Some introductory notes on the organization of the forebrain in tetrapods.

As an introduction to the main theme of this conference an overview of the organization of the tetrapod forebrain is presented with emphasis on the telencephalic representation of sensory and motor functions. In all classes of tetrapods, olfactory, visual, octavolateral, somatosensory and gustatory information reaches the telencephalon. Major differences exist in the telencephalic targets of sensory information between amphibians and amniotes. In amphibians, three targets are found: the lateral pallium for olfactory input, the medial pallium for visual and multisensory input, and the lateral subpallium for visual, octavolateral and somatosensory information. The forebrains of reptiles and mammals are similar in that the dorsal surface of their cerebral hemisphere is formed by a pallium with three major segments: (a) an olfactory, lateral cortex; (b) a 'limbic' cortex that forms the dorsomedial wall of the hemisphere, and (c) an intermediate cortex that is composed entirely of isocortex in mammals, but in reptiles (and birds) consists of at least part of the dorsal cortex (in birds the Wulst) and a large intraventricular protrusion, i.e. the dorsal ventricular ridge. In birds, the entire lateral wall of the hemisphere is involved in this expansion. The intermediate pallial segment receives sensory projections from the thalamus and contains modality-specific sensory areas in reptiles, birds and mammals. The most important differences between the intermediate pallial segment of amniotes concern motor systems.     

Comparison of the mammalian and avian telencephalon from the perspective of gene expression data.

Pallial and subpallial morphological subdivisions of the mouse and chicken telencephalon were examined from the new perspective given by gene markers expressed in these territories during development. The rationale of this approach is that common gene expression patterns may underlie similar histogenetic specification and, consequently, comparable morphological nature. The nested expression domains of the genes Dlx-2 and Nkx-2.1 are characteristic for the subpallium (lateral and medial ganglionic eminences). Similar expression of these markers in parts of the mouse septum and amygdala suggests that such parts may be considered subpallial. The genes Pax-6, Tbr-1 and Emx-1 are expressed in the pallium. Complementary areas of the septum and amygdala shared expression of these genes, suggesting these are the pallial parts of these units. Differences in the relative topography of pallial marker genes also define different regions of the pallium, which can be partially traced into the amygdala. Importantly, there is evidence of a novel "ventral pallium" subdivision, which is a molecularly distinct pallial territory intercalated between the striatum and the lateral pallium. Its derivatives in the mouse apparently belong to the claustroamygdaloid complex. Chicken genes homologous sequence-wise to these mouse developmental genes are expressed in topologically comparable patterns during development. The avian subpallium -the paleostriatum- expresses Dlx-2 and Nkx-2.1; expression extends as well into the septum and anterior and medial parts of the archistriatum. The avian pallium expresses Pax-6, Tbr-1 and Emx-1 and also contains a distinct ventral pallium, formed by the neostriatum and ventral intermediate parts of the archistriatum. The lateral pallium comprises the hyperstriatum ventrale, overlying temporo-parieto-occipital corticoid layer and piriform cortex, plus dorsal intermediate and posterior archistriatum. The dorsal pallium includes the dorsal, intercalated and accessory hyperstriatum, plus the dorsolateral corticoid area. The medial pallium contains the hippocampus and parahippocampal area. A dorsal part of the septum shares pallial molecular markers. Gene markers thus suggest common sets of molecular developmental determinants in either pallial or subpallial domains of the mouse and chicken telencephalon, extending all the way from the posterior pole (amygdala) to the septum. Ventral pallial derivatives identified as claustroamygdaloid in the mouse correlate with avian neostriatum and parts of the archistriatum.     

