Brain microstructural development in neonates with critical congenital heart disease: An atlas-based diffusion tensor imaging study.

BACKGROUND: Brain microstructural maturation progresses rapidly in the third trimester of gestation and first weeks of life, but typical microstructural development may be influenced by the presence of critical congenital heart disease (CHD). OBJECTIVE: The aim of this study was to investigate the pattern of white matter (WM) microstructural development in neonates with different types of critical CHD. The secondary aim was to examine whether there is an association between WM microstructural maturity and neonatal ischemic brain injury. METHODS: For this prospective, longitudinal cohort study, 74 term born neonates underwent diffusion tensor imaging (DTI) before (N = 56) and after (N = 71) cardiac surgery performed <30 days of life for transposition of the great arteries (TGA), single ventricle physiology with aortic arch obstruction (SVP-AO), left- (LVOTO) or right ventricle outflow tract obstruction (RVOTO). Microstructural integrity was investigated by fractional anisotropy (FA) and by mean diffusivity (MD) in 16 white matter (WM) structures in three WM regions with correction for postmenstrual age. Ischemic brain injury was defined as moderate-severe white matter injury or stroke. RESULTS: Before cardiac surgery, the posterior parts of the corona radiata and internal capsule showed significantly higher FA and lower MD compared to the anterior parts. Centrally-located WM structures demonstrated higher FA compared to peripherally-located structures. Neonates with TGA had higher FA in projection-, association- and commissural WM before surgery, when compared to other CHD groups. Neonates with LVOTO showed lower preoperative MD in these regions, and neonates with SVP-AO higher MD. Differences in FA/MD between CHD groups were most clear in centrally located WM structures. Between CHD groups, no differences in postoperative FA/MD or in change from pre- to postoperative FA/MD were seen. Neonatal ischemic brain injury was not associated with pre- or postoperative FA/MD. CONCLUSIONS: Collectively, these findings revealed brain microstructural WM development to follow the same organized pattern in critical CHD as reported in healthy and preterm neonates, from posterior-to-anterior and central-to-peripheral. Neonates with TGA and LVOTO showed the most mature WM microstructure before surgery and SVP-AO the least mature. Degree of WM microstructural immaturity was not associated with ischemic brain injury.

One diffusion acquisition and different white matter models: how does microstructure change in human early development based on WMTI and NODDI?

White matter microstructural changes during the first three years of healthy brain development are characterized using two different models developed for limited clinical diffusion data: White Matter Tract Integrity (WMTI) metrics from Diffusional Kurtosis Imaging (DKI) and Neurite Orientation Dispersion and Density Imaging (NODDI). Both models reveal a non-linear increase in intra-axonal water fraction and in tortuosity of the extra-axonal space as a function of age, in the genu and splenium of the corpus callosum and the posterior limb of the internal capsule. The changes are consistent with expected behavior related to myelination and asynchrony of fiber development. The intra- and extracellular axial diffusivities as estimated with WMTI do not change appreciably in normal brain development. The quantitative differences in parameter estimates between models are examined and explained in the light of each model's assumptions and consequent biases, as highlighted in simulations. Finally, we discuss the feasibility of a model with fewer assumptions.

Assessing sequence and relationship of regional maturation in corpus callosum and internal capsule in preterm and term newborns by diffusion-tensor imaging.

