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

Primary cilia control telencephalic patterning and morphogenesis via Gli3 proteolytic processing

Primary cilia have essential functions in vertebrate development and signaling. However, little is known about cilia function in brain morphogenesis, a process that is severely affected in human ciliopathies. Here, we study telencephalic morphogenesis in a mouse mutant for the ciliopathy gene Ftm (Rpgrip1l). We show that the olfactory bulbs are present in an ectopic location in the telencephalon of Ftm(-/-) fetuses and do not display morphological outgrowth at the end of gestation. Investigating the developmental origin of this defect, we have established that E12.5 Ftm(-/-) telencephalic neuroepithelial cells lack primary cilia. Moreover, in the anterior telencephalon, the subpallium is expanded at the expense of the pallium, a phenotype reminiscent of Gli3 mutants. This phenotype indeed correlates with a decreased production of the short form of the Gli3 protein. Introduction of a Gli3 mutant allele encoding the short form of Gli3 into Ftm mutants rescues both telencephalic patterning and olfactory bulb morphogenesis, despite the persistence of cilia defects. Together, our results show that olfactory bulb morphogenesis depends on primary cilia and that the essential role of cilia in this process is to produce processed Gli3R required for developmental patterning. Our analysis thus provides the first in vivo demonstration that primary cilia control a developmental process via production of the short, repressor form of Gli3. Moreover, our findings shed light on the developmental origin of olfactory bulb agenesis and of other brain morphogenetic defects found in human diseases affecting the primary cilium.

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.

Distal-less-like protein distribution in the larval lamprey forebrain

A polyclonal antibody against the Drosophila distal-less (DLL) protein, cross-reactive with cognate vertebrate proteins, was employed to map DLL-like expression in the midlarval lamprey forebrain. This work aimed to characterize in detail the separate diencephalic and telencephalic DLL expression domains, in order to test our previous modified definition of the lamprey prethalamus [Pombal and Puelles (1999) J Comp Neurol 414:391-422], adapt our earlier schema of prosomeric subdivisions in the lamprey forebrain to more recent versions of this model [Pombal et al. (2009) Brain Behav Evol 74:7-19] and reexamine the pallio-subpallial regionalization of the lamprey telencephalon. We observed a large-scale conservation of the topologic distribution of the DLL protein, in consonance with patterns of Dlx expression present in other vertebrates studied. Moreover, evidence was obtained of substantial numbers of DLL-positive neurons in the olfactory bulb and the cerebral hemispheres, in a pattern consistent with possible tangential migration out of the subpallium into the overlying pallium, as occurs in mammals, birds, frogs and teleost fishes.

Similarities and differences in the forebrain expression of Lhx1 and Lhx5 between chicken and mouse: Insights for understanding telencephalic development and evolution

We compared expression of the paralogous LIM-homeodomain genes Lhx1 and Lhx5 in the developing rostral forebrain of mouse and chicken. Both genes are expressed in similar patterns in the septum, preoptic region, and related areas of the basal telencephalon, including the medial septum/diagonal band nuclei and the medial extended amygdala. In the septum, the expression of Lhx5 and Lhx1 appears to be specifically related to the pallial septum and its derivatives in mouse and chicken, and may produce the glutamatergic neurons observed in the diagonal band/medial septum nuclei. The preoptic area expresses both Lhx1 and Lhx5 in mouse and chicken, and appears to produce gamma-aminobutyric acid (GABA)ergic, glutamatergic, and cholinergic cells for the preoptic region and basal telencephalon. In addition, in mouse and chicken Lhx5 is expressed in two extratelencephalic domains that appear to contribute Lhx5-expressing cells to the basal telencephalon, including the supraoptoparaventricular hypothalamic domain and the eminentia thalami. In contrast, there are striking differences in the pallial expression of Lhx1 and Lhx5 between mouse and chicken. Both genes are expressed in Cajal-Retzius cells, and Lhx5 is also present in most pallial sources of Cajal-Retzius cells (including the cortical hem and retrobulbar area) and in the olfactory bulbs in the mouse. In contrast, putative Cajal-Retzius cells, the retrobulbar area, and the olfactory bulb of chicken do not express the paralog genes cLhx1/cLhx5. Moreover, the cortical hem-although it expresses cLhx5-is very tiny in chicken. We discuss the consequences of these differences in Lhx1/Lhx5 expression between mouse and chicken for pallial/cortical evolution.

FGF8 acts as a classic diffusible morphogen to pattern the neocortex

Gain- and loss-of-function experiments have demonstrated that a source of fibroblast growth factor (FGF) 8 regulates anterior to posterior (A/P) patterning in the neocortical area map. Whether FGF8 controls patterning as a classic diffusible morphogen has not been directly tested. We report evidence that FGF8 diffuses through the mouse neocortical primordium from a discrete source in the anterior telencephalon, forms a protein gradient across the entire A/P extent of the primordium, and acts directly at a distance from its source to determine area identity. FGF8 immunofluorescence revealed FGF8 protein distributed in an A/P gradient. Fate-mapping experiments showed that outside the most anterior telencephalon, neocortical progenitor cells did not express Fgf8, nor were they derived from Fgf8-expressing cells, suggesting that graded distribution of FGF8 results from protein diffusion from the anterior source. Supporting this conclusion, a dominant-negative high-affinity FGF8 receptor captured endogenous FGF8 at a distance from the FGF8 source. New FGF8 sources introduced by electroporation showed haloes of FGF8 immunofluorescence indicative of FGF8 diffusion, and surrounding cells reacted to a new source of FGF8 by upregulating different FGF8-responsive genes in concentric domains around the source. Reducing endogenous FGF8 with the dominant-negative receptor in the central neocortical primordium induced cells to adopt a more posterior area identity, demonstrating long-range area patterning by FGF8. These observations support FGF8 as a classic diffusible morphogen in neocortex, thereby guiding future studies of neocortical pattern formation.

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.

