Edited by: Hao Huang, University of Pennsylvania, USA
Reviewed by: Marina Bentivoglio, University of Verona, Italy; Gavin John Clowry, Newcastle University, UK
*Correspondence: Emi Takahashi
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The radial and tangential neural migration pathways are two major neuronal migration streams in humans that are critical during corticogenesis. Corticogenesis is a complex process of neuronal proliferation that is followed by neuronal migration and the formation of axonal connections. Existing histological assessments of these two neuronal migration pathways have limitations inherent to microscopic studies and are confined to small anatomic regions of interest (ROIs). Thus, little evidence is available about their three-dimensional (3-D) fiber pathways and development throughout the entire brain. In this study, we imaged and analyzed radial and tangential migration pathways in the whole human brain using high-angular resolution diffusion MR imaging (HARDI) tractography. We imaged ten fixed, postmortem fetal (17 gestational weeks (GW), 18 GW, 19 GW, three 20 GW, three 21 GW and 22 GW) and eight
Corticogenesis is a complex process that occurs during a critical period of brain development. This process begins with subcortical neuroproliferation, followed by neuronal migration and the formation and refinement of axonal connections (Caviness et al.,
Radial glia are recognized as a primary source of progenitor cells and also serve as a scaffold for migrating neurons in the developing brain. During corticogenesis, a complex process of both symmetric and asymmetric divisions of radial glial cells from the ventricular zone (VZ) has been described, with symmetric divisions attributed to increasing the progenitor pool and asymmetric divisions resulting in neuronal generation (Cai et al.,
Another major neuronal migratory pathway is the tangential migration pathway, which runs tangential to the CP and from which inhibitory GABAergic interneuron progenitors arise (Wonders and Anderson,
Initial efforts to assess migratory streams include whole brain analysis by magnetic resonance imaging (MRI; Zhang et al.,
High-angular resolution diffusion imaging (HARDI) tractography is a technique that enables reconstruction of complicated crossing neural fibers in the brain (Tuch et al.,
Disorganized neuronal migration from the radial and tangential pathways has been implicated in many neurological and psychiatric disorders (Marín and Rubenstein,
Ten fetal brain specimens were imaged for this study. IRB committees at Partners and Boston Children’s Hospital (BCH) approved the use of postmortem specimens for MRI studies. The brains were acquired after obtaining informed consent by the parents. Table
Subject | Source | Age | Brain image | Age group |
---|---|---|---|---|
1 | BWH | 17 GW | Early | |
2 | BWH | 18 GW | Early | |
3 | AIBB | 19 GW | Early | |
4 | BWH | 20 GW | Early | |
5 | BWH | 20 GW | Early | |
6 | BWH | 20 GW | Early | |
7 | BWH | 21 GW | Early | |
8 | AIBB | 21 GW | Early | |
9 | BWH | 21 GW | Early | |
10 | AIBB | 22 GW | Early | |
11 | BCH | 30 GW | Late | |
12 | BCH | 30 GW | Late | |
13 | BCH | 34 GW | Late | |
14 | BCH | 35 GW | Late | |
15 | BCH | 40 GW | Late | |
16 | BCH | 40 GW | Late | |
17 | BCH | 40 GW | Late | |
18 | BCH | 40 GW | Late |
Eight living participants underwent clinically-indicated brain MRI studies that were interpreted to show no abnormalities. The IRB at BCH approved the retrospective use of
At the time of autopsy, all brains were immersion fixed. The brains from BWH were stored in 4% paraformaldehyde, and the brains from AIBB were stored in 4% periodate-lysine-paraformaldehyde (PLP). During MR image acquisition, BWH brains were placed in Fomblin solution (Ausimont, Thorofare, NJ, USA; Takahashi et al.,
Six postmortem brain specimens from BWH were imaged with a 4.7T Bruker Biospec MR system, and three specimens from the AIBB were imaged with a 3T Siemens MR system at the A. A. Martinos Center, Massachusetts General Hospital, Boston, MA, USA. The 3T system was used to accommodate the AIBB brains that were
