Prenatal stress and the developing brain: Risks for neurodevelopmental disorders.

The prenatal period is increasingly considered as a crucial target for the primary prevention of neurodevelopmental and psychiatric disorders. Understanding their pathophysiological mechanisms remains a great challenge. Our review reveals new insights from prenatal brain development research, involving (epi)genetic research, neuroscience, recent imaging techniques, physical modeling, and computational simulation studies. Studies examining the effect of prenatal exposure to maternal distress on offspring brain development, using brain imaging techniques, reveal effects at birth and up into adulthood. Structural and functional changes are observed in several brain regions including the prefrontal, parietal, and temporal lobes, as well as the cerebellum, hippocampus, and amygdala. Furthermore, alterations are seen in functional connectivity of amygdalar-thalamus networks and in intrinsic brain networks, including default mode and attentional networks. The observed changes underlie offspring behavioral, cognitive, emotional development, and susceptibility to neurodevelopmental and psychiatric disorders. It is concluded that used brain measures have not yet been validated with regard to sensitivity, specificity, accuracy, or robustness in predicting neurodevelopmental and psychiatric disorders. Therefore, more prospective long-term longitudinal follow-up studies starting early in pregnancy should be carried out, in order to examine brain developmental measures as mediators in mediating the link between prenatal stress and offspring behavioral, cognitive, and emotional problems and susceptibility for disorders.

The COUP-TFII/Neuropilin-2 is a molecular switch steering diencephalon-derived GABAergic neurons in the developing mouse brain.

The preoptic area (POa) of the rostral diencephalon supplies the neocortex and the amygdala with GABAergic neurons in the developing mouse brain. However, the molecular mechanisms that determine the pathway and destinations of POa-derived neurons have not yet been identified. Here we show that Chicken ovalbumin upstream promoter transcription factor II (COUP-TFII)-induced expression of Neuropilin-2 (Nrp2) and its down-regulation control the destination of POa-derived GABAergic neurons. Initially, a majority of the POa-derived migrating neurons express COUP-TFII and form a caudal migratory stream toward the caudal subpallium. When a subpopulation of cells steers toward the neocortex, they exhibit decreased expression of COUP-TFII and Nrp2. The present findings show that suppression of COUP-TFII/Nrp2 changed the destination of the cells into the neocortex, whereas overexpression of COUP-TFII/Nrp2 caused cells to end up in the medial part of the amygdala. Taken together, these results reveal that COUP-TFII/Nrp2 is a molecular switch determining the pathway and destination of migrating GABAergic neurons born in the POa.

COUP-TFII controls amygdala patterning by regulating neuropilin expression.

The development of the progenitor zones in the pallium, lateral ganglionic eminence (LGE) and medial ganglionic eminence (MGE) in the subpallium has been well studied; however, so far the role of the caudal ganglionic eminence (CGE), a posterior subpallial domain, in telencephalon patterning remains poorly understood. COUP-TFII, an orphan nuclear receptor, is preferentially expressed in the CGE. We generated COUP-TFII mouse mutants, using Rx-Cre (RxCre;COUP-TFII(F/F)), to study its function in telencephalon development. In these mutants, we found severe defects in the formation of the amygdala complex, including the lateral (LA), basolateral (BLA) and basomedial (BMA) amygdala nuclei. Molecular analysis provided evidence that the migration of CGE-derived Pax6(+) cells failed to settle into the BMA nucleus, owing to reduced expression of neuropilin 1 (Nrp1) and Nrp2, two semaphorin receptors that regulate neuronal cell migration and axon guidance. Our ChIP assays revealed that Nrp1 and Nrp2 genes are the direct targets of COUP-TFII in the telencephalon in vivo. Furthermore, our results showed that the coordinated development between the CGE originated subpallial population (Pax6(+) cells) and pallial populations (Tbr1(+) and Lhx2(+) cells) was essential for patterning the amygdala assembly. Our study presented novel genetic evidence that the caudal ganglionic eminence, a distinct subpallial progenitor zone, contributes cells to the basal telencephalon, such as the BMA nucleus.

Embryonic Origin of the Islet1 and Pax6 Neurons of the Chicken Central Extended Amygdala Using Cell Migration Assays and Relation to Different Neuropeptide-Containing Cells.