Conserved expression of mouse Six1 in the pre-placodal region (PPR) and identification of an enhancer for the rostral PPR

All cranial sensory organs and sensory neurons of vertebrates develop from cranial placodes. In chick, amphibians and zebrafish, all placodes originate from a common precursor domain, the pre-placodal region (PPR), marked by the expression of Six1/4 and Eya1/2. However, the PPR has never been described in mammals and the mechanism involved in the formation of PPR is poorly defined. Here, we report the expression of Six1 in the horseshoe-shaped mouse ectoderm surrounding the anterior neural plate in a pattern broadly similar to that of non-mammalian vertebrates. To elucidate the identity of Six1-positive mouse ectoderm, we searched for enhancers responsible for Six1 expression by in vivo enhancer assays. One conserved non-coding sequence, Six1-14, showed specific enhancer activity in the rostral PPR of chick and Xenopus and in the mouse ectoderm. These results strongly suggest the presence of PPR in mouse and that it is conserved in vertebrates. Moreover, we show the importance of the homeodomain protein-binding sites of Six1-14, the Six1 rostral PPR enhancer, for enhancer activity, and that Dlx5, Msx1 and Pax7 are candidate binding factors that regulate the level and area of Six1 expression, and thereby the location of the PPR. Our findings provide critical information and tools to elucidate the molecular mechanism of early sensory development and have implications for the development of sensory precursor/stem cells.

Incipient forebrain boundaries traced by differential gene expression and fate mapping in the chick neural plate

We correlated available fate maps for the avian neural plate at stages HH4 and HH8 with the progress of local molecular specification, aiming to determine when the molecular specification maps of the primary longitudinal and transversal domains of the anterior forebrain agree with the fate mapped data. To this end, we examined selected gene expression patterns as they normally evolved in whole mounts and sections between HH4 and HH8 (or HH10/11 in some cases), performed novel fate-mapping experiments within the anterior forebrain at HH4 and examined the results at HH8, and correlated grafts with expression of selected gene markers. The data provided new details to the HH4 fate map, and disclosed some genes (e.g., Six3 and Ganf) whose expression domains initially are very extensive and subsequently retract rostralwards. Apart from anteroposterior dynamics, some genes soon became downregulated at the prospective forebrain floor plate, or allowed to identify an early roof plate domain (dorsoventral pattern). Peculiarities of the telencephalon (initial specification and differentiation of pallium versus subpallium) are contemplated. The basic anterior forebrain subdivisions seem to acquire correlated specification and fate mapping patterns around stage HH8.

The embryonic preoptic area is a novel source of cortical GABAergic interneurons

GABA-containing (GABAergic) interneurons play an important role in the function of the cerebral cortex. Through mostly inhibitory mechanisms, interneurons control hyperexcitability and synchronize and shape the spatiotemporal dynamics of cortical activity underlying various brain functions. Studies over the past 10 years have demonstrated that, in most mammals, interneurons originate during development from the subcortical telencephalon--the subpallium--and reach the cerebral cortex through tangential migration. Until now, interneurons have been demonstrated to derive exclusively from two subpallial regions, the medial ganglionic eminence and the caudal ganglionic eminence. Here, we show that another subpallial structure, the preoptic area, is a novel source of cortical GABAergic interneurons in the mouse. In utero labeling and genetic lineage-tracing experiments demonstrate that neurons born in this region migrate to the neocortex and hippocampus, where they differentiate into a distinct population of GABAergic interneurons with relatively uniform neurochemical, morphological, and electrophysiological properties.

Fate map of the chick embryo neural tube

Fate-map studies have provided important information in relation to the regional topology of brain areas in different vertebrate species. Moreover, these studies have demonstrated that the distribution of presumptive territories in neural plate and neural tube are highly conserved in vertebrates. The aim of this review is to re-examine and correlate the distribution of presumptive neuroepithelial domains in the chick neural tube with molecular information and discuss recent data. First, we review descriptive fate map studies of neural plate in different vertebrate species that have been studied using diverse fate-mapping methods. Then, we summarize the available data on the localization of neuroepithelial progenitors for the brain subregions in the chick neural tube at stage HH10-11, the most used stage for experimental embryology. This analysis is mainly focused on experimental fate mapping results using quail-chick chimeras.

Neural crest contribution to forebrain development

The neural crest (NC), a defining feature of vertebrate embryo, generates most of the skeletal tissues encasing the developing forebrain and provides the prosencephalon with functional vasculature and meninges. Recent findings show that early in development, the cephalic NC is also essential for the pre-otic neural tube closure and promotes the development of the prosencephalic alar plate by regulating the morphogenetic activities of forebrain organizers.

Molecular and tissue interactions governing induction of cranial ectodermal placodes

Whereas neural crest cells are the source of the peripheral nervous system in the trunk of vertebrates, the "ectodermal placodes," together with neural crest, form the peripheral nervous system of the head. Cranial ectodermal placodes are thickenings in the ectoderm that subsequently ingress or invaginate to make important contributions to cranial ganglia, including epibranchial and trigeminal ganglia, and sensory structures, the ear, nose, lens, and adenohypophysis. Recent studies have uncovered a number of molecular signals mediating induction and differentiation of placodal cells. Here, we described recent advances in understanding the tissue interactions and signals underlying induction and neurogenesis of placodes, with emphasis on the trigeminal and epibranchial. Important roles of Fibroblast Growth Factors, Platelet Derived Growth Factors, Sonic Hedgehog, TGFbeta superfamily members, and Wnts are discussed.

