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PubMed1997

PMP-22 expression in the central nervous system of the embryonic mouse defines potential transverse segments and longitudinal columns.

PMP-22, a major constituent of peripheral nervous system (PNS) myelin, is also present in the central nervous system (CNS), in motoneurons of the cranial nerve motor nuclei and spinal cord (Parmantier et al. [1995] Eur. J. Neurosci. 7:1080-1088). The expression of PMP-22 in the CNS during embryonic and early postnatal development was investigated and showed a biphasic spatio-temporal pattern. The expression of PMP-22 started at embryonic day (E)11.5, in restricted longitudinal and transverse domains, in the ventricular zone of the spinal cord, rhombencephalon, mesencephalon and prosencephalon. In the mid- and forebrain, the PMP-22 signal was detectable in a longitudinal domain that followed ventrally the basal/alar boundary but could no longer be detected dorsally at some distance from the roof plate. Along the caudo-rostral axis, the territory in which PMP-22 was detected spanned the mesencephalon and the prosencephalon, extending caudally from the limit between the isthmus and the mesencephalon, and rostrally to the boundary between prosomeres 4 and 5 (p4/p5). In agreement with the prosomeric model of forebrain organization proposed by Puelles and Rubenstein ([1993] TINS 16:472-479), differences in the level of PMP-22 expression in p2, p3, and p4 clearly defined the p2/p3 and p3/p4 neuromeric boundaries. By E17.5, PMP-22 was no longer detected in the ventricular zone, but at E18.5 it began to be expressed in motoneurons of cranial nerve motor nuclei and, after birth, following a rostro-caudal gradient, in the ventral spinal cord.

Animals↗
PubMed2000

Expression and putative role of neuropilin-1 in the early scaffold of axon tracts in embryonic Xenopus brain.

Although the general principles of axon guidance in vitro are understood, little is known about how axons respond to the myriad of cues in vivo and navigate axon pathways within the complex milieu of the embryonic brain. Although neuropilin-1 is an axon guidance receptor for chemorepulsive ligands in the class 3 subfamily of semaphorins, its role in directing axon growth in vivo is unknown. In the present study, we have examined the expression and role of neuropilin-1 in the embryonic forebrain of Xenopus. Neuropilin-1 was selectively expressed by a subset of axons in the early scaffold of axon tracts. These axons arise from the presumptive telencephalic nucleus, cross the rostral midline by means of the postoptic commissure, and enter the major longitudinal tract of the prosencephalon, the tract of the postoptic commissure. At the level of the mesencephalon, these axons diverge and enter one of two axon tracts: either the ventral longitudinal tract or the ventral commissure. This same population of axons also expresses NOC-2, a novel glycoform of the neural cell adhesion molecule N-CAM. We have previously revealed the presence of a chemorepulsive activity underlying the pathway followed by these axons as they cross the ventral commissure. When neuropilin-1 was overexpressed after blastomere injections of synthetic RNA transcripts, NOC-2 axons entered the ventral commissure but failed to cross the midline. Instead, these axons were inhibited from growing ventrally within the commissural pathway. These results suggest that the level of neuropilin-1 in the NOC-2 subpopulation of axons is critical for determining whether these axons reach the ventral midline. Thus, neuropilin-1 may a specific role in directing the growth of NOC-2 axons across the ventral midline in the early embryonic mesencephalon.

Animals↗
PubMed1996

Initial organization of neurons and tracts in the embryonic mouse fore- and midbrain.