In a recent study, we tentatively identified different subdivisions of the central extended amygdala (EAce) in chicken based on the expression of region-specific transcription factors (including Pax6 and Islet1) and several phenotypic markers during embryonic development. Such a proposal was partially based on the suggestion that, similarly to the subdivisions of the EAce of mammals, the Pax6 and Islet1 neurons of the comparable chicken subdivisions derive from the dorsal (Std) or ventral striatal embryonic domains (Stv), respectively. To investigate whether this is true, in the present study, we carried out cell migration assays from chicken Std or Stv combined with immunofluorescence for Pax6 or Islet1. Our results showed that the cells of the proposed chicken EAce truly originate in either Std (expressing Pax6) or Stv (expressing Islet1). This includes lateral subdivisions previously compared to the intercalated amygdalar cells and the central amygdala of mammals, also rich in Std-derived Pax6 cells and/or Stv-derived Islet1 cells. In the medial region of the chicken EAce, the dorsal part of the lateral bed nucleus of the stria terminalis (BSTL) contains numerous cells expressing Nkx2.1 (mostly derived from the pallidal domain), but our migration assays showed that it also contains neuron subpopulations from the Stv (expressing Islet1) and Std (expressing Pax6), resembling the mouse BSTL. These findings, together with those previously published in different species of mammals, birds and reptiles, support the homology of the chicken EAce to that of other vertebrates, and reinforce the existence of several cell subcorridors inside the EAce. In addition, together with previously published data on neuropeptidergic cells, these results led us to propose the existence of at least seventeen neuron subtypes in the EAce in rodents and/or some birds (chicken and pigeon). The functional significance and the evolutionary origin of each subtype needs to be analyzed separately, and such studies are mandatory in order to understand the multifaceted modulation by the EAce of fear responses, ingestion, motivation and pain in different vertebrates.

Contribution of genoarchitecture to understanding forebrain evolution and development, with particular emphasis on the amygdala.

The amygdala is a forebrain center involved in functions and behaviors that are critical for survival (such as control of the neuroendocrine system and homeostasis, and reproduction and fear/escape responses) and in cognitive functions such as attention and emotional learning. In mammals, the amygdala is highly complex, with multiple subdivisions, neuronal subtypes, and connections, making it very difficult to understand its functional organization and evolutionary origin. Since evolution is the consequence of changes that occurred in development, herein we review developmental data based on genoarchitecture and fate mapping in mammals (in the mouse model) and other vertebrates in order to identify its basic components and embryonic origin in different species and understand how they changed in evolution. In all tetrapods studied, the amygdala includes at least 4 components: (1) a ventral pallial part, characterized by expression of Lhx2 and Lhx9, that includes part of the basal amygdalar complex in mammals and a caudal part of the dorsal ventricular ridge in sauropsids and also produces a cell subpopulation of the medial amygdala; (2) a striatal part, characterized by expression of Pax6 and/or Islet1, which includes the central amygdala in different species; (3) a pallidal part, characterized by expression of Nkx2.1 and, in amniotes, Lhx6, which includes part of the medial amygdala, and (4) a hypothalamic part (derived from the supraoptoparaventricular domain or SPV), characterized by Otp and/or Lhx5 expression, which produces an important subpopulation of cells of the medial extended amygdala (medial amygdala and/or medial bed nucleus of the stria terminalis). Importantly, the size of the SPV domain increases upon reduction or lack of Nkx2.1 function in the hypothalamus. It appears that Nkx2.1 expression was downregulated in the alar hypothalamus during evolution to mammals, which may have produced an enlargement of SPV and the amygdalar cell subpopulation derived from it.

Emx1-lineage progenitors differentially contribute to neural diversity in the striatum and amygdala.

In the developing mammalian basal telencephalon, neural progenitors from the subpallium generate the majority of inhibitory medium spiny neurons (MSNs) in the striatum, while both pallial- and subpallial-derived progenitors contribute to excitatory and inhibitory neuronal diversity in the amygdala. Using a combination of approaches, including genetic fate mapping, cell birth dating, cell migration assays, and electrophysiology, we find that cells derived from the Emx1 lineage contribute to two distinct neuronal populations in the mature basal forebrain: inhibitory MSNs in the striatum and functionally distinct subclasses of excitatory neurons in the amygdala. Our cell birth-dating studies reveal that these two populations are born at different times during early neurogenesis, with the amygdala population born before the MSNs. In the striatum, Emx1-lineage neurons represent a unique subpopulation of MSNs: they are disproportionately localized to the dorsal striatum, are found in dopamine receiving, reelin-positive patches, and are born throughout striatal neurogenesis. In addition, our data suggest that a subpopulation of these Emx1-lineage cells originate in the pallium and subsequently migrate to the developing striatum and amygdala. Our intersectional fate-mapping analysis further reveals that Emx1-lineage cells that coexpress Dlx exclusively generate MSNs but do not contribute to the excitatory neurons in the amygdala. Thus, both the timing of neurogenesis and differential combinatorial gene expression appear to be key determinants of striatal versus amygdala fate decisions of Emx1-lineage cells.

