Prenatal neural origins of infant motor development: Associations between fetal brain and infant motor development.

Functional circuits of the human brain emerge and change dramatically over the second half of gestation. It is possible that variation in neural functional system connectivity in utero predicts individual differences in infant behavioral development, but this possibility has yet to be examined. The current study examines the association between fetal sensorimotor brain system functional connectivity and infant postnatal motor ability. Resting-state functional connectivity data was obtained in 96 healthy human fetuses during the second and third trimesters of pregnancy. Infant motor ability was measured 7 months after birth using the Bayley Scales of Infant Development. Increased connectivity between the emerging motor network and regions of the prefrontal cortex, temporal lobes, posterior cingulate, and supplementary motor regions was observed in infants that showed more mature motor functions. In addition, females demonstrated stronger fetal-brain to infant-behavior associations. These observations extend prior longitudinal research back into prenatal brain development and raise exciting new ideas about the advent of risk and the ontogeny of early sex differences.

Postnatal development of the electrophysiological properties of somatostatin interneurons in the anterior cingulate cortex of mice.

Somatostatin (SST)-positive interneurons in the anterior cingulate cortex (ACC) play important roles in neuronal diseases, memory and cognitive functions. However, their development in the ACC remains unclear. Using postnatal day 3 (P3) to P45 GIN mice, we found that most of the intrinsic membrane properties of SST interneurons in the ACC were developmentally mature after the second postnatal week and that the development of these neurons differed from that of parvalbumin (PV) interneurons in the prefrontal cortex. In addition, electrical coupling between SST interneurons appeared primarily between P12-14. The coupling probability plateaued at approximately P21-30, with a non-age-dependent development of coupling strength. The development of excitatory chemical afferents to SST interneurons occurred earlier than the development of inhibitory chemical afferents. Furthermore, eye closure attenuated the development of electrical coupling probability at P21-30 but had no effect on coupling strength. Eye closure also delayed the development of inhibitory chemical afferent frequency but had no effect on the excitatory chemical afferent amplitude, frequency or rise time. Our data suggest that SST interneurons in the ACC exhibit inherent developmental characteristics distinct from other interneuron subtypes, such as PV interneurons, and that some of these characteristics are subject to environmental regulation.

Subcortical and cortical structural central nervous system changes and attention processing deficits in preschool-aged children with prenatal methamphetamine and tobacco exposure.

OBJECTIVE: To examine the independent contributions of prenatal methamphetamine exposure (PME) and prenatal tobacco exposure (PTE) on brain morphology among a sample of nonalcohol-exposed 3- to 5-year-old children followed prospectively since birth. STUDY DESIGN: The sample included 20 children with PME (19 with PTE) and 15 comparison children (7 with PTE), matched on race, birth weight, maternal education and type of insurance. Subcortical and cortical volumes and cortical thickness measures were derived through an automated segmentation procedure from T1-weighted structural magnetic resonance images obtained on unsedated children. Attention was assessed using the computerized Conners' Kiddie Continuous Performance Test Version 5 (K-CPT™ V.5). PME effects on subcortical and cortical brain volumes and cortical thickness were tested by general linear model with type III sum of squares, adjusting for PTE, prenatal marijuana exposure, age at time of scan, gender, handedness, pulse sequence and total intracranial volume (for volumetric outcomes). A similar analysis was done for PTE effects on subcortical and cortical brain volumes and thickness, adjusting for PME and the above covariates. RESULTS: Children with PME had significantly reduced caudate nucleus volumes and cortical thickness increases in perisylvian and orbital-frontal cortices. In contrast, children with PTE showed cortical thinning in perisylvian and lateral occipital cortices and volumetric increases in frontal regions and decreases in anterior cingulate. PME was positively related and caudate volume was inversely related to K-CPT reaction time by inter-stimulus interval, a measure of the ability to adjust to changing task demands, suggesting that children with PME may have subtle attentional deficits mediated by caudate volume reductions. CONCLUSIONS: Our results suggest that PME and PTE may have distinct differential cortical effects on the developing central nervous system. Additionally, PME may be associated with subtle deficits in attention mediated by caudate volume reductions.

Fetal gyrification in cynomolgus monkeys: a concept of developmental stages of gyrification.