Pallial origin of basal forebrain cholinergic neurons in the nucleus basalis of Meynert and horizontal limb of the diagonal band nucleus

The majority of the cortical cholinergic innervation implicated in attention and memory originates in the nucleus basalis of Meynert and in the horizontal limb of the diagonal band nucleus of the basal prosencephalon. Functional alterations in this system give rise to neuropsychiatric disorders as well as to the cognitive alterations described in Parkinson and Alzheimer's diseases. Despite the functional importance of these basal forebrain cholinergic neurons very little is known about their origin and development. Previous studies suggest that they originate in the medial ganglionic eminence of the telencephalic subpallium; however, our results identified Tbr1-expressing, reelin-positive neurons migrating from the ventral pallium to the subpallium that differentiate into cholinergic neurons in the basal forebrain nuclei projecting to the cortex. Experiments with Tbr1 knockout mice, which lack ventropallial structures, confirmed the pallial origin of cholinergic neurons in Meynert and horizontal diagonal band nuclei. Also, we demonstrate that Fgf8 signaling in the telencephalic midline attracts these neurons from the pallium to follow a tangential migratory route towards the basal forebrain.     

The corticostriatal junction: a crucial region for forebrain development and evolution

Most parts of the brain are conserved across reptiles and birds (sauropsids) and mammals. Two major qualitative differences occur in the upper part, or pallium, of the telencephalon, the most rostral part of the brain. Mammals have a six-layered neocortex and also exhibit a different morphological organization in the lateral half, or sector, of their pallium than do sauropsids. These differences of lateral pallial construction may derive from small but crucial differences in migration patterns of neuronal precursors generated at or above the corner of the lateral ventricle, the corticostriatal junction (CS). Sauropsids have a large structure, the dorsal ventricular ridge, that is proliferated from this region, and its anterior part (ADVR) receives ascending projections from the dorsal thalamus. Mammals have multiple structures in this same region-the lateral part of neocortex, amygdala, and claustrum-endopiriform formation. We propose here that, as the degree of development of structures that form the deeper tier of the pallium varies across the stages of embryology and across phylogeny, mutations may have occurred during evolution at the origin of mammals that had profound consequences for the fate of neural populations generated in the region of the CS and its neighboring pallial germinal zone.     

Fate map of the avian anterior forebrain at the four-somite stage, based on the analysis of quail-chick chimeras.

To better understand the topological organization of the primordia within the anterior forebrain, we made a fate map of the rostral neural plate in the chick. Homotopic grafts at the four-somite stage were allowed to survive for up to 9 days to enable an analysis of definitive brain structures. In some cases, the topography of the grafted neuroepithelia was compared with gene expression patterns. The midpoint of the anterior neural ridge maps upon the anterior commissure in the closed neural tube, continuing concentrically into the preoptic area and optic field. Non-neural epithelium just in front of this median ridge gives rise to the adenohypophysis. Areas for the presumptive pallial commissure, septum, and prosencephalic choroidal tissue lie progressively more posteriorly along the ridge, peripheral to the telencephalic entopeduncular and striatopallidal primordia (the subpallium), and the pallium (olfactory bulb, dorsal ventricular ridge, and cortical domains). Subpallial structures lie topologically anterior to the pallial formations, and both are concentric to the septum. Within the pallium, the major cortical domains (Wulst and caudolateral, parahippocampal, and hippocampal cortices) appear posterior to the dorsal ventricular ridge. The amygdaloid region appears concentrically across both the subpallial and pallial regions. This fate map shows that the arrangement of the prospective primordia in the neural plate is basically a flattened representation of topological relationships present in the mature brain, though marked phenomena of differential growth and selective tangential migration of some cell populations complicate the histogenetic constitution of the mature telencephalon.     