Functional mode of FoxD1/CBF2 for the establishment of temporal retinal specificity in the developing chick retina

Two winged-helix transcription factors, FoxG1 (previously called chick brain factor1, CBF1) and FoxD1 (chick brain factor2, CBF2), are expressed specifically in the nasal and temporal regions of the developing chick retina, respectively. We previously demonstrated that FoxG1 controls the expression of topographic molecules including FoxD1, and determines the regional specificity of the nasal retina. FoxD1 is known to prescribe temporal specificity, however, molecular mechanisms and downstream targets have not been elucidated. Here we addressed the genetic mechanisms for establishing temporal specificity in the developing retina using an in ovo electroporation technique. Fibroblast growth factor (Fgf) and Wnt first play pivotal roles in inducing the region-specific expression of FoxG1 and FoxD1 in the optic vesicle. Misexpression of FoxD1 represses the expression of FoxG1, GH6, SOHo1, and ephrin-A5, and induces that of EphA3 in the retina. GH6 and SOHo1 repress the expression of FoxD1. In contrast to the inhibitory effect of FoxG1 on bone morphogenic protein (BMP) signaling, FoxD1 does not alter the expression of BMP4 or BMP2. Studies with chimeric mutants of FoxD1 showed that FoxD1 acts as a transcription repressor in controlling its downstream targets in the retina. Taken together with previous findings, our data suggest that FoxG1 and FoxD1 are located at the top of the gene cascade for regional specification along the nasotemporal (anteroposterior) axis in the retina, and FoxD1 determines temporal specificity.

Integration of telencephalic Wnt and hedgehog signaling center activities by Foxg1

The forebrain is patterned along the dorsoventral (DV) axis by Sonic Hedgehog (Shh). However, previous studies have suggested the presence of an Shh-independent mechanism. Our study identifies Wnt/beta-catenin-activated from the telencephalic roof-as an Shh-independent pathway that is essential for telencephalic pallial (dorsal) specification during neurulation. We demonstrate that the transcription factor Foxg1 coordinates the activity of two signaling centers: Foxg1 is a key downstream effector of the Shh pathway during induction of subpallial (ventral) identity, and it inhibits Wnt/beta-catenin signaling through direct transcriptional repression of Wnt ligands. This inhibition restricts the dorsal Wnt signaling center to the roof plate and consequently limits pallial identities. Concomitantly to these roles, Foxg1 controls the formation of the compartment boundary between telencephalon and basal diencephalon. Altogether, these findings identify a key direct target of Foxg1, and uncover a simple molecular mechanism by which Foxg1 integrates two opposing signaling centers.

Rostral paraxial mesoderm regulates refinement of the eye field through the bone morphogenetic protein (BMP) pathway

The eye field is initially a large single domain at the anterior end of the neural plate and is the first indication of optic potential in the vertebrate embryo. During the course of development, this domain is subject to interactions that shape and refine the organogenic field. The action of the prechordal mesoderm in bisecting this single region into two bilateral domains has been well described, however the role of signalling interactions in the further restriction and refinement of this domain has not been previously characterised. Here we describe a role for the rostral cephalic paraxial mesoderm in limiting the extent of the eye field. The anterior transposition of this mesoderm or its ablation disrupted normal development of the eye. Importantly, perturbation of optic vesicle development occurred in the absence of any detectable changes in the pattern of neighbouring regions of the neural tube. Furthermore, negative regulation of eye development is a property unique to the rostral paraxial mesoderm. The rostral paraxial mesoderm expresses members of the bone morphogenetic protein (BMP) family of signalling molecules and manipulation of endogenous BMP signalling resulted in abnormalities of the early optic primordia.

Genetic mapping of Foxb1-cell lineage shows migration from caudal diencephalon to telencephalon and lateral hypothalamus

The hypothalamus is a brain region with vital functions, and alterations in its development can cause human disease. However, we still do not have a complete description of how this complex structure is put together during embryonic and early postnatal stages. Radially oriented, outside-in migration of cells is prevalent in the developing hypothalamus. In spite of this, cell contingents from outside the hypothalamus as well as tangential hypothalamic migrations also have an important role. Here we study migrations in the hypothalamic primordium by genetically labeling the Foxb1 diencephalic lineage. Foxb1 is a transcription factor gene expressed in the neuroepithelium of the developing neural tube with a rostral expression boundary between caudal and rostral diencephalon, and therefore appropriate for marking migrations from caudal levels into the hypothalamus. We have found a large, longitudinally oriented migration stream apparently originating in the thalamic region and following an axonal bundle to end in the anterior portion of the lateral hypothalamic area. Additionally, we have mapped a specific expansion of the neuroepithelium into the rostral diencephalon. The expanded neuroepithelium generates abundant neurons for the medial hypothalamus at the tuberal level. Finally, we have uncovered novel diencephalon-to-telencephalon migrations into septum, piriform cortex and amygdala.

Telencephalic morphogenesis during the process of neurulation: an experimental study using quail-chick chimeras

After gastrulation, during the process of neurulation, the anterior neural region undergoes important morphological transformations. The almost flat epithelium of the rostral neural plate becomes transformed into a spherical region, the prosencephalic vesicle, in the neural tube. Later in development, two bilateral areas (the optic and telencephalic vesicles) progressively protrude from the prosencephalon, generating the eyes and the cerebral hemispheres, respectively. Although the principal processes of neurulation have been well characterized, the growth patterns and evolution of topological relations between internal prosencephalic regions have not been experimentally analyzed. In order to better characterize morphogenetic transformations of the prosencephalon, we have realized and comparatively analyzed neuroepithelial fate maps before and after neurulation using quail/chick chimerical experiments. Since we have previously reported the fate map of the prosencephalon at the neural plate stage, in the present work we report the corresponding fate map at the neural tube stage. Comparative analysis of the two maps has allowed us to descriptively characterize the morphogenetic transformations of the alar prosencephalic regions during neurulation and to establish the topologic evolution of the principal areas of the vertebrate telencephalon.

The cells that fill the bill: neural crest and the evolution of craniofacial development

Avian embryos, which have been studied scientifically since Aristotle, continue to persevere as invaluable research tools, especially for our understanding of the development and evolution of the craniofacial skeleton. Whether the topic is beak shape in Darwin's finches or signaling interactions that underlie bone and tooth formation, birds offer advantages for craniofacial biology that uniquely complement the strengths of other vertebrate model systems, such as fish, frogs, and mice. Several papers published during the past few years have helped pinpoint molecular and cellular mechanisms that pattern the face and jaws through experiments that could only have been done together with our feathered friends. Ultimately, such knowledge will be essential for devising novel clinical approaches to treat and/or prevent diseases, injuries, and birth defects that affect the human craniofacial skeleton. Here we review recent insights plucked from avians on key developmental processes that generate craniofacial diversity.