BACKGROUND: Diffusion-tensor imaging (DTI) can be used to investigate water diffusion in living tissue. OBJECTIVE: To investigate sequence and relationship of regional maturation in corpus callosum (CC) and internal capsule (IC) in preterm and term. METHODS: DTI was performed on 11 preterm infants at less than 37 weeks of corrected gestational age (group I), 21 preterm infants at equivalent-term (group II), 11 term infants during neonatal period (group III). Apparent diffusion coefficient (ADC) and fractional anisotropy (FA) were measured in: anterior limb of IC (ALIC), posterior limb of IC (PLIC), genu and splenium of CC. RESULTS: FA in splenium was more than that in other regions except genu of group I. Differences of FA between genu and PLIC were significant only in group III. ADC in genu was more than that in other regions but in splenium of groups I and II. Differences of ADC between splenium and ALIC were insignificant except group II. Higher FA and lower ADC in PLIC were gotten compared with those in ALIC. Correlations of FA and of ADC existed in CC and IC. CONCLUSION: Maturation sequence was splenium followed by genu, then by PLIC and last by ALIC in term at neonatal period. Genu's maturation in preterm at equivalent-term was hindered. Regional maturation's correlations existed in CC and IC.

Diffusional kurtosis imaging of the developing brain.

BACKGROUND AND PURPOSE: Diffusional kurtosis imaging is an extension of DTI but includes non-Gaussian diffusion effects, allowing more comprehensive characterization of microstructural changes during brain development. Our purpose was to use diffusional kurtosis imaging to measure age-related microstructural changes in both the WM and GM of the developing human brain. MATERIALS AND METHODS: Diffusional kurtosis imaging was performed in 59 subjects ranging from birth to 4 years 7 months of age. Diffusion metrics, fractional anisotropy, and mean kurtosis were collected from VOIs within multiple WM and GM structures and subsequently analyzed with respect to age. Diffusional kurtosis tractography images at various stages of development were also generated. RESULTS: Fractional anisotropy and mean kurtosis both showed age-related increases in all WM regions, reflecting progression of diffusional anisotropy throughout development, predominantly in the first 2 years of life (eg, 70% and 157% increase in fractional anisotropy and mean kurtosis, respectively, from birth to 2 years for the splenium). However, mean kurtosis detected continued microstructural changes in WM past the fractional anisotropy plateau, accounting for more delayed isotropic changes (eg, 90% of maximum fractional anisotropy was reached at 5 months, whereas 90% of maximum mean kurtosis occurred at 18 months for the external capsule). Mean kurtosis may also provide greater characterization of GM maturation (eg, the putamen showed no change in fractional anisotropy but an 81% change in mean kurtosis from birth to 4 years 7 months). CONCLUSIONS: Mean kurtosis detects significant microstructural changes consistent with known patterns of brain maturation. In comparison with fractional anisotropy, mean kurtosis may offer a more comprehensive evaluation of age-related microstructural changes in both WM and GM and is potentially a valuable technique for studying brain development.

DTI values in key white matter tracts from infancy through adolescence.

BACKGROUND AND PURPOSE: DTI is an advanced neuroimaging technique that allows in vivo quantification of water diffusion properties as surrogate markers of the integrity of WM microstructure. In our study, we investigated normative data from a large number of pediatric and adolescent participants to examine the developmental trends in DTI during this conspicuous WM maturation period. MATERIALS AND METHODS: DTI data in 202 healthy pediatric and adolescent participants were analyzed retrospectively. Fractional anisotropy and mean diffusivity values in the corpus callosum and internal capsule were fitted to an exponential regression model to delineate age-dependent maturational changes across the WM structures. RESULTS: The DTI metrics demonstrated characteristic exponential patterns of progression during development and conspicuous age-dependent changes in the first 36 months, with rostral WM tracts experiencing the highest slope of the exponential function. In contrast, the highest final FA and lowest MD values were detected in the splenium of the corpus callosum and the posterior limb of the internal capsule. CONCLUSIONS: Our analysis shows that the more caudal portions of the corpus callosum and internal capsule begin the maturation process earlier than the rostral regions, but the rostral regions develop at a more accelerated pace, which may suggest that rostral regions rely on development of more caudal brain regions to instigate their development. Our normative DTI can be used as a reference to study normal spatiotemporal developmental profiles in the WM and help identify abnormal WM structures in patient populations.

Early gray-matter and white-matter concentration in infancy predict later language skills: a whole brain voxel-based morphometry study.