Central connections of the olfactory bulb in the bichir,Polypterus palmas,reexamined

Summary Central connections of the olfactory bulb of Polypterus palmas were studied with the use of horseradish peroxidase and cobalt-tracing techniques. The olfactory bulb projects to subpallial and palliai areas in the ipsilateral telencephalon; a projection to the contralateral subpallium is noted via the habenular commissure. A further target of secondary olfactory fibers is a caudal olfactory projection area in the ipsilateral hypothalamus. No labeling was seen in the anterior commissure and in the contralateral olfactory bulb. The medial and the lateral pallium receive secondary olfactory fibers in distinct areas. Neurons projecting to the bulb are found in the ipsilateral subpallium, mainly in one dorsal longitudinal nucleus. The main connection with the tel- and diencephalon is mediated via the medial olfactory tract. This tract also contains fibers to the contralateral telencephalon, and to the hypothalamus. The smaller lateral olfactory tract mediates fibers to the lateral pallium. The organization of pathways of secondary olfactory fibers in the telencephalon is described. The present findings are compared to those obtained in species possessing an inverted forebrain.This investigation was supported by grants from the Deutsche Forschungsgemeinschaft to DLM     

Effects of canonical Wnt signaling on dorso-ventral specification of the mouse telencephalon

Wnt signaling is involved in numerous processes during vertebrate CNS development. In this study, we used conditional Cre/loxP system in mouse to ablate or activate beta-catenin in the telencephalon in two time windows: before and after the onset of neurogenesis. We show that beta-catenin mediated Wnt signals are required to maintain the molecular identity of the pallium. Inactivation of beta-catenin in the telencephalon before neurogenesis results in downregulated expression of dorsal markers Emx1, Emx2 and Ngn2, and in ectopic up-regulation of ventral markers Gsh2, Mash1 and Dlx2 in the pallium. In contrast, ablation of ss-catenin after the onset of cortical neurogenesis (E11.5) does not result in a dorso-ventral fate shift. In addition, activation of canonical Wnt signaling in the subpallium leads to a repression of ventral telencephalic cell identities as shown by the down-regulation of subpallial markers Dlx2, Nkx2.1, Gsh2, Olig2 and Mash1. This was accompanied with an expansion of dorsal identities ventrally as shown by the expanded expression domains of pallial markers Pax6 and Ngn2. Thus, our data suggest that canonical Wnt signals are involved in maintaining the identity of the pallium by controlling expression of dorsal markers and by suppressing ventral programs from being activated in pallial progenitor cells.     

Development and evolution of the pallium

The neocortex is the most representative and elaborated structure of the mammalian brain and is related to the achievement of complex cognitive capabilities, which are disturbed following malformation or lesion. Searching for the evolutionary origin of this structure continues to be one of the most important and challenging questions in comparative neurobiology. However, this is extremely difficult because of the highly divergent evolution of the pallium in different vertebrates, which has obscured the comparison. Herein, we review developmental neurobiology data for trying to understand the genetic factors that define and underlie the parcellation of homologous pallial subdivisions in different vertebrates. According to these data, the pallium in all tetrapods parcellates during development into four major histogenetic subdivisions, which are homologous as fields across species. The neocortex derives from the dorsal pallium and, as such, is only comparable to the sauropsidian dorsal pallium (avian hyperpallium and lizard/turtle dorsal cortex). We also tried to identify developmental changes in phylogeny that may be responsible of pallial divergent evolution. In particular, we point out to evolutionary differences regarding the cortical hem (an important signaling center for pallial patterning, that also is a source of Cajal–Retzius cells, which are involved in cortical lamination), which may be behind the distinct organization of the pallium in mammals and non-mammals. In addition, we mention recent data suggesting a correlation between the appearance and elaboration of the subventricular zone (a new germinative cell layer of the developing neocortex), and the evolution of novel cell layers (the supragranular layers) and interneuron subtypes. Finally, we comment on epigenetic factors that modulate the developmental programs, leading to changes in the formation of functional areas in the pallium (within some constraints).     