The bHLH transcription factor Olig3 marks the dorsal neuroepithelium of the hindbrain and is essential for the development of brainstem nuclei

The Olig3 gene encodes a bHLH factor that is expressed in the ventricular zone of the dorsal alar plate of the hindbrain. We found that the Olig3(+) progenitor domain encompassed subdomains that co-expressed Math1, Ngn1, Mash1 and Ptf1a. Olig3(+) cells give rise to neuronal types in the dorsal alar plate that we denote as class A neurons. We used genetic lineage tracing to demonstrate that class A neurons contribute to the nucleus of the solitary tract and to precerebellar nuclei. The fate of class A neurons was not correctly determined in Olig3 mutant mice. As a consequence, the nucleus of the solitary tract did not form, and precerebellar nuclei, such as the inferior olivary nucleus, were absent or small. At the expense of class A neurons, ectopic Lbx1(+) neurons appeared in the alar plate in Olig3 mutant mice. By contrast, electroporation of an Olig3 expression vector in the chick hindbrain suppressed the emergence of Lbx1(+) neurons. Climbing fiber neurons of the inferior olivary nucleus express Foxd3 and require Olig3 as well as Ptf1a for the determination of their fate. We observed that electroporation of Olig3 and Ptf1a expression vectors, but not either alone, induced Foxd3. We therefore propose that Olig3 can cooperate with Ptf1a to determine the fate of climbing fiber neurons of the inferior olivary nucleus.

Early mammillary pouch specification in the course of prechordal ventralization of the forebrain tegmentum

The mammillary body, a ventral specialization of the caudal hypothalamus, lies close to the transition between epichordal and prechordal parts of the forebrain (Puelles and Rubenstein, 2003). This report examines its presumed causal connection with either prechordal or notochordal mesodermal induction, as well as the timing of its specification, in the context of early ventral forebrain patterning. It was recently found that the ephrin receptor gene EphA7 is selectively expressed in the mammillary pouch from early stages of development (HH14: García-Calero et al., 2006). We used mammillary EphA7 expression as well as ventral hypothalamic expression of the gene markers Nkx2.1 and Shh to analyze experimental effects on mammillary specification and morphogenesis after axial mesoderm ablation at stages HH4+ to HH6. Progressively delayed ablation of the prechordal plate revealed its sequential implication in molecular specification of the entire ventral forebrain, including the mammillary and tuberal regions of the hypothalamus. We observed differential contact requirements for induction by the prechordal plate of all the forebrain regions expressing Shh and Nkx2.1, including distant subpallial ones. In contrast, ablation of the anterior notochordal tip at these stages did not elicit significant patterning changes, particularly no effects on mammillary EphA7 expression or mammillary pouch development.

Regionalization of the telencephalon in urodele amphibians and its bearing on the identification of the amygdaloid complex

The brain of urodele amphibians has formed the basis for numerous comparative neuroanatomical studies because its simplified arrangement of neurons and fibers was considered to represent the basic pattern common to all tetrapods. However, on the basis of classical histological techniques many common features shared by the brain of amniotes could not be identified in the anamniotic amphibians. Recently, the combined analysis of the chemoarchitecture and hodology has demonstrated that the brain, and particularly the telencephalon, of anuran amphibians shares all major basic features with amniotes. In the present study, we have conducted a series of immunohistochemical detections for telencephalic regional markers (nitric oxide synthase (NOS), gamma-amino butyric acid (GABA), Islet-1 (Isl1), and Nkx2.1) that were useful tools for unraveling telencephalic organization in other vertebrates. In addition, the combination of tract-tracing techniques with dextran amines to demonstrate olfactory secondary centers, hypothalamic projections, and brainstem connections has served to propose subdivisions within the amygdaloid complex. The results of the present analysis of the urodele telencephalon using a multiple approach have demonstrated, among other features, the presence of a ventral pallial region, striatopallidal subdivision in the basal ganglia, and three main components of the amygdaloid complex. Therefore, in spite of its apparently simple organization, within the telencephalon of urodeles it is possible to identify most of the features observed in amniotes and anurans that are only revealed with the use of combined modern techniques in neuroanatomy.

Co-option of signaling mechanisms from neural induction to telencephalic patterning

This article provides an overview of signaling processes during early specification of the anterior neural tube, with special emphasis on the telencephalon. A series of signaling systems based on the action of distinct morphogens acts at different developmental stages, specifying interacting developmental fields that define axes of differentiation in the rostrocaudal and the dorsoventral domains. Interestingly, many of these signaling systems are co-opted for several differentiation processes. This strategy provides a simple and efficient mechanism to generate novel structures in evolution, and may have been especially important in the origin of the telencephalon and the mammalian cerebral cortex. For example, the action of fibroblast growth factor (FGF) secreted in early stages from the anterior neural ridge, but in later stages from the dorsal anterior forebrain, may have been a key factor in the early differentiation of the ventral telencephalon and in the eventual expansion of the mammalian neocortex. Likewise, bone morphogenetic proteins (BMPs) participate at several stages in neural patterning, even if early neural induction consists of the inhibition of the BMP pathway. BMPs, secreted dorsally, interact with FGFs in the frontal aspect of the hemispheres, and with PAX6-dependent signaling sources located laterally, to pattern the dorsal telencephalon. The actions of other morphogens are also described in this context, such as the ventralizing factor SHH, the dorsalizing element GLI3, and other factors related to the dorsomedial telencephalon such as WNTs and EMXs. The main conclusion we draw from this review is the well-known phylogenetic and developmental conservatism of signaling pathways, which in evolution have been applied in different embryological contexts, generating novel interactions between morphogenetic fields and leading to the generation of new morphological structures.