We investigated the potential role of rostral-caudal and dorsal-ventral subdivisions of the early rostral brain by relating these subdivisions to the early patterning of neuron cell bodies and their axon projections. The earliest neurons were mapped using the lipophilic axon tracers diI and diO on embryos fixed on embryonic days 9.5-10.5 (E9.5-E10.5); neuromeric boundaries were marked by diO. The tracts were small in number, were organized orthogonally (2 dorsal-ventral and 4 rostral-caudal), and originated from groups of cell bodies which we term "sources." Two parallel longitudinal axon systems, one dorsal (the tract of the postoptic commissure and the mesencephalic tract of the trigeminal nerve) and one ventral (the mammillotegmental tract and the medial longitudinal fasciculus), projected caudally from the prosencephalon into the rhombencephalon. We argue that the dorsal longitudinal pathway marked the boundary between the alar and basal plates along the entire neuraxis. The dorsal-ventral axons coursed circumferentially and either crossed the midline (forming the posterior and ventral tegmental commissures) or turned caudally without crossing the midline. The dorsal-ventral axons were not generally restricted to the interneuromeric boundaries, as others have suggested. Earlier, all neighboring neurons projected their axons together; later, nearby neurons projected into different pathways. Some tracts originated in single neuromeres, while other tracts had origins in two or more neuromeres. The dorsal longitudinal axons altered course at several of the borders, but the ventral longitudinal axons did not. In summary, the early subdivisions appeared to influence some, but not all, aspects of tract formation.

Animals↗
PubMed2001

Dynamic domains of gene expression in the early avian forebrain.

The expression domains of genes implicated in forebrain patterning often share borders at specific anteroposterior positions. This observation lies at the heart of the prosomeric model, which proposes that such shared borders coincide with proposed compartment boundaries and that specific combinations of genes expressed within each compartment are responsible for its patterning. Thus, genes such as Emx1, Emx2, Pax6, and qin (Bf1) are seen as being responsible for specifying different regions in the forebrain (diencephalon and telencephalon). However, the early expression of these genes, before the appearance of putative compartment boundaries, has not been characterized. In order to determine whether they have stable expression domains before this stage, we have compared mRNA expression of each of the above genes, relative both to one another and to morphological landmarks, in closely staged chick embryos. We find that, between HH stage 8 and HH stage 13, each of the genes has a dynamic spatial and temporal expression pattern. To test for autonomy of gene expression in the prosencephalon, we grafted tissue from this region to more caudal positions in the neural tube and analyzed for expression of Emx1, Emx2, qin, or Pax6. We find that gene expression is autonomous in prosencephalic tissue from as early as HH stage 8. In the case of Emx1, our data suggest that, from as early stage 8, presumptive telencephalic tissue also is committed to express this gene. We propose that early patterning along the anteroposterior axis of the presumptive telencephalon occurs across a field that is subdivided by different combinations of genes, with some overlapping areas, but without either sharp boundaries or stable interfaces between expression domains.

Amino Acid Sequence↗
PubMed1996

Otlx2, an Otx-related homeobox gene expressed in the pituitary gland and in a restricted pattern in the forebrain.

The vertebrate nervous system forms by the specification of, successively, neuroepithelial regions and cell groups. One of the proposed major histogenic steps is the subdivision of the neural tube in compartments along its caudorostral and dorsoventral axis. This event is reflected, and may be directed, by the restricted expression of many transcription factors. Here, we report on the isolation of a new homeobox gene of the paired superclass, Otlx2, whose early expression pattern in the mesencephalon and prosencephalon is congruent with proposed neuromeric models of brain morphogenesis. In addition, its late embryonic and postnatal expression, in clear continuity with the earlier pattern, suggests a role in the neuronal differentiation and the histogenesis of several prosencephalic and mesencephalic areas. Finally, Otlx2 is expressed from the earliest morphogenetic events in the Rathke's pouch, the anlage of the adenohypophysis, an expression site shared by a very close homologue, Otlx1/Ptx1/P-Otx.

Amino Acid Sequence↗
PubMed1994

Three-dimensional reconstructions of the developing forebrain in rat embryos.