Our article summarizes a series of studies about fetal gyrification and its relation to cerebral growth in cynomolgus monkeys. Based on the cerebral growth (i.e., brain weight, cerebral volume, and frontooccipital length of the cerebral hemisphere) and the developmental pattern of gyrification in each sulcus of cynomolgus monkeys, we divided the gyrification process into four stages: Stage 1. Demarcation of cerebral lobes and limbic gyri; Stage 2. Demarcation of neocortical gyri; Stage 3. Emergence of secondary and tertiary sulci; and Stage 4. Growth of sulcal length and depth. Each stage of those gyrification processes was influenced by different developmental events, such as the emergence of corticocortical long-associative fiber tracts, cortical maturations, and subcortical white-matter development. This is the first report to systematically propose gyrification processes closely related to the order of phyologenetical development of the cerebral cortex in primates.

The von Economo neurons in the frontoinsular and anterior cingulate cortex.

The von Economo neurons (VENs) are large bipolar neurons located in the frontoinsular cortex (FI) and limbic anterior (LA) area in great apes and humans but not in other primates. Our stereological counts of VENs in FI and LA show them to be more numerous in humans than in apes. In humans, small numbers of VENs appear the 36th week postconception, with numbers increasing during the first 8 months after birth. There are significantly more VENs in the right hemisphere in postnatal brains; this may be related to asymmetries in the autonomic nervous system. VENs are also present in elephants and whales and may be a specialization related to very large brain size. The large size and simple dendritic structure of these projection neurons suggest that they rapidly send basic information from FI and LA to other parts of the brain, while slower neighboring pyramids send more detailed information. Selective destruction of VENs in early stages of frontotemporal dementia (FTD) implies that they are involved in empathy, social awareness, and self-control, consistent with evidence from functional imaging.

Malformations of cortical development: current concepts and advanced neuroimaging review.

Malformations of cortical development (MCD) result from disruptions in the complex process of the human brain cortex formation and are highly associated to severe epilepsy, neurodevelopmental delay and motor dysfunction. Nowadays, magnetic resonance imaging (MRI) is the cornerstone of the work-up of patients with epilepsy and modern advanced imaging techniques have improved not only our ability to detect and characterize cortical malformations, but also in identifying associated functional abnormalities that are far beyond the structural visualized lesions. Herein, we address the most currently used classifications of MCD and make a concise review of the embryological process of cortical development. Our main goal is to summarize recent advances and new trends in diagnostic imaging techniques concerning MCD. Thereafter, follows a brief discussion of specific disorders and their radiological features.

Malformations of cortical development: current concepts and advanced neuroimaging review / Malformações do desenvolvimento cortical: conceitos atuais e revisão de neuro-imagem avançada

Malformations of cortical development (MCD) result from disruptions in the complex process of the human brain cortex formation and are highly associated to severe epilepsy, neurodevelopmental delay and motor dysfunction. Nowadays, magnetic resonance imaging (MRI) is the cornerstone of the work-up of patients with epilepsy and modern advanced imaging techniques have improved not only our ability to detect and characterize cortical malformations, but also in identifying associated functional abnormalities that are far beyond the structural visualized lesions. Herein, we address the most currently used classifications of MCD and make a concise review of the embryological process of cortical development. Our main goal is to summarize recent advances and new trends in diagnostic imaging techniques concerning MCD. Thereafter, follows a brief discussion of specific disorders and their radiological features.

As malformações do desenvolvimento cortical (MDC) resultam de distúrbios no complexo processo do desenvolvimento do córtex cerebral humano e estão comumente associadas a epilepsia severa e disfunções neuropsicomotoras. Atualmente, as imagens por ressonância magnética (RM) são a pedra angular no manejo de pacientes com epilepsia e modernas técnicas avançadas de imagem melhoraram não só a nossa capacidade de detectar e caracterizar as malformações corticais, mas também levaram ao reconhecimento de anomalias funcionais associadas que estão muito além das lesões estruturais visibilizadas. Abordaremos as classificações mais utilizadas de MDC e revisaremos a embriologia do desenvolvimento cortical. Nosso principal objetivo é destacar os avanços recentes e as novas tendências em técnicas de diagnóstico por imagens relacionadas às MDC. Em seguida, faremos uma breve discussão sobre alguns transtornos específicos e suas características radiológicas.

Impaired oligodendroglial development by decabromodiphenyl ether in rat offspring after maternal exposure from mid-gestation through lactation.