For the BWH brains, a three-dimensional (3-D) diffusion-weighted spin-echo echo-planar imaging (SE-EPI) sequence was used with a repetition time/echo time (TR/TE) of 1000/40 ms, with an imaging matrix of 112 × 112 × 112 pixels. Sixty diffusion-weighted measurements (with the strength of the diffusion weighting,
The brains of living participants were imaged on a 3T Siemens MR system, BCH, Boston, MA, USA. The diffusion pulse sequence used was a diffusion-weighted SE-EPI sequence, TR/TE 8320/88 ms, with an imaging matrix of 128 × 128 × 64 pixels. The spatial resolution was 2 × 2 × 2 mm. Thirty diffusion-weighted measurements (
We used Diffusion Toolkit and TrackVis
Trajectories were propagated by consistently pursuing the orientation vector of least curvature. Tracking was terminated when the angle between two consecutive orientation vectors was greater than the given threshold (40°) or when the fibers extended outside of the brain surface using a brain mask. The brain mask volumes were used to terminate tractography structures instead of the FA threshold (Schmahmann et al.,
Using previously reported methodology (Takahashi et al.,
Volume measurements are automatically derived by TrackVis for each migration pathway. The volume of pathways was calculated by counting the number of voxels that touched or passed through the detected pathways, and was compared at different developmental time points.
We defined left and right radial pathways separately between the VZ/SVZ and CP in each hemisphere. We also defined the anterior and posterior radial pathways as the front or posterior half of the radial pathway. Due to challenges in defining the “middle” coronal plane in early fetal stages from only anatomic information without gyral or other mature subcortical structures, we instead measured the length between the anterior and posterior edges of the brain and defined a middle coronal plane to separate the anterior and posterior radial pathways. Left/right hemispheric asymmetry was tested for both the radial and tangential migration pathways, and anterior-posterior differences were tested for the radial pathway.
We statistically analyzed asymmetry with a laterality index (LI), calculated by the following formula:
LIs range from −2 to 2, with positive and negative LI values corresponding to left- and rightward, and anterior and posterior asymmetries, respectively (Caviness et al.,
Development and regression of the radial and tangential migration pathways are shown in Figures
Figure
Reconstructed GE tractography pathways are shown in Figure
Next, we studied the volumetric laterality indices (LIs) for the radial and tangential migration pathways in both the early and late ages (Figures
Results of comparisons between the volume of the anterior and posterior regions of the identified radial pathways are shown in Figure
We investigated two major neural migration pathways, the radial and tangential migration pathways, in human fetal and newborn brains (17–40 GW) using HARDI tractography. Due to limitations of microscopy, previous studies have performed only limited regional analyses of these migration pathways in developing human brains. Our new findings are: (1) significant left-right hemispheric asymmetry and anterior-posterior asymmetry of radial migration pathways between 17 GW and 22 GW by analyzing whole human fetal brain with MR imaging; (2) there were no significant asymmetry of tangential migration pathways between 17 GW and 22 GW, and no significant asymmetry of radial migration pathways between 30 GW and 40 GW. Our previous study (Kolasinski et al.,
Many lines of evidence suggest that the majority of radial pathways contribute excitatory glutamatergic projection neurons (Rakic,
While the radial tractography pathways showed structural asymmetries, the tangential migration pathways revealed no significant hemispheric asymmetry in volume in this study. These results suggest that, at least in the pathways that supply inhibitory interneurons to the cortex, there is no demonstrable laterality by tractography.