Magnetic resonance imaging (MRI) brain scans were obtained from 19 infants at 7 months. Expressive and receptive language performance was assessed at 12 months. Voxel-based morphometry (VBM) identified brain regions where gray-matter and white-matter concentrations at 7 months correlated significantly with children's language scores at 12 months. Early gray-matter concentration in the right cerebellum, early white-matter concentration in the right cerebellum, and early white-matter concentration in the left posterior limb of the internal capsule (PLIC)/cerebral peduncle were positively and strongly associated with infants' receptive language ability at 12 months. Early gray-matter concentration in the right hippocampus was positively and strongly correlated with infants' expressive language ability at 12 months. Our results suggest that the cerebellum, PLIC/cerebral peduncle, and the hippocampus may be associated with early language development. Potential links between these structural predictors and infants' linguistic functions are discussed.

Tractography of developing white matter of the internal capsule and corpus callosum in very preterm infants.

OBJECTIVES: To investigate in preterm infants associations between Diffusion Tensor Imaging (DTI) parameters of the posterior limb of the internal capsule (PLIC) and corpus callosum (CC) and age, white matter (WM) injury and clinical factors. METHODS: In 84 preterm infants DTI was performed between 40-62 weeks postmenstrual age on 3 T MR. Fractional anisotropy (FA), apparent diffusion coefficient (ADC) values and fibre lengths through the PLIC and the genu and splenium were determined. WM injury was categorised as normal/mildly, moderately and severely abnormal. Associations between DTI parameters and age, WM injury and clinical factors were analysed. RESULTS: A positive association existed between FA and age at imaging for fibres through the PLIC (r = 0.48 p < 0.001) and splenium (r = 0.24 p < 0.01). A negative association existed between ADC and age at imaging for fibres through the PLIC (r = -0.65 p < 0.001), splenium (r = -0.35 p < 0.001) and genu (r = -0.53 p < 0.001). No association was found between DTI parameters and gestational age, degree of WM injury or categorical clinical factors. CONCLUSIONS: These results indicate that in our cohort of very preterm infants, at this young age, the development of the PLIC and CC is ongoing and independent of the degree of prematurity or WM injury.

The role of early life stress in development of the anterior limb of the internal capsule in nonhuman primates.

Deep brain stimulation (DBS) of the anterior limb of the internal capsule (ALIC) may be effective in treating depression. Parental verbal abuse has been linked to decreased fractional anisotropy (FA) of white matter and reduced FA correlated with depression and anxiety scores. Utilizing a nonhuman primate model of mood and anxiety disorders following disrupted mother-infant attachment, we examined whether adverse rearing conditions lead to white matter impairment of the ALIC. We examined white matter integrity using Diffusion Tensor Imaging (DTI) on a 3T-MRI. Twenty-one adult male Bonnet macaques participated in this study: 12 were reared under adverse [variable foraging demand (VFD)] conditions whereas 9 were reared under normative conditions. We examined ALIC, posterior limb of the internal capsule (PLIC) and occipital white matter. VFD rearing was associated with significant reductions in FA in the ALIC with no changes evident in the PLIC or occipital cortex white matter. Adverse rearing in monkeys persistently impaired frontal white matter tract integrity, a novel substrate for understanding affective susceptibility.

Diffusion tensor imaging study of the cortical origin and course of the corticospinal tract in healthy children.