High fetal testosterone and sexually dimorphic cerebral networks in females.

Active masculinization by fetal testosterone is believed to be a major factor behind sex differentiation of the brain. We tested this hypothesis in a 15O-H2O positron emission tomography study of 11 women with congenital adrenal hyperplasia (CAH), a condition with high fetal testosterone, and 26 controls. Two indices of cerebral dimorphism were measured--functional connectivity and cerebral activation by 2 putative pheromones (androstadienone [AND] and estratetraenol [EST]), previously reported to activate the hypothalamic networks in a sex-differentiated manner. Smelling of unscented air was the baseline condition, also used for measurements of functional connectivity from the amygdala. In CAH women and control women AND activated the anterior hypothalamus, and EST the amygdala, piriform, and anterior insular cortex. The pattern was reciprocal in the male controls. Also the functional connections were similar in CAH women and control women, but different in control men. Women displayed connections with the contralateral amygdala, cingulate, and the hypothalamus, men with the basal ganglia, the insular and the sensorimotor cortex. Furthermore, the connections were in CAH and control women more widespread from the left amygdala, in men from the right amygdala. Thus, we find no evidence for masculinization of the limbic circuits in women with high fetal testosterone.

The role of DCX and LIS1 in migration through the lateral cortical stream of developing forebrain.

During forebrain development the lateral cortical stream (LCS) supplies neurons to structures in the ventral telencephalon including the amygdala and piriform cortex. In the current study, we used spatially directed in utero electroporation and RNAi to investigate mechanisms of migration to the ventral telencephalon. Cells labeled by in utero electroporation of the lateral ventricular zone migrated into the LCS, and entered the lateral neocortex, piriform cortex and amygdala, where they differentiated primarily as pyramidal neurons. RNAi of DCX or LIS1 disrupted migration into amygdala and piriform cortex and caused many neurons to accumulate in the external and amygdalar capsules. RNAi of LIS1 and DCX had similar as well as distinguishable effects on the pattern of altered migration. Combinatorial RNAi of LIS1 and DCX further suggested interaction in the functions of LIS1 and DCX on the morphology and migration of migrating neurons in the LCS. Together, these results confirm that the LCS contributes pyramidal neurons to ventral forebrain structures and reveals that DCX and LIS1 have important functions in this major migratory pathway in the developing forebrain.

Dynamic patterns of colocalization of calbindin, parvalbumin and GABA in subpopulations of mouse basolateral amygdalar cells during development.

Calbindin cells represent a major interneuron subtype of the cortical/pallial regions, such as the basolateral amygdala, which are often analyzed in studies of tangential migration of interneurons from the subpallial ganglionic eminences to the pallium/cortex. However, previous evidence suggests that during development the calbindin cells may include more than one of the interneuron subtypes found in the adult pallium/cortex. Furthermore, in the adult basolateral amygdala, calbindin cells include a subpopulation of non-GABAergic (non-interneuron) cells. To better characterize these cells throughout development, in the present study we investigated the colocalization of calbindin, parvalbumin and GABA in cells of the mouse basolateral amygdala during late embryonic (E16.5) and several postnatal ages from birth until 4 weeks after birth (P0, P10 and P28). Our results indicate that CB, PV and GABA show a dynamic pattern of colocalization in cells of the mouse basolateral amygdalar nucleus throughout development. From E16.5 through P28, the majority of CB+ neurons and virtually all PV+ neurons are GABAergic. However, after P10, the percentage of GABAergic CB+ cells decline from 96% to 70%. Furthermore, while only 9% of CB+ neurons are PV+ at P10, this percentage raises to 42% at P28. At all postnatal ages studied, the majority of the PV+ cells are CB+, suggesting that PV+ interneurons develop postnatally mainly as a subpopulation within the CB+ cells of the basolateral amygdalar nucleus. These results are important for interpreting data from interneuron migration.

Distinct immunohistochemically defined areas in the medial amygdala in the developing and adult mouse.