Pregnant Sprague-Dawley rats were given diet containing decabromodiphenyl ether (DBDE) either at 0, 10, 100, or 1000 ppm from gestation day (GD) 10 until day 20 after delivery (PND 20). No significant alterations were observed in maternal and offspring reproductive parameters. At PND 20, serum triiodothyronine concentrations examined in males were slightly reduced at 1000 ppm (84.2% of the control value), and incidence of thyroid follicular cell hypertrophy was increased in both sexes with significant difference in males at 1000 ppm. Diffuse liver cell hypertrophy accompanying increased relative liver weight and increased cytoplasmic eosinophilia of the renal proximal tubules were observed in both sexes with significant difference from 10 ppm in males and females, respectively. At postnatal week 11, serum thyroxine concentrations examined in males were slightly reduced at 1000 ppm (85.9% of the control value), and the incidence of thyroid follicular cell hypertrophy was non-significantly increased from 10 ppm in males. There were reductions in the corpus callosum area and density of 2',3'-cyclic nucleotide 3'-phosphodiesterase-immunoreactive oligodendrocytes in the cingulate deep cortex in males from 100 ppm. Conversely, NeuN-immunoreactive neuronal distribution in the hippocampal CA1 was unchanged. This suggests that developmental DBDE-exposure caused irreversible white matter hypoplasia targeting oligodendrocytes from 100 ppm, accompanied with developmental hypothyroidism. The lowest-observed-adverse-effect level of DBDE was determined to be 10 ppm (0.7-2.4 mg/kg-body weight-d).

Multiple non-cell-autonomous defects underlie neocortical callosal dysgenesis in Nfib-deficient mice.

BACKGROUND: Agenesis of the corpus callosum is associated with many human developmental syndromes. Key mechanisms regulating callosal formation include the guidance of axons arising from pioneering neurons in the cingulate cortex and the development of cortical midline glial populations, but their molecular regulation remains poorly characterised. Recent data have shown that mice lacking the transcription factor Nfib exhibit callosal agenesis, yet neocortical callosal neurons express only low levels of Nfib. Therefore, we investigate here how Nfib functions to regulate non-cell-autonomous mechanisms of callosal formation. RESULTS: Our investigations confirmed a reduction in glial cells at the midline in Nfib-/- mice. To determine how this occurs, we examined radial progenitors at the cortical midline and found that they were specified correctly in Nfib mutant mice, but did not differentiate into mature glia. Cellular proliferation and apoptosis occurred normally at the midline of Nfib mutant mice, indicating that the decrease in midline glia observed was due to deficits in differentiation rather than proliferation or apoptosis. Next we investigated the development of callosal pioneering axons in Nfib-/- mice. Using retrograde tracer labelling, we found that Nfib is expressed in cingulate neurons and hence may regulate their development. In Nfib-/- mice, neuropilin 1-positive axons fail to cross the midline and expression of neuropilin 1 is diminished. Tract tracing and immunohistochemistry further revealed that, in late gestation, a minor population of neocortical axons does cross the midline in Nfib mutants on a C57Bl/6J background, forming a rudimentary corpus callosum. Finally, the development of other forebrain commissures in Nfib-deficient mice is also aberrant. CONCLUSION: The formation of the corpus callosum is severely delayed in the absence of Nfib, despite Nfib not being highly expressed in neocortical callosal neurons. Our results indicate that in addition to regulating the development of midline glial populations, Nfib also regulates the expression of neuropilin 1 within the cingulate cortex. Collectively, these data indicate that defects in midline glia and cingulate cortex neurons are associated with the callosal dysgenesis seen in Nfib-deficient mice, and provide insight into how the development of these cellular populations is controlled at a molecular level.

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.

Development of cerebral sulci and gyri in fetuses of cynomolgus monkeys (Macaca fascicularis). II. Gross observation of the medial surface.