BACKGROUND AND PURPOSE: Several studies have questioned the traditional belief that the corticospinal tract (CST) arises exclusively from the precentral gyrus and passes through the anterior half of the posterior limb of the internal capsule (PLIC) in humans; however, no direct evidence existed from structural imaging, and developmental aspects of CST origin have not been clarified. We used diffusion tensor imaging (DTI) tractography to test the hypotheses that CST can originate from both pre- and postcentral gyri and is located posteriorly in the PLIC, and we also determined how age, sex, or handedness affected these locations. MATERIALS AND METHODS: Forty-two healthy children (2.6-17.5 years of age; 20 girls) underwent DTI. Subsequently, tractography was performed on the basis of fiber assignment by continuous tracking (FACT) algorithm and brute force approach, with a fractional anisotropy (FA) threshold of <0.2 and an angle threshold of >50 degrees . The CST was isolated by using a knowledge-based region-of-interest approach, and its cortical origin and location on the PLIC was determined. RESULTS: DTI revealed that the CST originated from both pre- and postcentral gyri in 71.4% of hemispheres, from the precentral gyrus only in 19%, and from the postcentral gyrus only in 7.1%. The overall distribution was similar in both hemispheres. However, children with CST originating from both pre- and postcentral gyri were older (mean, 11.1 years of age) than those with precentral origin (mean, 5.8 years of age) or postcentral origin (mean, 7.8 years of age) only (P = .00003). The center of the CST was localized at 65% of the length (from its anterior margin) of the PLIC, and the CST occupied 26.5% of its anteroposterior length. There was a significant positive correlation between age and FA of the CST (r = 0.49; P = .002). The volume of the precentral portion of the left CST was significantly higher than that of its postcentral portion (P = .01) and that of the right CST (P = .0002). The pattern of cortical origin of CST, its location at the level of PLIC, and its volume and FA were unaffected by sex or handedness. CONCLUSIONS: The CST most frequently originates from both pre- and postcentral gyri, especially in older children, and is typically centered approximately two thirds of the distance from the anterior margin of the PLIC and occupies about a quarter of its anteroposterior length. In young children, the CST can often be seen originating exclusively from the precentral gyrus by DTI.

Neonatal brain structure on MRI and diffusion tensor imaging, sex, and neurodevelopment in very-low-birthweight preterm children.

The neurological basis of an increased incidence of cerebral palsy (CP) in preterm males is unknown. This study examined neonatal brain structure on magnetic resonance imaging (MRI) and diffusion tensor imaging (DTI) at term-equivalent age, sex, and neurodevelopment at 1 year 6 months on the basis of the Amiel-Tison neurological examination, Gross Motor Function Classification System, and Bayley Scales of Infant Development in 78 very-low-birthweight preterm children (41 males, 37 females; mean gestational age 27.6 wks, SD 2.5; mean birthweight 1021 g, SD 339). Brain abnormalities on MRI and DTI were not different between males and females except in the splenium of the corpus callosum, where males had lower DTI fractional anisotropy (p=0.025) and a higher apparent diffusion coefficient (p=0.013), indicating delayed splenium development. In the 26 infants who were at higher risk on the basis of DTI, males had more abnormalities on MRI (p=0.034) and had lower fractional anisotropy and a higher apparent diffusion coefficient in the splenium (p=0.049; p=0.025) and right posterior limb of the internal capsule (PLIC; p=0.003; p=0.033). Abnormal neurodevelopment was more common in males (n=9) than in females (n=2; p=0.036). Children with abnormal neurodevelopment had more abnormalities on MRI (p=0.014) and reduced splenium and right PLIC fractional anisotropy (p=0.001; p=0.035). In children with abnormal neurodevelopment, right PLIC fractional anisotropy was lower than left (p=0.035), whereas in those with normal neurodevelopment right PLIC fractional anisotropy was higher than left (p=0.001). Right PLIC fractional anisotropy correlated to neurodevelopment (rho=0.371, p=0.002). Logistic regression predicted neurodevelopment with 94% accuracy; only right PLIC fractional anisotropy was a significant logistic coefficient. Results indicate that the higher incidence of abnormal neurodevelopment in preterm males relates to greater incidence and severity of brain abnormalities, including reduced PLIC and splenium development.

Role of the atypical cadherin Celsr3 during development of the internal capsule.