The prethalamus is established during gastrulation and influences diencephalic regionalization

The vertebrate neural plate contains distinct domains of gene expression, prefiguring the future brain areas. In this study, we draw an extended expression map of the rostral neural plate that reveals discrete domains inside the presumptive posterior forebrain. We show, by fate mapping, that these well-defined cell populations will develop into specific diencephalic regions. To address whether these early subterritories are already committed to restricted identities, we began to analyse the consequences of ablation and transplantation of these specific cell populations. We found that precursors of the prethalamus are already specified and irreplaceable at late gastrula stage, because ablation of these cells results in loss of prethalamic markers. Moreover, when transplanted into the ectopic environment of the presumptive hindbrain, these cells still pursue their prethalamic differentiation program. Finally, transplantation of these precursors, in the rostral-most neural epithelium, induces changes in cell identity in the surrounding host forebrain. This cell–non-autonomous property led us to propose that these committed prethalamic precursors may play an instructive role in the regionalization of the developing diencephalon.

During the earliest stages of development, the brain is first formed as a simple sheet of cells called the neural plate. Although the plate looks homogenous, it contains distinct domains that can be identified by differential gene expression. These domains correspond to distinct future brain areas. In this study, we examined gene expression patterns in an area of the neural plate that later forms the forebrain to show that well-defined cell populations will develop into specific forebrain regions, such as the prethalamus, thalamus, hypothalamus, and epithalamus. We then tested whether these early neural plate subterritories are fully committed to a particular forebrain identity. We found that precursors of the prethalamus are not replaceable by other neighbouring cells, because ablation of these cells results in loss of prethalamus development. Moreover, when prethalamus precursors were moved into the environment of the presumptive hindbrain, the cells still pursued their prethalamic differentiation program. Finally, when the prethalamic precursors were moved to areas of the future forebrain, they transformed the surrounding host forebrain. We propose that the committed prethalamic precursors play an instructive role in the regionalization of the developing forebrain.

This study shows that prethalamic identity is established as early as the end of gastrulation, thereby elucidating the developmental stage at which prethalamus identity is assigned to a specific cell population inside the neural plate.

Developmental mechanisms and experimental models to understand forebrain malformative diseases

The development of the central nervous system can be divided into a number of phases, each of which can be subject of genetic or epigenetic alterations that may originate particular developmental disorders. In recent years, much progress has been made in elucidating the molecular and cellular mechanisms by which the vertebrate forebrain develops. Therefore, our understanding of major developmental brain disorders such as cortical malformations and neuronal migration disorders has significantly increased. In this review, we will describe the major stages in forebrain morphogenesis and regionalization, with special emphasis on developmental molecular mechanisms derailing telencephalic development with subsequent damage to cortical function. Because animal models, mainly mouse, have been fundamental for this progress, we will also describe some characteristic mouse models that have been capital to explore these molecular mechanisms of malformative diseases of the human brain. Although most of the genes involved in the regulation of basic developmental processes are conserved among vertebrates, the extrapolation of mouse data to corresponding gene expression and function in humans needs careful individual analysis in each functional system.

Conserved pattern of tangential neuronal migration during forebrain development

Origin, timing and direction of neuronal migration during brain development determine the distinct organization of adult structures. Changes in these processes might have driven the evolution of the forebrain in vertebrates. GABAergic neurons originate from the ganglionic eminence in mammals and migrate tangentially to the cortex. We are interested in differences and similarities in tangential migration patterns across corresponding telencephalic territories in mammals and reptiles. Using morphological criteria and expression patterns of Darpp-32, Tbr1, Nkx2.1 and Pax6 genes, we show in slice cultures of turtle embryos that early cohorts of tangentially migrating cells are released from the medial ganglionic eminence between stages 14 and 18. Additional populations migrate tangentially from the dorsal subpallium. Large cohorts of tangentially migrating neurons originate ventral to the dorsal ventricular ridge at stage 14 and from the lateral ganglionic eminence from stage 15. Release of GABAergic cells from these regions was investigated further in explant cultures. Tangential migration in turtle proceeds in a fashion similar to mammals. In chimeric slice culture and in ovo graft experiments, the tangentially migrating cells behaved according to the host environment - turtle cells responded to the available cues in mouse slices and mouse cells assumed characteristic migratory routes in turtle brains, indicating highly conserved embryonic signals between these distant species. Our study contributes to the evaluation of theories on the origin of the dorsal cortex and indicates that tangential migration is universal in mammals and sauropsids.

Evolutionary modification of mouth position in deuterostomes

In chordates, the oral ectoderm is positioned at the anterior neural boundary and is characterized by pituitary homeobox (Pitx) and overlapping Dlx and Six3 expressions. Recent studies have shown that the ectoderm molecular map is also conserved in hemichordates and echinoderms. However, the mouth develops in a more posterior position in these animals, in a domain characterized by Nkx2.1 and Goosecoid expression, in a manner similar to that observed in protostomes. Furthermore, BMP signaling antagonizes mouth development in echinoderms and hemichordates, but seems to promote oral ectoderm specification in chordates. Conversely, Nodal signaling appears to be required for oral ectoderm specification in sea urchins but not in chordates. The Nodal/BMP antagonism at work during ectoderm patterning thus seems to constitute a conserved feature in deuterostomes, and mouth relocation may have been accompanied by a change in the influence of BMP/Nodal signals on oral ectoderm specification. We suggest that the mouth primordium was located at the anterior neural boundary, in early chordate evolution. In extant chordate embryos, subsequent mouth positioning differ between urochordates and vertebrates, presumably as a consequence of surrounding tissues remodelling. We illustrate these morphogenetic movements by means of morphological data obtained by the confocal imaging of ascidian tailbud embryos, and provide a table for determining the tailbud stages of this model organism.