The volume of the radial pathway in the left hemisphere was significantly larger than that of the right hemisphere in the early age group (17–22 GW) in our study. In contrast, no significant difference between the left- and right hemispheres was observed in the late age group (30–40 GW). Previous studies reported that the left hemisphere is volumetrically larger than the right hemisphere in right-handed adults (Büchel et al.,
In the current study, the volume of the radial pathway in the posterior region was significantly larger compared to the anterior region when examining ages up to 22 GW in our study. This quantitative analysis was in agreement with the visual tractography of the radial pathways, which revealed a gradual thinning of this pathway in the parieto-occipital regions at 21 GW. This thinning of pathways appeared to progress towards the frontal region by 22 GW. Previous studies reported a higher neural density in the posterior than in the anterior brain regions in primates (Cahalane et al.,
While the regression of the radial pathways in the dorsal parietal and frontal regions continued through 21 GW, the regression of the radial pathways in the inferior frontal regions seemed to begin after 22 GW. Although radial pathways in the occipitoparietal regions also decreased in these ages, the late-onset regression in the inferior frontal regions seemed greater than in the occipitoparietal regions, which likely contributed to the relative increase in volume of the posterior radial pathways compared to those in the anterior region. At 30 GW, almost no anterior radial pathways were visible, while there were some residual radial pathways in the posterior regions. By 34 GW and 40 GW, radial pathways almost all disappeared throughout the brain. One reason why no significant difference between the anterior and posterior regions was observed in the late age group may be due to the uneven number of specimens in each age group within the late age group. For instance, in the late age group, only 30 GW (
The tangential migration pathways were observed in the early age group (17–22 GW) and at the beginning of the late age group (30–34 GW), but were not visible at 40 GW. This observation is consistent with the literature describing the gradual disappearance of the GE by 34 GW (Rakić and Sidman,
While Arshad et al. (
The coherent diffusivity within the GE visualized by tractography during the early fetal ages is likely attributable to the tangential migratory routes on scaffolds of corticofugal fibers. This directionality of migration has been shown in investigational studies using tracer-labeling (Denaxa et al.,
In this study, we examined whether volumetric asymmetries by tractography were present in two critical neuronal migratory pathways, the radial and tangential migration pathways, from which progenitor neurons have been reported to migrate in the process of corticogenesis in developing human fetal brains. Several conclusions may be drawn from this study. First, our results confirm a postero-dorsal to antero-ventral direction of migration of the radial pathway based on the gradual disappearance of tractography pathways. By tractography, the volume of radial pathways was greater in the posterior region than in the anterior brain region at early stages of development (22 GW and younger). Second, volumetric asymmetry for only the radial pathways was statistically significant in the early age group (22 GW and younger). Third, no volumetric hemispheric asymmetry of the tangential pathway emerged, contrasting with the radial pathway. Taken together, these findings suggest hemispheric volumetric asymmetry related to neural progenitors from the radial glial pathway is established early in development, which contrasts with the GE. Describing the anatomic development and regression of these neuronal migratory pathways is critical as many neurologic and psychological disorders are described to have underlying disorganized corticogenesis. Future investigations aim to include a more comprehensive range of developmental stages to enable a more thorough description of the developmental changes, as well as comparisons of normal and diseased brains to understand what may contribute to the underlying problems in the formation of brain connectivity.
ET performed the experiments. YM, JWS, and ET analyzed the data and wrote the paper.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.
This work was supported by Boston Children’s Hospital (BCH), NICHD (R01HD078561, R21HD069001, R03NS091587; ET). This research was carried out in part at the Athinoula A. Martinos Center for Biomedical Imaging at the Massachusetts General Hospital, using resources provided by the Center for Functional Neuroimaging Technologies, NIH P41RR14075, a P41 Regional Resource supported by the Biomedical Technology Program of the National Center for Research Resources (NCRR), National Institutes of Health. This work also involved the use of instrumentation supported by the NCRR Shared Instrumentation Grant Program (NIH S10RR023401, S10RR019307, and S10RR023043) and High-End Instrumentation Grant Program (NIH S10RR016811). This study was conducted partly using postmortem human brain specimens from the tissue collection at the Department of Neurobiology at Yale University School of Medicine (supported by grant NIH MH081896), which form a part of the BrainSpan Consortium collection (
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