Developmental dynamics of the periventricular parietal crossroads of growing cortical pathways in the fetal brain - In vivo fetal MRI with histological correlation

The periventricular crossroads have been described as transient structures of the fetal brain where major systems of developing fibers intersect. The triangular parietal crossroad constitutes one major crossroad region. By combining in vivo and post-mortem fetal MRI with histological and immunohistochemical methods, we aimed to characterize these structures. Data from 529 in vivo and 66 post-mortem MRI examinations of fetal brains between gestational weeks (GW) 18-39 were retrospectively reviewed. In each fetus, the area adjacent to the trigone of the lateral ventricles at the exit of the posterior limb of the internal capsule (PLIC) was assessed with respect to signal intensity, size, and shape on T2-weighted images. In addition, by using in vivo diffusion tensor imaging (DTI), the main fiber pathways that intersect in these areas were identified. In order to explain the in vivo features of the parietal crossroads (signal intensity and developmental profile), we analyzed 23 post-mortem fetal human brains, between 16 and ​40 GW of age, processed by histological and immunohistochemical methods. The parietal crossroads were triangular-shaped areas with the base in the continuity of the PLIC, adjacent to the germinal matrix and the trigone of the lateral ventricles, with the tip pointing toward the subplate. These areas appeared hyperintense to the subplate, and corresponded to a convergence zone of the developing external capsule, the PLIC, and the fronto-occipital association fibers. They were best detected between GW 25-26, and, at term, they became isointense to the adjacent structures. The immunohistochemical results showed a distinct cellular, fibrillar, and extracellular matrix arrangement in the parietal crossroads, depending on the stage of development, which influenced the MRI features. The parietal crossroads are transient, but important structures in white matter maturation and their damage may be indicative of a poor prognosis for a fetus with regard to neurological development. In addition, impairment of this region may explain the complex neurodevelopmental deficits in preterm infants with periventricular hypoxic/ischemic or inflammatory lesions.

In utero transplantation of neural stem cells ameliorates maternal inflammation-induced prenatal white matter injury

Prenatal white matter injury is a serious problem due to maternal inflammation leading to postnatal disabilities. In this study, we used the periventricular leukomalacia (PVL) model as a common prenatal white matter injury by maternal administration of lipopolysaccharide (LPS). Neural stem cells (NSCs) have shown therapeutic ability in neurological disorders through a different mechanism such as immunomodulation. Here, we studied the preventive potential of NSCs following in utero transplantation into the embryonic lateral ventricle in an LPS-induced white matter injury model. Pregnant animals were divided into three groups and received phosphate buffered saline, LPS, or LPS + NSCs. The brains of offspring were obtained and evaluated by real-time polymerase chain reaction (PCR), immunohistochemy, enzyme-linked immunosorbent assay (ELISA), terminal deoxynucleotidyl transferase-mediated biotinylated-dUTP nick-end labeling (TUNEL), and caspase-3 activity assay. The LPS-induced maternal inflammation degenerated the myelin sheath in the offspring periventricular region which was associated with an increased microglial number, oligodendrocytes degeneration, proinflammatory cytokine secretion, and cell apoptosis. The transplanted NSCs homed into the brain and ameliorated the evaluated parameters. The expression of proinflammatory cytokines interleukin-1β (IL-1β), IL-6, and tumor necrosis factor-α (TNF-α), cell apoptosis and caspase-3 activity were inhibited by NSCs. In addition, Olig2 and myelin basic protein immunohistochemy staining showed that prenatal NSCs transplantation augmented the myelination in the periventricular white matter of offspring. In conclusion, we think that prenatal therapeutic strategies, such as in utero NSCs transplantation, may prevent prenatal white matter injury after birth.