Chicken lateral septal organ and other circumventricular organs form in a striatal subdomain abutting the molecular striatopallidal border

The avian lateral septal organ (LSO) is a telencephalic circumventricular specialization with liquor-contacting neurons (Kuenzel and van Tienhoven [1982] J. Comp. Neurol. 206:293-313). We studied the topological position of the chicken LSO relative to molecular borders defined previously within the telencephalic subpallium (Puelles et al. [2000] J. Comp. Neurol. 424:409-438). Differential expression of Dlx5 and Nkx2.1 homeobox genes, or the Shh gene encoding a secreted morphogen, allows distinction of striatal, pallidal, and preoptic subpallial sectors. The chicken LSO complex was characterized chemoarchitectonically from embryonic to posthatching stages, by using immunohistochemistry for calbindin, tyrosine hydroxylase, NKX2.1, and BEN proteins and in situ hybridization for Nkx2.1, Nkx2.2, Nkx6.1, Shh, and Dlx5 mRNA. Medial and lateral parts of LSO appear, respectively, at the striatal part of the septum and adjacent bottom of the lateral ventricle (accumbens), in lateral continuity with another circumventricular organ that forms along a thin subregion of the entire striatum, abutting the molecular striatopallidal boundary; we called this the "striatopallidal organ" (SPO). The SPO displays associated distal periventricular cells, which are lacking in the LSO. Moreover, the SPO is continuous caudomedially with a thin, linear ependymal specialization found around the extended amygdala and preoptic areas. This differs from SPO and LSO in some molecular aspects. We tentatively identified this structure as being composed of an "extended amygdala organ" (EAO) and a "preoptohypothalamic organ" (PHO). The position of LSO, SPO, EAO, and PHO within a linear Dlx5-expressing ventricular domain that surrounds the Nkx2.1-expressing pallidopreoptic domain provides an unexpected insight into possible common and differential causal mechanisms underlying their formation.

A dynamic fate map of the forebrain shows how vertebrate eyes form and explains two causes of cyclopia

Mechanisms for shaping and folding sheets of cells during development are poorly understood. An example is the complex reorganisation of the forebrain neural plate during neurulation, which must fold a sheet into a tube while evaginating two eyes from a single contiguous domain within the neural plate. We, for the first time, track these cell rearrangements to show that forebrain morphogenesis differs significantly from prior hypotheses. We postulate a new model for forebrain neurulation and demonstrate how mutations affecting two signalling pathways can generate cyclopic phenotypes by disrupting normal cell movements or introducing new erroneous behaviours.

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.

Dose-dependent functions of Fgf8 in regulating telencephalic patterning centers

Mouse embryos bearing hypomorphic and conditional null Fgf8mutations have small and abnormally patterned telencephalons. We provide evidence that the hypoplasia results from decreased Foxg1 expression,reduced cell proliferation and increased cell death. In addition, alterations in the expression of Bmp4, Wnt8b, Nkx2.1 and Shh are associated with abnormal development of dorsal and ventral structures. Furthermore, nonlinear effects of Fgf8 gene dose on the expression of a subset of genes, including Bmp4 and Msx1, correlate with a holoprosencephaly phenotype and with the nonlinear expression of transcription factors that regulate neocortical patterning. These data suggest that Fgf8 functions to coordinate multiple patterning centers, and that modifications in the relative strength of FGF signaling can have profound effects on the relative size and nature of telencephalic subdivisions.

Organization and evolution of the avian forebrain

Early 20th-century comparative anatomists regarded the avian telencephalon as largely consisting of a hypertrophied basal ganglia, with thalamotelencephalic circuitry thus being taken to be akin to thalamostriatal circuitry in mammals. Although this view has been disproved for more than 40 years, only with the recent replacement of the old telencephalic terminology that perpetuated this view by a new terminology reflecting more accurate understanding of avian brain organization has the modern view of avian forebrain organization begun to become more widely appreciated. The modern view, reviewed in the present article, recognizes that the avian basal ganglia occupies no more of the telencephalon than is typically the case in mammals, and that it plays a role in motor control and motor learning as in mammals. Moreover, the vast majority of the telencephalon in birds is pallial in nature and, as true of cerebral cortex in mammals, provides the substrate for the substantial perceptual and cognitive abilities evident among birds. While the evolutionary relationship of the pallium of the avian telencephalon and its thalamic input to mammalian cerebral cortex and its thalamic input remains a topic of intense interest, the evidence currently favors the view that they had a common origin from forerunners in the stem amniotes ancestral to birds and mammals.

Craniofacial dermoids: an embryological theory unifying nasal dermoid sinus cysts

OBJECTIVE

The nasal dermoid sinus cyst (NDSC) is an uncommon congenital lesion presenting as a large panel of midline craniofacial anomalies. The objective of this study was to review and reanalyze embryological hypotheses concerning NDSCs and to propose an embryological theory unifying the various anatomical characteristics of these lesions. The first case of frontal localization of a NDSC extending within the diploetic bone in a 9-month-old boy, presenting as a median frontal fistula with recurrent frontal swelling, 6 months after a mild frontal trauma is presented.

RESULTS

Complete surgical removal was performed, and there was no evidence of either persistent or recurrent disease 2 years after his surgery. The embryological and anatomical origins of NDSCs are reviewed. This article reexamines and discusses major embryological theories on NDSC pathogenesis and proposes to refute the "prenasal space" theory of Grunwald and rehabilitate a forgotten embryological hypothesis, which unifies the main various clinical presentations of NDSCs.

Correlation of a chicken stage 4 neural plate fate map with early gene expression patterns

A number of gene markers are currently claimed to allow positive or negative visualization of the early chick neural plate at stages 3d/4, when its fate becomes determined. Some markers labeled by various authors as either "neural" or "non-neural" indeed show ectodermal expression patterns roughly correlative with widespread yet vague ideas on the shape and size of the early neural plate, based on previous fate maps. However, for technical reasons, it is not clear how precisely these expression patterns correlate with any experimentally determined fate boundaries. An eventual mismatch between fate and marker interpretation might bear importantly on ideas about gene functions and causal hypotheses in issues such as the establishment of the neural/non-neural border or the earliest mechanisms of neural regionalization. In this review, we correlated a set of epiblastic and mesendodermal gene expression patterns with the novel neuroectoderm proportions suggested by our recent fate map of the chick neural plate at stages HH 3d/4 [P. Fernández-Garre, L. Rodriguez-Gallardo, V. Gallego-Diaz, I.S. Alvarez, L. Puelles, Fate map of the chicken neural plate at stage 4, Development 129 (2002) 2807-2822.]. This analysis suggests the existence of various nested subregions of the epiblast with boundaries codefined by given sets of gene patterns. No gene expression studied reproduces exactly or even approximately the entire neural plate shape, leading to a combinatorial hypothesis on its specification. This kind of analysis (fate and molecular maps), jointly with competence maps, provides the basis for understanding gene functions and the mechanisms of neural induction, specification and regionalization. Several gene patterns observed are consistent with precocious incipient regionalization of the neural plate along the dorsoventral and anteroposterior axes.