The development of axonal tracts requires interactions between growth cones and the environment. Major bundles, particularly in the internal capsule, are completely defective in mice with constitutive mutation of Celsr3. In order to understand better how Celsr3 controls axonal tract formation, we generated a conditional allele that allowed inactivation of Celsr3 in different sectors of the forebrain. Effects of Celsr3 inactivation specifically in the telencephalon, in the ventral forebrain, or in the cortex, demonstrate essential roles for the gene, both in the neurons that project their axons to subcerebral targets such as the spinal cord, as well as in cells that guide projecting axons through the ventral forebrain. These observations provide unequivocal in vivo evidence that heterotypic interactions between axons and guidepost cells govern axonal path formation in mammals, and that Celsr3 plays a key role in this process. In absence of cortico-subcortical connections, mice can survive up to P20, allowing studies of behavior and cortical maturation. Mutant mice with defective corticospinal tracts survive normally and provide a model to evaluate in vivo the role of this tract in motor function in rodents.

Gray matter density and white matter integrity in pianists' brain: a combined structural and diffusion tensor MRI study.

The current study combined structural magnetic resonance imaging (sMRI) and diffusion tensor MRI (DT-MRI) to investigate both gray matter density (GMD) and white matter integrity (WMI) in 18 pianists and 21 age-matched non-musicians. The pianists began their piano training at a mean age of 12. Voxel-based morphometry of the sMRI data showed that the pianists had higher GMD in the left primary sensorimotor cortex and right cerebellum. Voxel-based analysis of the DT-MRI data showed that pianists had higher fractional anisotropy (FA) (indicating higher WMI) in the right posterior limb of the internal capsule. The sMRI and DT-MRI results indicate that both the GMD and WMI of pianists may exhibit movement-related increases during adolescence or even early adulthood compared with non-musicians.

Substantial migration of SVZ cells to the cortex results in the generation of new neurons in the excitotoxically damaged immature rat brain.

Mammalian SVZ progenitors continuously generate new neurons in the olfactory bulb. After injury, changes in SVZ cell number suggest injury-induced migration. Studies that trace the migration of SVZ precursors into neurodegenerating areas are lacking. Previously, we showed a decrease in BrdU+SVZ cells following excitotoxic damage to the immature rat cortex. Here, we demonstrate that NMDA-induced injury forces endogenous Cell Tracker Green (CTG) labeled VZ/SVZ precursors out of the SVZ into the neurodegenerating cortex. CTG+/Nestin+/Filamin A+ precursors are closely associated with vimentin+/GFAP+/GLAST+ filaments and express both chemokine receptor CXCR4 and Robo1. In the cortex, SVZ-derived progenitors show a progressive expression of developing, migrating and mature neurons and glial markers. CTG+/GFAP+ astrocytes greatly outnumber CTG+/MAP2+/NeuN+ neurons. SVZ-derived progenitors differentiate into both tbr1+ cortical glutamatergic neurons and calretinin+ interneurons. But, there is little integration of these neurons into the existing circuitry, as seen by Fluorogold retrograde tracing from the internal capsule.

Developmental differences in white matter architecture between boys and girls.

Previous studies have found developmental differences between males and females in brain structure. During childhood and adolescence, relative white matter volume increases faster in boys than in girls. Sex differences in the development of white matter microstructure were investigated in a cohort of normal children ages 5-18 in a cross-sectional diffusion tensor imaging (DTI) study. Greater fractional anisotropy (FA) in boys was shown in associative white matter regions (including the frontal lobes), while greater FA in girls was shown in the splenium of the corpus callosum. Greater mean diffusivity (MD) in boys was shown in the corticospinal tract and in frontal white matter in the right hemisphere; greater MD in girls was shown in occipito-parietal regions and the most superior aspect of the corticospinal tract in the right hemisphere. Significant sex-age interactions on FA and MD were also shown. Girls displayed a greater rate of fiber density increase with age when compared with boys in associative regions (reflected in MD values). However, girls displayed a trend toward increased organization with age (reflected in FA values) only in the right hemisphere, while boys displayed this trend only in the left hemisphere. These results indicate differing developmental trajectories in white matter for boys and girls and the importance of taking sex into account in developmental DTI studies. The results also may have implications for the study of the relationship of brain architecture with intelligence.