Using a computerized three-dimensional reconstruction technique with serially sectioned rat embryos, changes in the size and form of the forebrain were studied on Embryonic Days (E) 12 (1 day after closure of the neural tube), E15, E18, and E21 (2 days before birth). During this time, the forebrain changes from a relatively simple tubular structure with thin walls surrounding a large ventricular system to a thick-walled brain with a highly convoluted but reduced ventricular system. On E12, the two components of the forebrain, the telencephalon and the diencephalon, cannot be distinguished. Considering the forebrain as a whole (the embryonic prosencephalon), its volume continually increases between E12 and E21 due to the generation, differentiation, and maturation of neurons and glia. Attention was paid to changes in the sizes of the ventricles, the neuroepithelium and the parenchyma. Volumes of the ventricles and the surrounding neuroepithelium rapidly expanded from E12 to E18 and then decreased by E21, while the volume of the parenchyma continually increased. Differential growth of the telencephalon and that of the diencephalon were compared between E15 and E21. The expansion of the telencephalon was much larger than that of the diencephalon. In the telencephalon, the volumes of the lateral ventricles and the surrounding neuroepithelium increased between E15 and E18 and decreased by E21, while in the diencephalon the volumes of the third ventricle and its surrounding neuroepithelium continually declined between E15 and E21. That observation is compatible with previous work showing that the majority of diencephalic structures develop earlier than those in the telencephalon. It is important to note that volume changes in the ventricles and the neuroepithelium are maintained in "lock-step," suggesting a close relationship between the size of the ventricle and the size of the neuroepithelium.

Animals↗
PubMed2001

Aprosencephaly: histopathological features of the rudimentary forebrain and retina.

Aprosencephaly/atelencephaly is an extremely rare central nervous system defect of the human fetus, which has been attributed to an encephaloclastic process. We report here the histopathological features of the rudimentary forebrain and retina of a case of aprosencephaly in a male fetus that was examined at 20 weeks of gestation, in which there was no evidence of destruction. The prosencephalon was excessively small and replaced by a solid, cylindrical mass without a hemispheric cleavage or a ventricle. Histologically, the rudimentary forebrain tissue consisted of primitive cells that had accumulated in the anterior portion, and of differentiated neurons and glia that were distributed in a random-manner in the posterior portion. The most rostral portion of the tissue penetrated the sphenoidal bone through a single canal: an extracranial ectopic nest consisted of primitive cells that were located at the posterior portion of the face. There were no eyelid openings, eyeballs, or optic nerves, although a nest of dysplastic retinal rosettes and two nests of melanin-pigmented epithelium were identified. This case seems to represent a primary malformation, and the features imply that the failure occurs after neurulation and before the prosencephalic medium began its growth.

Abnormalities, Multiple↗
PubMed1998

Atelencephalic microcephaly: a case report and review of the literature.

UNLABELLED: Atelencephalic microcephaly is a lethal form of abnormal cerebral development. In atelencephaly there is a rudimentary prosencephalon; in aprosencephaly, a more severe form of cerebral malformation, both prosencephalic and diencephalic derivatives fail to develop; both conditions form the aprosencephaly/atelencephaly spectrum (AAS). In the literature 20 cases with atelencephaly or aprosencephaly have been described. Except for the brain malformation other congenital abnormalities seem to be present more often in patients with aprosencephaly. In two patients (one with atelencephaly and one with aprosencephaly) an aberration of chromosome 13 was found. We report on a prematurely born microcephalic male infant with a severely malformed calvarium with overlying rugged skin, non-fused cranial sutures, absent fontanelles, and multiple contractures. CT scan of the brain revealed neither cerebral hemispheres, nor ventricles and a diagnosis of atelencephalic microcephaly was made. In the literature two sibs have been described, products of consanguineous parents, who were the only ones with cerebellar dysgenesis. Aprosencephaly/atelencephaly spectrum in combination with cerebellar dysgenesis seems to be an autosomal recessive syndrome. CONCLUSIONS: Atelencephalic microcephaly is a distinct entity and should be differentiated from anencephaly and the fetal brain disruption sequence. The aetiology of the disorder is unknown.