The medial amygdala has been considered a subpallial structure, but various studies have shown that is a somewhat complex structure expressing both pallial and subpallial gene markers. In this regard, we analyzed the immunohistochemical expression of the neurotransmitter GABA, the vesicular glutamate transporter type 2 (VGLUT2), the neuronal nitric oxide synthase (nNOS), and the calcium-binding proteins calbindin-D28k (CB) and calretinin (CR) in the developing and adult mouse medial amygdala. From intermediate embryonic stages on, neurochemical data show a distinctive superficial region forming a band all along the medial amygdalar surface. This superficial band displays a strong VGLUT2-immunoreactive neuropil and numerous CR-immunoreactive fibers, as well as some nNOS-, CR- or CB-positive cells. In contrast, the superficial region of the posterior medial amygdala appears to be non-GABA immunoreactive. This band in the posterior medial amygdala matches a Tbr1-expressing territory. Our results also show differences between dorsal and ventral parts of the postnatal and adult posterior medial amygdala. Especially, a compact cell aggregate of nNOS immunoreactive cells was found in the ventral portion of the medial amygdala, whereas the dorsal part is occupied by scattered weakly stained cells. Comparison of our results with gene expression patterns and fiber-tracing studies, let us to propose that the superficial band is a pallial derivative, whereas the dorsal and ventral nuclei of the posterior medial amygdala receive each neurons from different subpallial compartments, and the latter one a subset of excitatory, pallial projection neurons.

Expression of cLhx6 and cLhx7/8 suggests a pallido-pedunculo-preoptic origin for the lateral and medial parts of the avian bed nucleus of the stria terminalis.

We investigated the origin of the avian bed nucleus of the stria terminalis (BST) and other parts of the avian subpallial amygdala, by studying the expression of the LIM-homeobox chick genes Lhx6 (cLhx6) and Lhx7/8 (cLhx7/8) in the embryonic chicken telencephalon. Our results indicate that these genes are expressed in a subpallial subdomain partially overlapping the expression of Nkx2.1, which includes pallidal, peduncular, commissural preoptic and pallidoseptal subdivisions comparable to those of mammals. The lateral and medial parts of the avian BST express cLhx6 and/or cLhx7/8, suggesting that they derive from the Nkx2.1-expressing subpallial domain. Our results indicate that the avian lateral BST (BSTL) contains two components, a dorsal part rich in cLhx6 and lacking cLhx7/8 expression that may derive from the pallidal subdivision, and a ventral part showing moderate or light expression of cLhx6 and cLhx7/8, which may derive from the peduncular subdivision. Moreover, the medial BST (BSTM1 and BSTM2) shows moderate to strong expression of cLhx6 and very strong expression of cLhx7/8 throughout development, and appears to derive from both the peduncular and the commissural preoptic subdivisions. Based on this, the avian dorsal BSTL appears comparable to the mammalian BSTL, whereas the avian ventral BSTL and at least part of BSTM may be comparable to the anterior and posteromedial parts of the mammalian BSTM. We also identified a ventrolateral portion of BSTM (BSTM3) and other cell corridors expressing cLhx6 and/or cLhx7/8 in chicken and propose their homology with specific parts of the extended amygdala of mammals.

Prenatal stress generates deficits in rat social behavior: Reversal by oxytocin.

Neurodevelopmental changes induced by environmental stress exposure play a significant but poorly defined role in the etiology of schizophrenia. Exposure of pregnant female rats to a series of unpredictable stresses during the final week of pregnancy generates behavioral deficits and molecular changes in the offspring similar to those observed in schizophrenic individuals. We used this rat prenatal stress preparation to investigate social withdrawal behaviors that may have relevance to the negative symptoms of schizophrenia. The cumulative time adult male offspring of stress-exposed pregnant female rats actively interacted with a weight-matched, same-sex peer was decreased approximately 76% relative to non-stress exposed control rats. Prenatal stress exposure also diminished the quality of the social interaction behavior indicative of reduced social drive. Analysis of the oxytocinergic system in the prenatally stressed male rats revealed significantly less oxytocin mRNA in the paraventricular nucleus and increased oxytocin receptor binding in the central amygdala. Moreover, oxytocin, but not vasopressin, administration into the central amygdala reversed the social incompetence of the prenatally stressed rats without increasing behavior in non-stressed control animals. In addition, cross-fostering pups from prenatally stressed mothers to non-stressed mothers failed to improve the social deficit of the prenatally stressed male offspring. Two behavioral assays designed to measure anxiety did not differentiate the prenatally stressed rats from non-stressed controls. These data indicate that prenatal stress may be an etiologically appropriate animal model for some aspects of schizophrenic social withdrawal. Furthermore, unpredictable prenatal stress exposure selectively degrades social interaction behaviors without increasing anxiety measures.