Asynchrony of the early maturation of white matter bundles in healthy infants: quantitative landmarks revealed noninvasively by diffusion tensor imaging.

Normal cognitive development in infants follows a well-known temporal sequence, which is assumed to be correlated with the structural maturation of underlying functional networks. Postmortem studies and, more recently, structural MR imaging studies have described qualitatively the heterogeneous spatiotemporal progression of white matter myelination. However, in vivo quantification of the maturation phases of fiber bundles is still lacking. We used noninvasive diffusion tensor MR imaging and tractography in twenty-three 1-4-month-old healthy infants to quantify the early maturation of the main cerebral fascicles. A specific maturation model, based on the respective roles of different maturational processes on the diffusion phenomena, was designed to highlight asynchronous maturation across bundles by evaluating the time-course of mean diffusivity and anisotropy changes over the considered developmental period. Using an original approach, a progression of maturation in four relative stages was determined in each tract by estimating the maturation state and speed, from the diffusion indices over the infants group compared with an adults group on one hand, and in each tract compared with the average over bundles on the other hand. Results were coherent with, and extended previous findings in 8 of 11 bundles, showing the anterior limb of the internal capsule and cingulum as the most immature, followed by the optic radiations, arcuate and inferior longitudinal fascicles, then the spinothalamic tract and fornix, and finally the corticospinal tract as the most mature bundle. Thus, this approach provides new quantitative landmarks for further noninvasive research on brain-behavior relationships during normal and abnormal development.

A developmental study of the structural integrity of white matter in autism.

Diffusion tensor imaging was used to examine developmental changes in the organization of white matter in a large sample of male participants with autism and controls between the ages of 10 and 35 years. Participants with autism had lower fractional anisotropy in areas within and near the corpus callosum and in the right retrolenticular portion of the internal capsule. Only one area, in the posterior limb of the right internal capsule, showed an interaction between age and group. The findings suggest that reductions in the structural integrity of white matter in autism persist into adulthood. These reductions may underlie the behavioral pattern observed in autism, as well as findings of reduced functional connectivity in functional magnetic resonance imaging signal between activating cortical areas.

The internal capsule in neonatal imaging.

The internal capsule is highly visible on conventional magnetic resonance imaging (MRI). It is myelinating rapidly at term, and the time course of its maturation is well known. It carries the major motor and sensory pathways to and from the cortex and the spinal cord. Additionally, fibres from the thalamus pass through it connecting to most regions of the cortex. It is therefore of vital importance, and damage to it has severe consequences. Its abnormal appearance on conventional MRI is a good predictor of an abnormal motor outcome in different clinical situations encountered in perinatal medicine. Its normal appearance on conventional MR images at term age is usually associated with a relatively normal motor outcome. More recently, diffusion-weighted and diffusion tensor imaging have allowed a much more sophisticated assessment of its maturation and connectivity; this has already led to a better understanding of how its development is affected by preterm birth and by hypoxic-ischaemic brain injury. Future studies will assess the relevance of these findings not only for motor outcome but also for cognitive, visual and sensory abilities.

Diffusion-tensor MR imaging of gray and white matter development during normal human brain maturation.