Evidences for tangential migrations in Xenopus telencephalon: developmental patterns and cell tracking experiments

Extensive tangential cell migrations have been described in the developing mammalian, avian, and reptilian forebrain, and they are viewed as a powerful developmental mechanism to increase neuronal complexity in a given brain structure. Here, we report for the first time anatomical and cell tracking evidence for the presence of important migratory processes in the developing forebrain of the anamniote Xenopus laevis. Combining developmental gene expression patterns (Pax6, Nkx2.1, Isl1, Lhx5, Lhx9, and Dll3), neurotransmitter identity (GABA, NOS, ChAT), and connectivity information, several types of putative migratory cell populations and migration routes originating in the ventral pallium and the subpallium are proposed. By means of in vivo cell tracking experiments, pallio-subpallial and subpallio-pallial migrating neurons are visualized. Among them, populations of Nkx2.1(+) striatal interneurons and pallial GABAergic interneurons, which also express the migratory marker doublecortin, are identified. Finally, we find that these tangentially migrating pallial interneurons travel through an "isl1-free channel" that may guide their course through the subpallium. Our findings strongly suggest that the developing Xenopus telencephalon shares many similarities with amniotes in terms of neuronal specification and migrations. However, some differences are discussed, particularly with regard to the evolution of the pallium.     

Activity of acetyl- and butyrylcholinesterase of the hemispheres and several subcortical zones of the human brain in prenatal ontogenesis

Histochemical investigations of acetyl- and butyrilcholinesterase (AChE and BChE) activity in the cortical plate and in some subcortical areas of the human brain have demonstrated that on the 8th week of the prenatal development of the greatest AChE and BChE activity is observed in the dorsal thalamus, epithalamus and in the ependymal layer of various cerebral parts, the forebrain including. The data obtained, prove previous observations, concerning predominant localization of AChE in the intermediate layer of the isocortex (10 weeks). In a 10-week-old human fetus a total high level of AChE and BChE activity is demonstrated in various nuclei of the thalamus and in subcortical structures of the forebrain (nucl. caudatus, Meynert nucl.).     

Extra-lemniscal afferent projections of dorsal spinal cord nuclei to the ventrobasal nuclear complex structure in the contralateral optic thalamus

It has been found that section of half the midbrain tegmentum in cats failed to prevent the afferent somatosensory projections from the foreleg to the ventrobasal nuclear complex of the contralateral thalamus. Specific evoked responses to the stimulation of the contralateral foreleg were recorded in this structure. These specific EP have the same latency as "lemniscal responses" (4-5 ms) and diminish the amplitude and duration of both components of the responses. Simultaneously, we have observed terminal axonal degeneration into the ventrobasal nuclear complex of the thalamus 5-7 days after the section of the contralateral midbrain tegmentum, using the electron microscopy method. All the results obtained indicate that the dorsal column nuclei have extra-lemniscal afferent connections with ventrolateral nuclear complex of the contralateral thalamus. These connections ascend in the back parts of the brainstem ipsilaterally to the corresponding pair of the dorsal column nuclei and rostrally to the midbrain on the contralateral side.     

Evidence that descending cortical axons are essential for thalamocortical axons to cross the pallial-subpallial boundary in the embryonic forebrain

Developing thalamocortical axons traverse the subpallium to reach the cortex located in the pallium. We tested the hypothesis that descending corticofugal axons are important for guiding thalamocortical axons across the pallial-subpallial boundary, using conditional mutagenesis to assess the effects of blocking corticofugal axonal development without disrupting thalamus, subpallium or the pallial-subpallial boundary. We found that thalamic axons still traversed the subpallium in topographic order but did not cross the pallial-subpallial boundary. Co-culture experiments indicated that the inability of thalamic axons to cross the boundary was not explained by mutant cortex developing a long-range chemorepulsive action on thalamic axons. On the contrary, cortex from conditional mutants retained its thalamic axonal growth-promoting activity and continued to express Nrg-1, which is responsible for this stimulatory effect. When mutant cortex was replaced with control cortex, corticofugal efferents were restored and thalamic axons from conditional mutants associated with them and crossed the pallial-subpallial boundary. Our study provides the most compelling evidence to date that cortical efferents are required to guide thalamocortical axons across the pallial-subpallial boundary, which is otherwise hostile to thalamic axons. These results support the hypothesis that thalamic axons grow from subpallium to cortex guided by cortical efferents, with stimulation from diffusible cortical growth-promoting factors.     