This study aimed to clarify chronological sequences of the appearances of sulci and gyri on the medial cerebral surface and its relation to the regional development of the cerebrum in cynomolgus monkeys. The lengths of cingulate and calcarine sulci were measured, and the ratios of these lengths to fronto-occipital length were estimated as indices of the size of the "frontoparietal" and "occipital" regions, respectively. The relative length of cingulate sulcus showed a biphasic increase: a slow phase from EDs 100 to 110, and a rapid phase from EDs 110 to 130. The gyri in the "frontoparietal region" were convoluted in the limbic cortex during the initial slow phase and in the neocortical region during the rapid phase. The relative length of calcarine sulcus lineally increased between EDs 90 and 130, and the gyri in the "occipital region" generated in a dorso-ventral manner: the gyrus convolutions occurred first in the "phylogenetically older" striate and dorsal extrastriate cortices, and then in the "phylogenetically newer" ventral extrastriate cortex. The results suggest that the chronological order of appearance of sulci and gyri is closely associated with the order of phylogenetical development of the cerebral cortex. The present study provides a standard reference for the development of cerebral sulci and gyri of cynomolgus monkeys together with our previous study (Fukunishi et al. Anat Embryol 211:757-764, 2006).

Development of cerebral sulci and gyri in fetuses of cynomolgus monkeys (Macaca fascicularis).

This study aimed to clarify the development of sulci and gyri on the external surface of the cerebrum of cynomolgus monkeys. Sulcus formation began with the appearance of the lateral fissure on embryonic day (ED) 70, followed by delineations of four cerebral lobes by the emergence of the parietooccipital sulcus, central sulcus, and preoccipital notch on EDs 80-90. The following primary sulci were then visible until ED 120: the superior temporal sulcus on ED 90; the intraparietal sulcus, lunate sulcus, inferior occipital sulcus, and arcuate sulcus on ED 100; and the principle sulcus on ED 110; the occipitotemporal sulcus, anterior middle temporal sulcus, and superior postcentral dimple on ED 120. These sulci demarcated the superior temporal gyrus on ED 90, the precentral gyrus, supramarginal gyrus, and angular gyrus on ED 100, and the inferior and middle temporal gyri, postocentral gyrus, superior parietal lobule, superior, middle and inferior frontal gyri, and inferior occipital gyrus on ED 120. Except for the intermediate and lateral orbitofrontal sulci, the sulci that appeared on ED 130 and thereafter were not related to the gyrus demarcations. Intriguingly, the brain markedly gained weight on EDs 100 and 120, corresponding to the embryonic ages when almost all gyri were visible. The results suggest that a rapid growth of the cerebrum involves convolutions of the gyri by a regular sequence of the sulcus formation in cynomolgus monkeys. This study further provides a standard of reference for normal development in the cerebral cortical morphology of cynomolgus monkeys.

Leptin deficiency causes pycnotic change in fetal cingulate cortical cells.

Leptin is an obese gene product, and leptin-deficient ob/ob mice develop hyperphagia and reduced locomotor activity. Leptin is thought to be related to brain development, because leptin receptors are widely expressed in the brain, and because brain weight as well as brain protein and DNA contents were reduced in adult ob/ob mice. In this study, we investigated the effect of leptin on the fetal cingulate cortex, since the leptin receptor is expressed in the neurons of the cingulate cortex, which is involved in emotion as well as in sensory, motor, and cognitive processes. The ob/ob fetuses had more pycnotic cells than wild-type fetuses in the cingulate cortex at embryonic day (E) 18. Many pycnotic cells were observed in the intermediate zone of the cingulate cortex. Most cells observed in this area were neuronal lineage cells, while few undifferentiated cells and oligodendrocyte precursor cells were found. At E18 there was no significant difference in the rostrocaudal length of the corpus callosum, which contains the neuronal projection from the cingulate cortex, between ob/ob and wild-type fetuses. We also showed that the length of the cerebrum was greater and the width of the cerebrum and cerebellum were lesser in ob/ob fetuses than in wild-type at E16. These results suggest an increased cell death in neuronal lineage cells in the intermediate zone of the cingulate cortex in leptin-deficient ob/ob mice. Leptin deficiency may also alter the gross morphology of the brain in development, but not the formation of the corpus callosum.

Sonographic appearance of the normal and abnormal insula of Reil.