Comparative analysis of Six 3 and Six 6 distribution in the developing and adult mouse brain

Six 3 and Six 6 genes are two closely related members of the Six/sine oculis family of homeobox containing transcription factors. Their expression and function at early stages of embryonic development has been widely addressed in a variety of species. However, their mRNA distribution during late embryonic, postnatal, and adult brain barely has been analyzed. Here, we show that despite their initial overlap in the anterior neural plate, the expression of Six 3 and Six 6 progressively segregates to different regions during mammalian brain development, maintaining only few areas of partial overlap in the thalamic and hypothalamic regions. Six 3, but not Six 6, is additionally expressed in the olfactory bulb, cerebral cortex, hippocampus, midbrain, and cerebellum. These distinct patterns support the idea that Six 3 and Six 6 are differentially required during forebrain development.

Thalamic development induced by Shh in the chick embryo

Patterning of the early neural tube is achieved in part by the inductive signals, which arise from neuroepithelial signaling centers. The zona limitans intrathalamica (ZLI) is a neuroepithelial domain in the alar plate of the diencephalon which separates the prethalamus from the thalamus. The ZLI has recently been considered to be a possible secondary organizer, effecting its inductions via sonic hedgehog (Shh), a signaling molecule which drives morphogenetic information for the thalamus. Using experimental embryological techniques involving the generation of chimeric embryos, we show that the formation of the ZLI in the diencephalic alar plate is due to an interaction between the prechordal and epichordal plate neuroepithelia. We also provide evidence that Shh expression in the ZLI underlies the morphogenetic activity of this putative diencephalic organizer. Ectopic Shh led to the auto-induction of its own gene expression in host cells, as well as to the expression of other genes involved in diencephalic regionalization and histogenesis. Analysis of long-term surviving embryos after Shh ectopic expression demonstrated that Shh was able to induce thalamic structures and local overgrowth. Overall, these results indicate that Shh expressed in the ZLI plays an important role in diencephalic growth and in the development of the thalamus.

Agreement and disagreement among fate maps of the chick neural plate

Fate maps are essential to understand embryonic development; they provide a background for deducing maps of differential cellular specification in the context of other experimental data and molecular expression patterns. Due to its accessibility, the chick neural plate has been fate-mapped many times, albeit without complete agreement with respect to its shape, extent and fated subdivisions. In this review, we first comment about avian neural plate fate maps reported since the early period of experimental embryology, referring to the different methods followed. We next review a perfected fate-mapping methodology, which recently allowed us rather precise delimitation of the chick neural plate at stages 3d/4. This leads to a general discussion about the apparent border of the neural plate and the prospective main rostrocaudal and longitudinal divisions of the neural tube.

The rostral and caudal boundaries of the diencephalon

Knowledge of nature and features of the boundaries between the main neural regions seems to be essential to understand the rules of brain regionalization. On the light of several current and classical criteria used to define cerebral boundaries, we examine the features of the places recognized as rostral and caudal boundaries in the developing diencephalon and provide new images about the glial features of these boundaries. One demonstrated property of some embryonic boundaries is the prevention of the crossing cells in the early ventricular zone (clonal restriction), while the intermediate zone seems to lack it. Data available so far indicate that the early boundary between diencephalon and mesencephalon (d/m) is a clonal restriction limit, but not between diencephalon and telencephalon (d/t). Later, while diencephalic nuclei form, cellular dispersion does not occur through the alar part of d/m, but it achieves in the corresponding d/t alar portion. The relationship between origin, migration, and cell-type specification of neural cells is being the object of special attention in the telencephalon, where specific cellular fenotipes can migrate to distant regions following non-radial routes. Such is the case of most GABAergic interneurons of avian and mammalian pallium and oligodendrocytes of the forebrain. In this regard, little attention has been devoted to the diencephalon, where this type of migration, specially those through the rostral boundary, has been reported by different authors. We introduce increasing evidence about non-conventional neuronal migration in the developing diencephalon and compare the reported migratory behavior with respect to both boundaries.

Genesis of alcohol-induced craniofacial dysmorphism

The initial diagnosis of fetal alcohol syndrome (FAS) in the United States was made because of the facial features common to the first cohort of patients. This article reviews the development of an FAS mouse model whose craniofacial features are remarkably similar to those of affected humans. The model is based on short-term maternal treatment with a high dosage of ethanol at stages of pregnancy that are equivalent to Weeks 3 and 4 of human gestation. At these early stages of development, alcohol's insult to the developing face is concurrent with that to the brain, eyes, and inner ear. That facial and central nervous system defects consistent with FAS can be induced by more "realistic" alcohol dosages as illustrated with data from an oral alcohol intake mouse model in which maternal blood alcohol levels do not exceed 200 mg/dl. The ethanol-induced pathogenesis involves apoptosis that occurs within 12 hrs of alcohol exposure in selected cell populations of Day 7, 8, and 9 mouse embryos. Experimental evidence from other species also shows that apoptosis underlies ethanol-induced malformations. With knowledge of sensitive and resistant cell populations at specific developmental stages, studies designed to identify the basis for these differing cellular responses and, therefore, to determine the primary mechanisms of ethanol's teratogenesis are possible. For example, microarray comparisons of sensitive and resistant embryonic cell populations have been made, as have in situ studies of gene expression patterns in the populations of interest. Studies that illustrate agents that are effective in diminishing or exacerbating ethanol's teratogenesis have also been helpful in determining mechanisms. Among these agents are antioxidants, sonic hedgehog protein, retinoids, and the peptides SAL and NAP.