Humans↗
PubMed1992

Regional expression of the homeobox gene Nkx-2.2 in the developing mammalian forebrain.

A novel mouse homeobox-containing gene, Nkx-2.2, has been isolated. Nkx-2.2 is a member of a family of genes whose homeodomains are homologous to that of the Drosophila NK-2 gene. Nkx-2.2 transcripts are found in localized domains of the brain during mouse embryogenesis. Nkx-2.2 expression in the brain abuts and partially overlaps with the expression domains of two other related homeobox-containing genes, TTF-1 and Dlx. The expression domains of the three genes in the developing prosencephalon coincide with anatomical boundaries, particularly apparent in the diencephalon. This result raises the possibility that these genes may specify regional differentiation of the developing diencephalon into its anatomically and functionally defined subregions. Nkx-2.2 may be involved in specifying diencephalic neuromeric boundaries.

Animals↗
PubMed2004

Involvement of Pax6 and Otx2 in the forebrain-specific regulation of the vertebrate homeobox gene ANF/Hesx1.

During early vertebrate development, ANF homeobox genes are expressed in the prospective forebrain. Their regulation is essential for correct morphogenesis and function of the prosencephalon. We identified a 1-kb fragment upstream of the chicken GANF gene sufficient to drive lacZ expression in the endogenous expression domain. Concordant with the high conservation of this sequence in five investigated species, this element is also active in the corresponding expression domain of the zebrafish orthologue. In vivo analysis of two in vitro-identified Otx2 binding sites in this conserved sequence revealed their necessity for activation of the chicken ANF promoter. In addition, we identified a Pax6-binding site close to the transcriptional start site that is occupied in vivo by Pax6 protein. Pax6 and GANF exhibit mutually exclusive expression domains in the anterior embryonic region. Overexpression of Pax6 in chick embryos inhibited the endogenous GANF expression, and in Pax6(-/-) mice the expression domain of the murine ANF orthologue Hesx1 was expanded and sustained, indicating inhibitory effects of Pax6 on GANF. However, a mutation of the Pax6 site did not abolish reporter activity from an electroporated vector. We conclude that Otx2 and Pax6 are key molecules involved in conserved mechanisms of ANF gene regulation.

Amino Acid Sequence↗
PubMed1997

The effect of unilateral neurotoxic lesions to serotonin fibres in the medial forebrain bundle on the metabolism of [3H]DOPA in the telencephalon of the living rat.

We used quantitative autoradiography to measure the contribution of the 5-hydroxytryptamine (5-HT, serotonin) innervation of rat telencephalon to the synthesis of dopamine (DA) from exogenous L-DOPA. One week after stereotaxic infusions of 5,7-dihydroxy-tryptamine (5,7-DHT, 1.6 micrograms) into the right medial forebrain bundle (MFB), rats received [3H]DOPA (200 microCi,i.v.), which circulated for 90 min. The specific bindings in vitro of the 5-HT uptake site ligand [3H]citalopram and the DA uptake site ligand [125I]RTI-55 were measured in cryostat sections from the prosencephalon. In most structures ipsilateral to the lesion, [3H]citalopram specific binding was substantially reduced (50-90%). In the lateral habenula specific binding declined by only 30-40%, reflecting the presence of a 5-HT pathway deviating from the MFB at the mesencephalic flexure. [125I]RTI-55 binding in the basal ganglia was reduced by 50% on the side of the 5,7-DHT lesion, but was unperturbed in rats pretreated with desmethylimipramine (DMI). 5,7-DHT infusions decreased the synthesis of [3H]DA from [3H]DOPA in vivo in the basal ganglia by (40-90%). Pretreatment with DMI protected [3H]DA synthesis in the basal ganglia, but not in the olfactory tubercle and amygdala ipsilateral to the lesion. Whereas the 5-HT innervation does not contribute greatly to [3H]DA synthesis in the basal ganglia, a substantial proportion of [3H]DA synthesis in olfactory tubercle and amygdala requires an intact 5-HT innervation.