Exploring early human brain development with structural and physiological neuroimaging

Early brain development, from the embryonic period to infancy, is characterized by rapid structural and functional changes. These changes can be studied using structural and physiological neuroimaging methods. In order to optimally acquire and accurately interpret this data, concepts from adult neuroimaging cannot be directly transferred. Instead, one must have a basic understanding of fetal and neonatal structural and physiological brain development, and the important modulators of this process. Here, we first review the major developmental milestones of transient cerebral structures and structural connectivity (axonal connectivity) followed by a summary of the contributions from ex vivo and in vivo MRI. Next, we discuss the basic biology of neuronal circuitry development (synaptic connectivity, i.e. ensemble of direct chemical and electrical connections between neurons), physiology of neurovascular coupling, baseline metabolic needs of the fetus and the infant, and functional connectivity (defined as statistical dependence of low-frequency spontaneous fluctuations seen with functional magnetic resonance imaging (fMRI)). The complementary roles of magnetic resonance imaging (MRI), electroencephalography (EEG), magnetoencephalography (MEG), and near-infrared spectroscopy (NIRS) are discussed. We include a section on modulators of brain development where we focus on the placenta and emerging placental MRI approaches. In each section we discuss key technical limitations of the imaging modalities and some of the limitations arising due to the biology of the system. Although neuroimaging approaches have contributed significantly to our understanding of early brain development, there is much yet to be done and a dire need for technical innovations and scientific discoveries to realize the future potential of early fetal and infant interventions to avert long term disease.

Age-specific gray and white matter DTI atlas for human brain at 33, 36 and 39 postmenstrual weeks

During the 3rd trimester, dramatic structural changes take place in the human brain, underlying the neural circuit formation. The survival rate of premature infants has increased significantly in recent years. The large morphological differences of the preterm brain at 33 or 36 postmenstrual weeks (PMW) from the brain at 40PMW (full term) make it necessary to establish age-specific atlases for preterm brains. In this study, with high quality (1.5 × 1.5 × 1.6 mm3 imaging resolution) diffusion tensor imaging (DTI) data obtained from 84 healthy preterm and term-born neonates, we established age-specific preterm and term-born brain templates and atlases at 33, 36 and 39PMW. Age-specific DTI templates include a single-subject template, a population-averaged template with linear transformation and a population-averaged template with nonlinear transformation. Each of the age-specific DTI atlases includes comprehensive labeling of 126 major gray matter (GM) and white matter (WM) structures, specifically 52 cerebral cortical structures, 40 cerebral WM structures, 22 brainstem and cerebellar structures and 12 subcortical GM structures. From 33 to 39 PMW, dramatic morphological changes of delineated individual neural structures such as ganglionic eminence and uncinate fasciculus were revealed. The evaluation based on measurements of Dice ratio and L1 error suggested reliable and reproducible automated labels from the age-matched atlases compared to labels from manual delineation. Applying these atlases to automatically and effectively delineate microstructural changes of major WM tracts during the 3rd trimester was demonstrated. The established age-specific DTI templates and atlases of 33, 36 and 39 PMW brains may be used for not only understanding normal functional and structural maturational processes but also detecting biomarkers of neural disorders in the preterm brains.

Neuronal Migration and Axonal Pathways Linked to Human Fetal Insular Development Revealed by Diffusion MR Tractography

The insula is a multimodal sensory integration structure that, in addition to serving as a gateway between somatosensory areas and limbic structures, plays a crucial role in autonomic nervous system function. While anatomical studies following the development of the insula have been conducted, currently, no studies have been published in human fetuses tracking the development of neuronal migration or of white matter tracts in the cortex. In this study, we aimed to follow the neuronal migration and subsequent maturation of axons in and around the insula in human fetal ages. Using high-angular resolution diffusion magnetic resonance imaging tractography, major white matter pathways to/from the insula and its surrounding operculum were identified at a number of time points during human gestation. Pathways likely linked to neuronal migration from the ventricular zone to the inferior frontal gyrus, superior temporal region, and the insular cortex were detected in the earliest gestational age studied (15 GW). Tractography reveals neuronal migration to areas surrounding the insula occurred at different time points. These results, in addition to demonstrating key time points for neuronal migration, suggest that neurons and axonal fiber pathways underlying the insula and its surrounding gyri mature differentially despite their relationship during cortical folding.