Three planes are discerned during ultrasonographic screening of the insula in parasagittal view: opercular, insular, and fissural. Six newborn infants with normal brain anatomy, including two each of 28, 34, and 40 weeks' gestation, were selected for a description of the evolution of these parasagittal planes. Opercularization of the insula begins to be detected on a sonogram at about the 24th gestational week and progresses cranially. On coronal section the insular space forms a shallow groove at 24 weeks, becoming a slit at 28 weeks that grows longer and develops branches after 32 weeks. At 28 weeks the ascending anterior branch of the circular groove and the lateral fissure at the bottom of the insula are clearly seen in parasagittal section. Subsequent change consists of undulation and bifurcation of the lateral fissure, together with elongation of the anterior margin. Secondary gyri become visible in the insular dome between 28 and 34 weeks, forming short anterior and long posterior insular gyri. Five term newborn infants with perisylvian polymicrogyria were observed with both neonatal ultrasound and magnetic resonance imaging: two of unknown cause, and one each due to cytomegalovirus, bifunctional peroxisomal protein deficiency, and monozygous twinning. With polymicrogyria, on parasagittal sonographic examination at fissural level the anterior margin of the circular groove is a short rudiment. In insular view, only rudimentary sulci ascend from the lateral fissure; short and long gyri are not seen. In opercular view, no secondary branches are seen from the lateral fissure, an image akin to that seen at 28 weeks. A complete insular triangle is not recognized in any of these sections. A deep abnormal sulcus may prolong the lateral fissure when schizencephaly is associated with polymicrogyria. The insula in glutaric aciduria type II has no secondary gyri at term and is reduced to a simple and small triangle. Not all instances of polymicrogyria lead to macroscopically recognizable alteration of insular gyri.

Developmental regulation of the 5-HT7 serotonin receptor and transcription factor NGFI-A in the fetal guinea-pig limbic system: influence of GCs.

Fetal exposure to excess glucocorticoids (GCs) programs the developing hypothalamo-pituitary-adrenal (HPA) axis, and may predispose offspring to adult-onset disease. During development, serotonin (5-HT) influences transcription of hippocampal GR mRNA via the 5-HT7 receptor. The effect of 5-HT on GR involves the transcription factor NGFI-A. Given the developmental changes which we have previously reported in hippocampal GR mRNA expression, we hypothesized that (1) there are progressive developmental changes in 5-HT7 receptor and NGFI-A mRNA expression in the fetal guinea-pig limbic system, and (2) repeated exposure to synthetic GC treatment will significantly modify developmental expression of these genes. 5-HT7 receptor mRNA was highly expressed in the hippocampus and thalamus at gestational day (gd) 40 (term approximately 70 days), and significantly decreased (P < 0.05) with advancing gestation. Conversely, NGFI-A mRNA expression in the hippocampus and frontal cortex was almost undetectable at gd40, but was dramatically elevated (P < 0.05; 8-fold) near term. Changes in mRNA were refelected by NGFI-A protein levels. These changes were significantly correlated to hippocampal GR expression and fetal plasma cortisol concentrations. Synthetic GC treatment increased NGFI-A mRNA levels in CA1 and the cingulate cortex, but had no effect on 5-HT7 receptor expression. In conclusion our results suggest that (1) limbic 5-HT7 receptor expression is not directly linked to maturation of hippocampal GR in late gestation; (2) the up-regulation of NGFI-A expression near term is driven by glucocorticoid; and (3) premature exposure to synthetic glucocorticoid significantly increases NGFI-A-related transcriptional activity in the fetal limbic system.

Paracingulate sulcus morphology in men with early-onset schizophrenia.

BACKGROUND: Cingulate dysfunction has been reported in schizophrenia. Although the paracingulate sulcus (PCS) is known to be asymmetric in healthy people, little information is available about its morphology in schizophrenia. AIMS: To search for morphological anomalies of the PCS in men with early-onset schizophrenia. METHOD: The PCS was examined in magnetic resonance images of the brains of men with schizophrenia and 100 healthy men. RESULTS: A significant asymmetry was found in the brains of healthy volunteers, whose sulci were more frequent and more marked in the left hemisphere. In contrast, the sulcus was as frequent in the right as in the left hemisphere in the patient group. Moreover, patients displayed significantly more rightward asymmetry, and overall less-asymmetrical patterns than the comparison group. CONCLUSIONS: Since the PCS has developed at 36 weeks of gestation, these findings suggest an impaired maturation of the cingulate region during the third trimester.

Diaphorase-positive neurons in the cingulate cortex of human fetuses during the second half of gestation.