Tlx and Pax6 co-operate genetically to establish the pallio-subpallial boundary in the embryonic mouse telencephalon

We have examined the role of Tlx, an orphan nuclear receptor, in dorsal-ventral patterning of the mouse telencephalon. Tlx is expressed broadly in the ventricular zone, with the exception of the dorsomedial and ventromedial regions. The expression spans the pallio-subpallial boundary, which separates the dorsal (i.e. pallium) and ventral (i.e. subpallium) telencephalon. Despite being expressed on both sides of the pallio-subpallial boundary, Tlx homozygous mutants display alterations in the development of this boundary. These alterations include a dorsal shift in the expression limits of certain genes that abut at the pallio-subpallial boundary as well as the abnormal formation of the radial glial palisade that normally marks this boundary. The Tlx mutant phenotype is similar to, but less severe than, that seen in Small eye (i.e. Pax6) mutants. Interestingly, removal of one allele of Pax6 on the homozygous Tlx mutant background significantly worsens the phenotype. Thus Tlx and Pax6 cooperate genetically to regulate the establishment of the pallio-subpallial boundary. The patterning defects in the Tlx mutant telencephalon result in a loss of region-specific gene expression in the ventral-most pallial region. This correlates well with the malformation of the lateral and basolateral amygdala in Tlx mutants, both of which have been suggested to derive from ventral portions of the pallium.     

Expression of Zash-1a in the postembryonic zebrafish brain allows comparison to mouse Mash1 domains

Four areas in the late embryonic murine forebrain, i.e. the subpallium (striatum), the preoptic region, the ventral thalamus, and the hypothalamus, have been described to express the basic helix-loop-helix (bHLH) gene mammalian achaete-scute homolog Mash1 (Ascl1, Mouse Genome Informatics) in a complementary fashion to another bHLH gene, neurogenin1 (ngn1) (Neurod3, Mouse Genome Informatics), which is expressed in directly adjacent forebrain regions. We report here that the four regions previously identified as subpallium, preoptic region, ventral thalamus and hypothalamus (i.e. ventral inferior lobe) in the postembryonic zebrafish brain show Zash-1a expression at 3 days postfertilization (dpf), whereas none of those areas express the bHLH gene neuroD (nrd) between 2 and 5 dpf. This indicates that two well established alternative genetic pathways involved in neurogenesis in the amniote (mammalian) brain are present in homologous phenotypic locations in the anamniote (zebrafish) brain as well and that these pathways possibly act similarly in the generation of different neuronal phenotypes (e.g. subpallial GABAergic interneurons versus pallial glutamatergic projection neurons, or dopaminergic neurons versus other neurotransmitter phenotypes). Furthermore, previous initial identification of early postembryonic brain subdivisions in the zebrafish is strongly corroborated by these expression patterns.     

Telencephalic sources of the afferents of the main olfactory bulb in the frog Rana temporaria

Following horseradish peroxidase iontophoretic application into the main olfactory bulb (MOB) retrograde neuronal labeling was examined in the telencephalon in the frog. Labeled neurons, the sources of the MOB afferents are found in the mitral cell layer of the contralateral MOB, pallial and some subpallial areas. Very heavy labeling is observed in the pars ventralis of the lateral pallium, and to a lesser extent in the medial pallium, pars dorsalis of the lateral pallium and in the dorsal pallium. In subpallium labeled neurons are found in the eminentia postolfactoria, the rostral part of the medial septal nucleus, and in the nucleus of the ventro-medial telencephalic wall, which is probably homologous to the nucleus of the diagonal band (Broca) of mammals. No labelled neurons were found in the caudal portion of the MOB granular layer, usually referred to as the anterior olfactory nucleus. The arrangement of the MOB centrifugal innervation in amphibians is discussed in comparison with that in mammals.