5,7-Dihydroxytryptamine↗
PubMed2003

Forebrain gene expression domains and the evolving prosomeric model.

The prosomeric model attributes morphological meaning to gene expression patterns and other data in the forebrain. It divides this territory into the same transverse segments (prosomeres) and longitudinal zones in all vertebrates. The axis and longitudinal zones of this model are widely accepted but controversy subsists about the number of prosomeres and their nature as segments. We describe difficulties encountered in establishing continuity between prosomeric limits postulated in the hypothalamus and intra-telencephalic limits. Such difficulties throw doubt on the intersegmental nature of these limits. We sketch a simplified model, in which the secondary prosencephalon (telencephalon plus hypothalamus) is a complex protosegment not subdivided into prosomeres, which exhibits patterning singularities. By contrast, we continue to postulate that prosomeres p1-p3 (i.e. the pretectum, thalamus and prethalamus) are the caudal forebrain.

Animals↗
PubMed2001

Brain segmentation and forebrain development in amniotes.

This essay contains a general introduction to the segmental paradigm postulated for interpreting morphologically cellular and molecular data on the developing forebrain of vertebrates. The introduction examines the nature of the problem, indicating the role of topological analysis in conjunction with analysis of various developmental cell processes in the developing brain. Another section explains how morphological analysis in essence depends on assumptions (paradigms), which should be reasonable and well founded in other research, but must remain tentative until time reveals their necessary status as facts for evolving theories (or leads to their substitution by alternative assumptions). The chosen paradigm affects many aspects of the analysis, including the sectioning planes one wants to use and the meaning of what one sees in brain sections. Dorsoventral patterning is presented as the fundament for defining what is longitudinal, whereas less well-understood anteroposterior patterning results from transversal regionalization. The concept of neural segmentation is covered, first historically, and then step by step, explaining the prosomeric model in basic detail, stopping at the diencephalon, the extratelencephalic secondary prosencephalon, and the telencephalon. A new pallial model for telencephalic development and evolution is presented as well, updating the proposed homologies between the sauropsidian and mammalian telencephalon.

Animals↗
PubMedDate not supplied

Continuity and discontinuity of the radial scaffolding in the forebrain of a lizard embryo.

We have used ventricular injections of the fluorescent carbocyanine dye DiI (Molecular Probes, Eugene, OR, USA) to demonstrate the scaffolding of the radial ventricular cells in the embryonic forebrain of lizards. The results reveal changes of density, orientation, or thickness of the radial processes between adjacent regions. On the whole, they support the idea that the hypothalamus together with the telencephalic stalk and the telencephalic hemispheres make up the anterior region of the neural tube or secondary prosencephalon, as proposed by the prosomeric model. In contrast, the thalamic eminence seems part of the diencephalon.

Animals↗
PubMed1998

Distribution and effects of testosterone on aromatase mRNA in the quail forebrain: a non-radioactive in situ hybridization study.

A number of studies have been devoted to the analysis of the anatomical distribution, control by steroids and functional significance of aromatase (the enzyme metabolizing testosterone into 17beta-estradiol) in the quail brain. In particular, the sexually dimorphic nucleus preopticus medialis has been the main focus of investigation because testosterone aromatization in this structure mediates the activation of male sexual behavior and aromatase activity is itself testosterone-dependent in this nucleus. No information on the anatomical distribution of aromatase gene expression is, however, available so far in this avian species. In the present study we applied a non-radioactive in situ hybridization technique to describe the distribution of aromatase mRNA containing neurons in the quail prosencephalon. We also analyzed, at a neuronal level of resolution, the induction by testosterone of this mRNA in the medial preoptic nucleus. Dense clusters of aromatase gene expressing neurons were observed within the medial preoptic nucleus, the nucleus of the stria terminalis, the ventro-medial hypothalamus and the tuberal region. Scattered neurons expressing lower levels of aromatase mRNA were also found in the dorsal thalamic area and central gray. The specificity of the staining was confirmed by demonstrating the absence of signal in sections that had been hybridized with a sense probe. Moreover, the distribution of the aromatase mRNA containing cells completely overlapped with the distribution of the aromatase-immunoreactive cells. Aromatase-mRNA expression was controlled by testosterone (or its metabolites) in the entire medial preoptic nucleus. Castration resulted in a decrease in the number of aromatase mRNA-containing cells and this effect was totally reversed by testosterone treatment. These data further support the idea that testosterone regulates the rate of its own aromatization by modulating the expression of aromatase rather than by acting at a post transcriptional level.

