda (RPTPlambda) is part of the complex molecular network that maintains and shapes the MHB region. RPTPlambda is expressed in a tight band of cells in the caudal midbrain, anterior to the transverse ring of Wnt1 expression. Forced expression of RPTPlambda across the mid-hindbrain region repressed expression of Wnt1, whereas RNA interference-mediated knock-down of RPTPlambda resulted in expansion and distortion of the Wnt1 domain. When ectopically expressed in the mesencephalon, RPTPlambda specifically inhibited the induction of Wnt1 expression after subsequent stimulation with Fgf8. Reduced Wnt1 expression after RPTPlambda transfection correlated with a decrease in Ras- mitogen-activated protein kinase activity at the MHB. We further show that in the embryonic midbrain, RPTPlambda can bind to beta-catenin, a central component of the canonical Wnt signaling pathway. Overexpression of RPTPlambda suppressed the activity of a beta-catenin responsive promoter in the midbrain and reduced progenitor cell proliferation. Cotransfection of Wnt1 or of a stabilized form of beta-catenin together with RPTPlambda partially rescued the RPTPlambda-mediated proliferation defect. Together, these data suggest that RPTPlambda may play a dual role in the control of midbrain development: as a negative modulator of Fgf8-induced Wnt1 expression at the MHB, which may help to confine the Wnt1 domain to it characteristic tight ring at the MHB; and as an inhibitor of canonical Wnt signaling through interaction with and presumably sequestration of beta-catenin.

Genetic control of basal midbrain development

Recent breakthroughs in the understanding of the genetic program that controls specification of ventral cell fate in the spinal cord and hindbrain have produced useful tools for the study of similar genetic networks in the more complex rostral regions of the central nervous system. Several research groups have elucidated key factors in the potential signaling processes, as well as transcription factors necessary for differentiation of various basal midbrain nuclei. Importantly, there has been substantial progress in understanding the genetic cascade involved in specifying dopaminergic neurons. This knowledge will be crucial in the understanding and possible treatment of Parkinson's disease.

A model of early molecular regionalization in the chicken embryonic pretectum

The pretectal region of the brain is visualized as a dorsal region of prosomere 1 in the caudal diencephalon, including derivatives from both the roof and alar plates. Its neuronal derivatives in the adult brain are known as pretectal nuclei. The literature is inconsistent about the precise anteroposterior delimitation of this region and on the number of specific histogenetic domains and subdomains that it contains. We performed a cross-correlated gene-expression map of this brain area in chicken embryos, with the aim of identifying differently fated pretectal domains on the basis of combinatorial gene expression patterns. We examined in detail Pax3, Pax6, Pax7, Tcf4, Meis1, Meis2, Nkx2.2, Lim1, Dmbx1, Dbx1, Six3, FoxP2, Zic1, Ebf1, and Shh mRNA expression, as well as PAX3 and PAX7 immunoreaction, between stages HH11 and HH28. The patterns analyzed serve to fix the cephalic and caudal boundaries of the pretectum and to define three molecularly distinct anteroposterior pretectal domains (precommissural, juxtacommissural, and commissural) and several dorsoventral subdomains. These molecular specification patterns are established step by step between stages HH10 and HH18, largely before neurogenesis begins. This set of gene-architectonic data constitutes a useful scaffold for correlations with fate maps and other experimental embryologic results and may serve as well for inquiries on homologies in this part of the brain.

Expression of Lrrn1 marks the prospective site of the zona limitans thalami in the early embryonic chicken diencephalon

An unknown chicken gene selected from a published substractive hybridization screen (GenBank Accession No. ; [Christiansen, J.H., Coles, E.G., Robinson, V., Pasini, A., Wilkinson, D.G., 2001. Screening from a subtracted embryonic chick hindbrain cDNA library: identification of genes expressed during hindbrain, midbrain and cranial neural crest development. Mech. Dev. 102, 119-133.]) was deemed of interest because of its dynamic pattern of expression across the forebrain and midbrain regions. A 528bp fragment cloned from early chick embryo cDNA and used for in situ hybridization corresponded to part of the 3' untranslated region of the chicken gene Leucine-rich repeat neuronal protein 1 (Lrrn1). The expression of this gene, mapped in the embryonic chick brain between stages HH10 and HH26, apparently preconfigures the zona limitans thalami site before overt formation of this boundary structure. Apart of colateral expression in the forebrain, midbrain and hindbrain basal plate, the most significant expression of Lrrn1 was found early on across the entire alar plate of midbrain and forebrain (HH10). This unitary domain soon divides at HH14 into a rostral part, across alar secondary prosencephalon and prospective alar prosomere 3 (prethalamus; caudal limit at the prospective zona limitans), and a caudal part in alar prosomere 1 (pretectum) and midbrain. The rostral forebrain domain later downregulates gradually most extratelencephalic signal of Lrrn1, but the rostral shell of zona limitans retains expression longer. Expression in the caudal alar domain also changes by downregulation within its pretectal subdomain. Caudally, the midbrain domain ends at the isthmo-mesencephalic junction throughout the studied period. Embryonic Lrrn1 signal also appears in the somites and in the otic vesicle.

Temporal and spatial delineation of mouse Otx2 functions by conditional self-knockout

To identify the independent spatial and temporal activities of the essential developmental gene the Otx2, the germline mutation of which is lethal at embryonic day 8.5, we floxed one allele and substituted the other with an inducible CreER recombinase gene. This makes 'trans' self-knockout possible at any developmental stage. The transient action of tamoxifen pulses allows time-course mutation. We demonstrate efficient temporal knockout and demarcate spatio-temporal windows in which Otx2 controls the head, brain structures and body development.

Locus coeruleus neurons originate in alar rhombomere 1 and migrate into the basal plate: Studies in chick and mouse embryos

We investigated in the mouse and chick the neuroepithelial origin and development of the locus coeruleus (LoC), the most important noradrenergic neuronal population in the brain. We first studied the topography of the developing LoC in the hindbrain, using as markers the key noradrenergic marker gene Dbh and the transcription factors Phox2a and Phox2b (upstream of Dbh). In both mouse and chicken, LoC neurons first appear arranged linearly along the middle one-third of the alar plate of rhombomere 1 (r1), collinear to a reference ventricular longitudinal band that early on expresses Phox2a and Phox2b in the alar plate of r2 and later expands to r1. Double-labeling experiments with LoC markers (Dbh or Phox2a) and either alar (Pax7 and Rnx3) or basal (Otp) genetic markers suggested that LoC cells migrate from their origin in the alar plate to a final position in the lateral basal plate. To corroborate these suggestions experimentally and determine the precise origin of the LoC, we fate mapped the LoC in the chick at stage HH11 by using quail-chick homotopic grafts. The experimental results confirmed that the LoC originates in the alar plate throughout the rostrocaudal extent of r1 and ruled out a rostrocaudal translocation. They also corroborated a ventralward tangential migration of LoC cells into the lateral basal plate, where the postmigratory LoC primordium is located. Comparisons with neighboring alar r1-derived cell populations established that LoC neurons originate outside the cerebellum, in a matrix area intercalated dorsoventrally between the sources of the prospective vestibular and trigeminal columns.