The distribution and cellular localization of glutamic acid decarboxylase-65 (GAD65) mRNA in the forebrain and midbrain of domestic chick

The distribution and cellular localization of GAD65 mRNA in the forebrain and midbrain of domestic chick were examined by in situ hybridization histochemistry with (35)[S]-UTP labeled cRNA probes, using film and emulsion autoradiography. Film autoradiograms showed intense GAD65 labeling in many structures of the basal telencephalon, such as the medial and lateral striatum, the septum, the olfactory tubercle, the lateral bed nucleus of the stria terminalis, and the intrapeduncular nucleus, while the pallial telencephalon showed only a low level of labeling. Emulsion-coated sections revealed that GAD65 mRNA-containing neurons were at least six times more abundant in striatum than pallium, with only a uniformly scattered subpopulation labeled in pallium, and that the vast majority of the large scattered projection neurons of globus pallidus were heavily labeled for GAD65. Prominent labeling was also evident in the nucleus taeniae and subpallial amygdala, but not in the arcopallium in film autoradiograms. Within the diencephalon, the hypothalamus was more GAD65-rich than the thalamus. Additional subtelencephalic cell groups showing prominent labeling included the thalamic reticular nucleus and ventral lateral geniculate nucleus of the diencephalon, the nucleus pretectalis, subpretectalis and spiriformis lateralis of the pretectum, and the magnocellular isthmic nucleus of the optic lobe. Tectal layers 9-10 were also rich in GAD65. These results further clarify GABAergic circuits of the avian forebrain and midbrain, and show them to closely resemble those in mammals.

Regulatory gene expressions in the ascidian ventral sensory vesicle: evolutionary relationships with the vertebrate hypothalamus

In extant chordates, the overall patterning along the anteroposterior and dorsoventral axes of the neural tube is remarkably conserved. It has thus been proposed that four domains corresponding to the vertebrate presumptive forebrain, midbrain-hindbrain transition, hindbrain, and spinal cord were already present in the common chordate ancestor. To obtain insights on the evolution of the patterning of the anterior neural tube, we performed a study aimed at characterizing the expression of regulatory genes in the sensory vesicle of Ciona intestinalis, the anteriormost part of the central nervous system (CNS) related to the vertebrate forebrain, at tailbud stages. Selected genes encoded primarily for homologues of transcription factors involved in vertebrate forebrain patterning. Seven of these genes were expressed in the ventral sensory vesicle. A prominent feature of these ascidian genes is their restricted and complementary domains of expression at tailbud stages. These patterning markers thus refine the map of the developing sensory vesicle. Furthermore, they allow us to propose that a large part of the ventral and lateral sensory vesicle consists in a patterning domain corresponding to the vertebrate presumptive hypothalamus.

Avian brains and a new understanding of vertebrate brain evolution

We believe that names have a powerful influence on the experiments we do and the way in which we think. For this reason, and in the light of new evidence about the function and evolution of the vertebrate brain, an international consortium of neuroscientists has reconsidered the traditional, 100-year-old terminology that is used to describe the avian cerebrum. Our current understanding of the avian brain - in particular the neocortex-like cognitive functions of the avian pallium - requires a new terminology that better reflects these functions and the homologies between avian and mammalian brains.

Avian brains and a new understanding of vertebrate brain evolution

We believe that names have a powerful influence on the experiments we do and the way in which we think. For this reason, and in the light of new evidence about the function and evolution of the vertebrate brain, an international consortium of neuroscientists has reconsidered the traditional, 100-year-old terminology that is used to describe the avian cerebrum. Our current understanding of the avian brain - in particular the neocortex-like cognitive functions of the avian pallium - requires a new terminology that better reflects these functions and the homologies between avian and mammalian brains.

Sensory organs: making and breaking the pre-placodal region

Sensory placodes are unique domains of thickened ectoderm in the vertebrate head that form important parts of the cranial sensory nervous system, contributing to sense organs and cranial ganglia. They generate many different cell types, ranging from simple lens fibers to neurons and sensory cells. Although progress has been made to identify cell interactions and signaling pathways that induce placodes at precise positions along the neural tube, little is known about how their precursors are specified. Here, we review the evidence that placodes arise from a unique territory, the pre-placodal region, distinct from other ectodermal derivatives. We summarize the cellular and molecular mechanisms that confer pre-placode character and differentiate placode precursors from future neural and neural crest cells. We then examine the events that subdivide the pre-placodal region into individual placodes with distinct identity. Finally, we discuss the hypothesis that pre-placodal cells have acquired a state of "placode bias" that is necessary for their progression to mature placodes and how such bias may be established molecularly.

Expression of regulatory genes during differentiation of thalamic nuclei in mouse and monkey

Expression patterns of genes implicated in development of the thalamus were examined in mice and monkeys, using in situ hybridization with RNA probes complementary to Cad6, Dlx1, Dlx2, Dlx5, Gbx2, Id2, and Lef1 cDNAs. Expression patterns were related to the evolving cytoarchitecture in mice at birth (P0) and in adulthood, and in fetal monkeys early and late in the period of gestation when thalamic nuclei are becoming histologically differentiated out of a series of pronuclear masses. At the earlier developmental stage, each gene was expressed in a pattern that appeared to be pronucleus-specific and maintained a nucleus-specific pattern into adulthood, with the possible exception of Gbx2. Each gene displayed a unique expression pattern in the dorsal thalamus, ventral thalamus, and epithalamus, and no gene was expressed throughout all three divisions or in every nucleus of a division. With the exception of Dlx2, whose expression disappeared at the later time point, all continued to be expressed into adulthood at higher levels and with identical patterns. Despite late appearance of gamma-aminobutyric acid (GABA)ergic cells in the dorsal lateral geniculate nucleus of mice, no Dlx genes, which promote formation of a GABAergic phenotype elsewhere, were detected in dorsal thalamus. Each thalamic nucleus was distinguished by expression of a combination of genes, and homologous nuclei in mouse and monkey exhibited the same combination. The presence of a centre médian nucleus and four pulvinar nuclei in monkeys was marked by patterns of expression not found in mice. The centre médian nucleus was marked by high expression of Id2, which was expressed only weakly in very few nuclei of mice.