In this work, the time course of appearance, distribution and morphology of diaphorase-positive neurons were studied in the developing cingulate cortex of the human brain during the second half of gestation. Five human fetuses at 18, 20, 25, 30 and 35 weeks postovulatory (wpo) were examined. The brain tissue was reacted by an indirect histochemistry protocol for detection of NADPH-diaphorase activity. Labeled neurons were identified at the microscope and documented photographically or by computer-aided charts. We have found that heavily labeled neurons (type I) first appear in the subplate (SP) between 20 and 25 wpo, and in the cortical plate (CP) between 25 and 35 wpo. By 35 wpo, CP neurons were both type I and type II (lightly labeled neurons). In addition, we observed 4 different morphological types among subplate neurons, very similar to callosally-projecting subplate cells (as described previously by our group). We concluded that medial nitridergic neurons of humans appear prenatally according to the usual gradient of cortical maturation -- first in the subplate and later in the cortical plate. Also, we suggest that some of the diaphorase-positive neurons in the transient subplate could possibly be callosal.

Prenatal cocaine exposure increases mesoprefrontal dopamine neuron responsivity to mild stress.

Children whose mothers used cocaine during pregnancy appear to have an increased incidence of certain neurobehavioral deficits. Rodent models of prenatal cocaine exposure have mimicked these deficits in the offspring, yet the biochemical basis of the behavioral abnormalities is unknown. We have been able to reproduce short-term memory deficits in our rat intravenous model of prenatal cocaine exposure, and as short-term memory is dependent on the function of dopamine neurons innervating the medial prefrontal cortex, we hypothesized that prenatal cocaine induces a dysfunction in the regulation of this pathway. Here we report that mild footshock stress, which preferentially activates the mesoprefrontal dopamine system, leads to an enhanced increase in dopamine turnover in the ventromedial prefrontal cortex of adolescent (postnatal day 35-37) rats exposed to cocaine in utero, suggesting that the dopamine neurons innervating this region are hyperresponsive in these rats. Thus, this biochemical alteration may be central to some of the cognitive deficits exhibited by offspring that were exposed to cocaine during fetal development.

The spindle neurons are present in the cingulate cortex of chimpanzee fetus.

We report the existence of the spindle neurons in layer Vb of the anterior cingulate cortex (Brodman's area 24b) in a chimpanzee fetus (embryonic day 224), which was stillborn. About 5.3% of neuronal cells in layer Vb were the spindle neurons at this stage. The width of the spindle neurons was 10-15 microm. In layer V of the prefrontal cortex (Brodman's area 46) in the chimpanzee fetus, and in layer Vb of the cingulate cortex in adult macaque monkeys, no spindle neurons were observed. The immunoreactivity against brain-derived neurotrophic factor (BDNF) was detected only in the pyramidal neurons at this stage. These findings suggest that the existence of the spindle neurons in layer Vb of the anterior cingulate cortex and the presence of BDNF in the pyramidal neurons are intrinsically characterized during the embryonic stage of the chimpanzee.

A role for cingulate pioneering axons in the development of the corpus callosum.

In many vertebrate and invertebrate systems, pioneering axons play a crucial role in establishing large axon tracts. Previous studies have addressed whether the first axons to cross the midline to from the corpus callosum arise from neurons in either the cingulate cortex (Koester and O'Leary [1994] J. Neurosci. 11:6608-6620) or the rostrolateral neocortex (Ozaki and Wahlsten [1998] J. Comp. Neurol. 400:197-206). However, these studies have not provided a consensus on which populations pioneer the corpus callosum. We have found that neurons within the cingulate cortex project axons that cross the midline and enter the contralateral hemisphere at E15.5. By using different carbocyanine dyes injected into either the cingulate cortex or the neocortex of the same brain, we found that cingulate axons crossed the midline before neocortical axons and projected into the contralateral cortex. Furthermore, the first neocortical axons to reach the midline crossed within the tract formed by these cingulate callosal axons, and appeared to fasciculate with them as they crossed the midline. These data indicate that axons from the cingulate cortex might pioneer a pathway for later arriving neocortical axons that form the corpus callosum. We also found that a small number of cingulate axons project to the septum as well as to the ipsilateral hippocampus via the fornix. In addition, we found that neurons in the cingulate cortex projected laterally to the rostrolateral neocortex at least 1 day before the neocortical axons reach the midline. Because the rostrolateral neocortex is the first neocortical region to develop, it sends the first neocortical axons to the midline to form the corpus callosum. We postulate that, together, both laterally and medially projecting cingulate axons may pioneer a path for the medially directed neocortical axons, thus helping to guide these axons toward and across the midline during the formation of the corpus callosum.