Genoarchitecture of the extended amygdala in zebra finch, and expression of FoxP2 in cell corridors of different genetic profile.

We used a battery of genes encoding transcription factors (Pax6, Islet1, Nkx2.1, Lhx6, Lhx5, Lhx9, FoxP2) and neuropeptides to study the extended amygdala in developing zebra finches. We identified different components of the central extended amygdala comparable to those found in mice and chickens, including the intercalated amygdalar cells, the central amygdala, and the lateral bed nucleus of the stria terminalis. Many cells likely originate in the dorsal striatal domain, ventral striatal domain, or the pallidal domain, as is the case in mice and chickens. Moreover, a cell subpopulation of the central extended amygdala appears to originate in the prethalamic eminence. As a general principle, these different cells with specific genetic profiles and embryonic origin form separate or partially intermingled cell corridors along the extended amygdala, which may be involved in different functional pathways. In addition, we identified the medial amygdala of the zebra finch. Like in the chickens and mice, it is located in the subpallium and is rich in cells of pallido-preoptic origin, containing minor subpopulations of immigrant cells from the ventral pallium, alar hypothalamus and prethalamic eminence. We also proposed that the medial bed nucleus of the stria terminalis is composed of several parallel cell corridors with different genetic profile and embryonic origin: preoptic, pallidal, hypothalamic, and prethalamic. Several of these cell corridors with distinct origin express FoxP2, a transcription factor implicated in synaptic plasticity. Our results pave the way for studies using zebra finches to understand the neural basis of social behavior, in which the extended amygdala is involved.

Regulation of aromatase expression in the anterior amygdala of the developing mouse brain depends on ERß and sex chromosome complement.

During development sex differences in aromatase expression in limbic regions of mouse brain depend on sex chromosome factors. Genes on the sex chromosomes may affect the hormonal regulation of aromatase expression and this study was undertaken to explore that possibility. Male E15 anterior amygdala neuronal cultures expressed higher levels of aromatase (mRNA and protein) than female cultures. Furthermore, treatment with oestradiol (E2) or dihydrotestosterone (DHT) increased Cyp19a1 expression and aromatase protein levels only in female neuronal cultures. The effect of E2 on aromatase expression was not imitated by oestrogen receptor (ER) α agonist PPT or the GPER agonist G1, but it was fully reproduced by DPN, a specific ligand of ERß. By contrast, the effect of DHT on aromatase expression was not blocked by the anti-androgen flutamide, but completely abrogated by the ERß antagonist PHTPP. Experiments using the four core genotype model showed a sex chromosome effect in ERß expression (XY > XX) and regulation by E2 or DHT (only XX respond) in amygdala neurons. In conclusion, sex chromosome complement governs the hormonal regulation of aromatase expression through activation of ERß in developing mouse brain.

Developmental exposure to an environmental PCB mixture delays the propagation of electrical kindling from the amygdala.

Developmental PCB exposure impairs hearing and induces brainstem audiogenic seizures in adult offspring. The degree to which this enhanced susceptibility to seizure is manifest in other brain regions has not been examined. Thus, electrical kindling of the amygdala was used to evaluate the effect of developmental exposure to an environmentally relevant PCB mixture on seizure susceptibility in the rat. Female Long-Evans rats were dosed orally with 0 or 6mg/kg/day of the PCB mixture dissolved in corn oil vehicle 4 weeks prior to mating and continued through gestation and up until postnatal day (PND) 21. On PND 21, pups were weaned, and two males from each litter were randomly selected for the kindling study. As adults, the male rats were implanted bilaterally with electrodes in the basolateral amygdala. For each animal, afterdischarge (AD) thresholds in the amygdala were determined on the first day of testing followed by once daily stimulation at a standard 200µA stimulus intensity until three stage 5 generalized seizures (GS) ensued. Developmental PCB exposure did not affect the AD threshold or total cumulative AD duration, but PCB exposure did increase the latency to behavioral manifestations of seizure propagation. PCB exposed animals required significantly more stimulations to reach stage 2 seizures compared to control animals, indicating attenuated focal (amygdala) excitability. A delay in kindling progression in the amygdala stands in contrast to our previous finding of increased susceptibility to brainstem-mediated audiogenic seizures in PCB-exposed animals in response to a an intense auditory stimulus. These seemingly divergent results are not unexpected given the distinct source, type, and mechanistic underpinnings of these different seizure models. A delay in epileptogenesis following focal amygdala stimulation may reflect a decrease in neuroplasticity following developmental PCB exposure consistent with reductions in use-dependent synaptic plasticity that have been reported in the hippocampus of developmentally PCB exposed animals.