Affected astrocytes in the spinal cord of the leukodystrophy vanishing white matter

Leukodystrophies are often devastating diseases, presented with progressive clinical signs as spasticity, ataxia and cognitive decline, and lack proper treatment options. New therapy strategies for leukodystrophies mostly focus on oligodendrocyte replacement to rescue lack of myelin in the brain, even though disease pathology also often involves other glial cells and the spinal cord. In this study we investigated spinal cord pathology in a mouse model for Vanishing White Matter disease (VWM) and show that astrocytes in the white matter are severely affected. Astrocyte pathology starts postnatally in the sensory tracts, followed by changes in the astrocytic populations in the motor tracts. Studies in post-mortem tissue of two VWM patients, a 13-year-old boy and a 6-year-old girl, confirmed astrocyte abnormalities in the spinal cord. For proper development of new treatment options for VWM and, possibly, other leukodystrophies, future studies should investigate spinal cord involvement.

T2* relaxometry of fetal brain at 1.5 Tesla using a motion tolerant method

PURPOSE

The aim of this study was to determine T2* values for the fetal brain in utero and to compare them with previously reported values in preterm and term neonates. Knowledge of T2* may be useful for assessing brain development, brain abnormalities, and for optimizing functional imaging studies.

METHODS

Maternal respiration and unpredictable fetal motion mean that conventional multishot acquisition techniques used in adult T2* relaxometry studies are not practical. Single shot multiecho echo planar imaging was used as a rapid method for measuring fetal T2* by effectively freezing intra-slice motion.

RESULTS

T2* determined from a sample of 24 subjects correlated negatively with gestational age with mean values of 220 ms (±45) for frontal white matter, 159 ms (±32) for thalamic gray matter, and 236 ms (±45) for occipital white matter.

CONCLUSION

Fetal T2* values are higher than those previously reported for preterm neonates and decline with a consistent trend across gestational age. The data suggest that longer than usual echo times or direct T2* measurement should be considered when performing fetal fMRI to reach optimal BOLD sensitivity.

The early development of brain white matter: a review of imaging studies in fetuses, newborns and infants

Studying how the healthy human brain develops is important to understand early pathological mechanisms and to assess the influence of fetal or perinatal events on later life. Brain development relies on complex and intermingled mechanisms especially during gestation and first post-natal months, with intense interactions between genetic, epigenetic and environmental factors. Although the baby's brain is organized early on, it is not a miniature adult brain: regional brain changes are asynchronous and protracted, i.e. sensory-motor regions develop early and quickly, whereas associative regions develop later and slowly over decades. Concurrently, the infant/child gradually achieves new performances, but how brain maturation relates to changes in behavior is poorly understood, requiring non-invasive in vivo imaging studies such as magnetic resonance imaging (MRI). Two main processes of early white matter development are reviewed: (1) establishment of connections between brain regions within functional networks, leading to adult-like organization during the last trimester of gestation, (2) maturation (myelination) of these connections during infancy to provide efficient transfers of information. Current knowledge from post-mortem descriptions and in vivo MRI studies is summed up, focusing on T1- and T2-weighted imaging, diffusion tensor imaging, and quantitative mapping of T1/T2 relaxation times, myelin water fraction and magnetization transfer ratio.

Impact of inhaled nitric oxide on the sulfatide profile of neonatal rat brain studied by TOF-SIMS imaging

Despite advances in neonatal intensive care leading to an increased survival rate in preterm infants, brain lesions and subsequent neurological handicaps following preterm birth remain a critical issue. To prevent brain injury and/or enhance repair, one of the most promising therapies investigated in preclinical models is inhaled nitric oxide (iNO). We have assessed the effect of this therapy on brain lipid content in air- and iNO-exposed rat pups by mass spectrometry imaging using a time-of-flight secondary ion mass spectrometry (TOF-SIMS) method. This technique was used to map the variations in lipid composition of the rat brain and, particularly, of the white matter. Triplicate analysis showed a significant increase of sulfatides (25%–50%) in the white matter on Day 10 of life in iNO-exposed animals from Day 0–7 of life. These robust, repeatable and semi-quantitative data demonstrate a potent effect of iNO at the molecular level.