BACKGROUND AND PURPOSE: Conventional MR imaging findings of human brain development are thought to result from decreasing water content, increasing macromolecular concentration, and myelination. We use diffusion-tensor MR imaging to test theoretical models that incorporate hypotheses regarding how these maturational processes influence water diffusion in developing gray and white matter. METHODS: Experimental data were derived from diffusion-tensor imaging of 167 participants, ages 31 gestational weeks to 11 postnatal years. An isotropic diffusion model was applied to the gray matter of the basal ganglia and thalamus. A model that assumes changes in the magnitude of diffusion while maintaining cylindrically symmetric anisotropy was applied to the white matter of the corpus callosum and internal capsule. Deviations of the diffusion tensor from the ideal model predictions, due to measurement noise, were estimated by using Monte Carlo simulations. RESULTS: Developing gray matter of the basal ganglia and developing white matter of the internal capsule and corpus callosum largely conformed to theory, with only small departures from model predictions in older children. However, data from the thalamus substantially diverged from predicted values, with progressively larger deviations from the model with increasing participant age. CONCLUSION: Changes in water diffusion during maturation of central gray and white matter structures can largely be explained by theoretical models incorporating simple assumptions regarding the influence of brain water content and myelination, although deviations from theory increase as the brain matures. Diffusion-tensor MR imaging is a powerful method for studying the process of brain development, with both scientific and clinical applications.

The thalamic reticular and perireticular nuclei in developing rabbits: patterns of parvalbumin expression.

Due to its strategic position, the thalamic reticular nucleus (TRN) plays an important role within the thalamo-cortical circuits. The perireticular thalamic nucleus (PRN) is a smaller group of cells, which is associated with the TRN and lies among the fibres of the internal capsule (IC). Studies of nuclei in rodents and carnivores have been conducted employing a number of different tools. The use of calcium-binding proteins is one example. It needs to be noted that rabbits have been regarded as intermediate between rodents and carnivores in relation to local GABAergic circuits. In the present study, sections from rabbits at different ages (prenatal, postnatal and adult) were examined to determine the parvalbumin (PV) expression in the developing TRN and PRN. In the TRN, there is one wave of PV expression during development, from caudal parts of the nucleus towards the rostral pole. At E22 there is already an incipient PV expression. In the adult stage, the TRN is completely positive to PV. The present study clearly indicates the presence of the PRN in the developing rabbit. The first PV positive cells were visible at E24, meanwhile the immunoreactivity was at its maximum at early postnatal stages (P0-P8). Two different types of perireticular cells in the IC were identified and the changes concerning neuronal morphology and orientation were described. The comparison between these results and previous data obtained in rats, ferrets or cats suggest that rabbits could represent an intermediate stage in the evolution of thalamic circuits and could be considered as useful neurobiological model.

Cortical and thalamic axon pathfinding defects in Tbr1, Gbx2, and Pax6 mutant mice: evidence that cortical and thalamic axons interact and guide each other.

During development, cortical areas establish precise reciprocal projections with corresponding thalamic nuclei. Pioneer axons from the cortex and thalamus first meet in the intermediate zone of the subcortical telencephalon (subpallium). Their close interactions in the subpallium suggest that they may use each other for guidance. To test this hypothesis, the development of corticothalamic and thalamocortical connections was studied in mice with mutations of transcription factor genes expressed specifically in the cortex (Tbr1), the dorsal thalamus (Gbx2), or both (Pax6). In Tbr1 mutants, cortical pioneer axons entered the subpallium at the appropriate time, but most stopped growing without entering the diencephalon. Surprisingly, thalamic axons (which do not express Tbr1) deviated into the external capsule and amygdala regions, without entering the cortex. Conversely, in most Gbx2 mutants, thalamic axons were reduced in number and grew no farther than the subpallium. Cortical axons (which do not express Gbx2) grew into the subpallium but did not enter the diencephalon. In one Gbx2- /- case, sparse thalamocortical and corticothalamic projections both developed, but in no case did one projection reach its target and not the other. In Pax6 mutants, neither corticothalamic nor thalamocortical axons reached their targets. These results suggest that thalamocortical and corticothalamic projections may not form independently. After reaching the subpallium, each projection may require a molecularly intact reciprocal projection for further guidance. This type of mechanism ensures that thalamocortical and corticothalamic axons project reciprocally. However, the exact nature of the interaction between cortical and thalamic pioneer axons remains to be elucidated.