Selective requirement of Pax6, but not Emx2, in the specification and development of several nuclei of the amygdaloid complex.

The amygdaloid complex is a group of nuclei that are thought to originate from multiple sites of the dorsal and ventral telencephalic neuroepithelium. The mechanisms that regulate their development are essentially unknown. We studied the role of Pax6 and Emx2, two transcription factors that regulate regional specification and growth of the telencephalon, in the morphogenesis of the amygdaloid complex. We used a set of specific marker genes that identify distinct amygdaloid nuclei to analyze Pax6/Small eye and Emx2 knock-out mutant mouse brains. We found that there is a selective requirement for Pax6, but not Emx2, in the formation a subset of nuclei within the amygdaloid complex. Specifically, structures that were not previously considered to be developmentally linked, the nucleus of the lateral olfactory tract and the lateral, basolateral, and basomedial nuclei, all appear to have a common requirement for Pax6. Together, our findings provide new insights into the origins and mechanisms underlying the development of the amygdaloid complex.

Perinatal corticosteroid effect on amygdala and hippocampus volume during brain development in the rat model.

Exposure of the fetus to corticosteroid during brain development has been suggested to cause permanent change in brain structure and has been associated with long term cognitive, behavioral and emotional impairment. We evaluated the effect of perinatal corticosteroid, at a dose similar to that which human fetuses are exposed, on cerebral cortex, corpus collosum, hippocampus, dentate gyrus and amygdala in a rat model. Rat pups were given betamethasone at day 1 (P1). Brain sections from the rat pups at postnatal day 45 (P45) were then analyzed. No differences were noted in the volumes of cerebral cortex, corpus collosum, hippocampus, dentate gyrus, or three nuclei of the amygdala compared to the control and sham groups. We concluded that a single course of betamethasone, at a comparable dose to that which the human fetus is exposed in clinical practice, had no effect on these regional brain volumes at this stage of development.

LIM genes parcellate the embryonic amygdala and regulate its development.

The mechanisms that regulate the development of the amygdaloid complex are as yet poorly understood. Here, we show that in the absence of the LIM-homeodomain (LIM-HD) gene Lhx2, a particular amygdaloid nucleus, the nucleus of the lateral olfactory tract (nLOT), is selectively disrupted. LIM family members are well suited for multiple roles in the development of complex structures because they participate in regulatory interactions that permit a diversity of function. To investigate the possible role for other LIM-HD genes as well as LIM-only (Lmo) genes in the developing amygdala, we examined their expression in the embryo. We show that amygdaloid nuclei upregulate distinct patterns of LIM gene expression from embryonic stages. This supports the hypothesis that LIM genes may participate in the mechanisms that control the development of the amygdala. The disruption of the nLOT in the Lhx2 mutant is the first evidence of a role for LIM-HD genes in the development of the amygdaloid complex. The combinatorial expression patterns of LIM genes suggest a comprehensive mechanism for patterning this structure.

Cortical excitatory neurons and glia, but not GABAergic neurons, are produced in the Emx1-expressing lineage.

By homologous recombination of an internal ribosome entry site and Cre recombinase coding region into the 3'-untranslated region of the mouse Emx1 gene, we have generated a strain of mice, Emx1(IRES)cre, that expresses the Cre recombinase in a spatial and temporal pattern like that observed for Emx1. When mated to reporter strains, these mice are a sensitive means to fate-map the Emx1-expressing cells of the developing forebrain. Our results demonstrate that radial glia, Cajal-Retzius cells, glutamatergic neurons, astrocytes, and oligodendrocytes of most pallial structures originate from an Emx1-expressing lineage. On the other hand, most of the pallial GABAergic neurons arise outside the Emx1-expressing lineage. Structures that are located near the basal ganglia (e.g., the amygdala and endopiriform nuclei) are not uniformly derived from Emx1-expressing cells.