Animals↗
PubMed2000

The organizer factors Chordin and Noggin are required for mouse forebrain development.

In mice, there is evidence suggesting that the development of head and trunk structures is organized by distinctly separated cell populations. The head organizer is located in the anterior visceral endoderm (AVE) and the trunk organizer in the node and anterior primitive streak. In amphibians, Spemann's organizer, which is homologous to the node, partially overlaps with anterior endoderm cells expressing homologues of the AVE markers cerberus, Hex and Hesx1. For mice, this raises the question of whether the AVE and node are independent of each other, as suggested by their anatomical separation, or functionally interdependent as is the case in amphibians. Chordin and Noggin are secreted bone morphogenetic protein (BMP) antagonists expressed in the mouse node, but not in the AVE. Here we show that mice double-homozygous mutants that are for chordin and noggin display severe defects in the development of the prosencephalon. The results show that BMP antagonists in the node and its derivatives are required for head development.

Animals↗
PubMed1998

Correlation between tectum formation and expression of two PAX family genes, PAX7 and PAX6, in avian brains.

Heterotopic transplantation of brain vesicles between chick and quail were performed, and the correlation between tectum formation and the expression of two PAX family genes, PAX7 and PAX6, analyzed. Reciprocal transplantation between the prosencephalon and mesencephalon showed that formation of the tectum always coincided with induction/maintenance of PAX7 and suppression of PAX6, indicating that switch-on or -off of these two PAX family genes in region specific manners are responsible for the differentiation of brain vesicles into the tectum. On the other hand, transplantation of the mesencephalic floor plate into the dorsal mesencephalon suppressed PAX7 expression in the dorsal mesencephalon and changed its fate from the tectum to the tegmentum, indicating that factors in the mesencephalic floor plate suppress PAX7 and limit tectum territory to the dorsal part of the mesencephalon.

Animals↗
PubMed2006

The cephalic neural crest exerts a critical effect on forebrain and midbrain development.

Encephalisation is the most important characteristic in the evolutionary transition leading from protochordates to vertebrates. This event has coincided with the emergence of a transient and pluripotent structure, the neural crest (NC), which is absent in protochordates. In vertebrates, NC provides the rostral cephalic vesicles with skeletal protection and functional vascularization. The surgical extirpation of the cephalic NC, which is responsible for building up the craniofacial skeleton, results in the absence of facial skeleton together with severe defects of preotic brain development, leading to exencephaly. Here, we have analyzed the role of the NC in forebrain and midbrain development. We show that (i) NC cells (NCC) control Fgf8 expression in the anterior neural ridge, which is considered the prosencephalic organizer; (ii) the cephalic NCC are necessary for the closure of the neural tube; and (iii) NCC contribute to the proper patterning of genes that are expressed in the prosencephalic and mesencephalic alar plate. Along with the development of the roof plate, NCC also concur to the patterning of the pallial and subpallial structures. We show that the NC-dependent production of FGF8 in anterior neural ridge is able to restrict Shh expression to the ventral prosencephalon. All together, these findings support the notion that the cephalic NC controls the formation of craniofacial structures and the development of preotic brain.

Animals↗