Diffusion tensor imaging of the developing human cerebrum

Diffusion tensor imaging (DTI) was performed on 15 fresh spontaneously or therapeutically aborted normal fetuses and five term infants at different gestational ages. Regional cortical fractional anisotropy (FA) values were observed to increase with gestational age (GA) from 15 to 28 weeks, followed by a decrease through 36 weeks. The early increase in the cortical FA value, which has never been reported before, is consistent with neuronal migration from the germinal matrix. A statistically significant inverse correlation between GA and the FA values in the germinal matrix was observed (r = -0.81, P = 0.004). In addition, there was a significant difference in the FA values in the right and left frontal cortices (P = 0.007, sign test), suggesting cortical lateralization during the early stage of development. Our studies suggest that the DTI-estimated anisotropy could be useful in following neuronal migration, cortical maturation, and associated changes in the germinal matrix during early brain development.     

In utero exposure to brief hyperthermia interferes with the production and migration of neocortical neurons and induces apoptotic neuronal death in the fetal mouse brain.

To investigate the pathogenetic mechanisms of brain maldevelopment induced by maternal hyperthermia, we exposed pregnant ICR mice to 43 degrees C for 12.5 min on day 13.5 or 14.5 of gestation and examined the proliferation and migration of neuronal precursor cells in the telencephalon of their fetuses. The brain weight was significantly decreased in heat-stressed fetuses when examined at 72 h after treatment. Histological examination revealed that the thickness of the neopallium, especially that of the intermediate (migratory) zone and the cortical plate, was decreased in the heated group. BrdU/anti-BrdU immunohistochemistry showed that cell proliferation in the matrix cell zone was suppressed for up to 8 h after hyperthermia and that the migration of BrdU-labeled neurons from the matrix cell zone to the primordial cortex was decelerated significantly. In addition, apoptotic cell death which is rarely observed in the brain of control animals increased in the brain of heat-stressed fetuses at 8-12 h after treatment. Thus, it seems that brief hyperthermia at critical stages of neuronal differentiation can interfere with the production and migration of neuronal precursor cells and result in abnormal brain development and neurobehavioural disturbances.     

Neuroimaging in disorders of cortical development

Malformations of cortical development are important causes of developmental delay and epilepsy. They are classified by the presumed stage during which normal development is interrupted: neuronal proliferation and differentiation, neuronal migration, and late migration/cortical organization. This article discusses the important malformations in each of these groups, how and why the malformations develop, and their imaging findings. A better understanding of these disorders helps in genetic counseling of the parents and may help in the treatment of associated epilepsy.     

Evolution of cerebral cortical development

Understanding how the human cerebral cortex evolved to its present complex state is a fascinating topic for neuroscience, genetics, bioinformatics and comparative biology. To gain further insights into the origins of the mammalian neocortex and to understand how the cortex evolved to be able to serve more complex cognitive functions, we study the development of various extant species. Our aim is to correlate cortical cell numbers and neuronal cell types with the elaboration of cortical progenitor populations and their modes of proliferation in different species. There are several progenitors, i.e. the ventricular radial glia, the subventricular intermediate progenitors and subventricular (outer) radial glia types, but the contribution of each to cortical layers and cell types through specific lineages is not fully understood. Recent comparisons of the proportions of these progenitors in various species during embryonic neurogenesis have revealed the elaboration and cytoarchitectonic compartmentalization of the germinal zone, with alterations in the proportions of various types that can be included among the intermediate progenitors. Across species, larger and more diverse intermediate progenitor populations correlate with brain size and cortical cell diversity. Understanding the molecular and cellular interactions regulating the divisions of these intermediate progenitors not only has implications for cortical evolution but also relates to stem cell biology and illuminates the pathomechanisms of several cortical developmental disorders.     

Survival of Cajal-Retzius cells after cortical lesions in newborn mice: a possible role for Cajal-Retzius cells in brain repair

Transient Cajal-Retzius (CR) cells in layer I of the mammalian cerebral cortex are the first postmitotic neurons and they are believed to play a role in neuronal migration and lamination during cortical development. Freezing insults to the cortex of newborn mice produce cortical malformations similar to those observed in human brain disorders. Here we have used calretinin immunostaining to investigate the response of CR cells to freezing lesions of the cortical surface. Shortly after injury, CR cells disappeared from the lesioned zone. Moreover, CR cells located near the lesioned area adopted extremely fusiform shapes. At later postnatal stages (P12), CR cells were still abundant in layer I of the lesioned zone, in contrast to their almost complete loss in control animals. These results show that CR cells survive for longer developmental periods following cortical injury. Furthermore, the initial loss and later re-appearance of CR cells suggest that these neurons might migrate tangentially from the cortical areas surrounding the lesioned zone. These findings imply a role for CR cells in brain repair after cortical injury during development.     

Molecular neuropathology of epilepsy-associated glioneuronal malformations

Glioneuronal malformations (malformations of cortical development [MCD]) include focal cortical dysplasias (FCD) as well as highly differentiated glioneuronal tumors (i.e. gangliogliomas) and constitute frequent findings in patients with pharmacoresistent focal epilepsies. Tailored resection strategies evolved as promising treatment options and allow a systematic neuropathologic and molecular biologic examination of the epileptogenic area in these patients. The histopathologic appearance and immunophenotype of glioneuronal lesions are, however, characterized by numerous similarities and suggest impaired proliferation, migration, and differentiation of neural precursor cells to play a pathogenetic role. Recent studies point toward molecular alterations within a variety of genes and pathways involved in development of the central nervous system, neuronal growth, and maturation. Compromised signaling within insulin- or reelin-transduction cascades are common findings and were associated with specific MCD entities. Unraveling pathogenic mechanisms may advance refined classification systems for epilepsy-associated malformations and open new avenues for the development of targeted treatment strategies in pharmacoresistent focal epilepsies associated with cortical malformations.     

Evidence for tangential migration disturbances in human lissencephaly resulting from a defect in LIS1, DCX and ARX genes

During corticogenesis, neurons adopt different migration pathways to reach their final position. The precursors of pyramidal neurons migrate radially, whereas most of the GABA-containing interneurons are generated in the ventral telencephalon and migrate tangentially into the neocortex. Then, they use a radial migration mode to establish themselves in an inside-out manner in the neocortex, similarly to pyramidal neurons. In humans, the most severe defects in radial migration result in lissencephaly. Lately, a few studies suggested that lissencephaly was also associated with tangential neuronal migration deficits. In the present report, we investigated potential anomalies of this migration mode in three agyric/pachygyric syndromes due to defects in the LIS1, DCX and ARX genes. Immunohistochemistry was performed on paraffin-embedded supra- and infratentorial structures using calretinin, calbindin and parvalbumin antisera. The results were compared with age-matched control brain tissue. In the Miller–Dieker syndrome, GABAergic neurons were found both in upper layers of the cortex and in heterotopic positions in the intermediate zone and in ganglionic eminences. In the DCX mutant brain, few interneurons were dispersed in the cortical plate, with a massive accumulation in the intermediate zone and subventricular zone as well as in the ganglionic eminences. In the ARX-mutated brain, the cortical plate contained almost exclusively pyramidal cells and was devoid of interneurons. The ganglionic eminences and basal ganglia were poorly cellular, suggesting an interneuron production and/or differentiation defect. These data argue for different mechanisms of telencephalic tangential migration impairment in these three agyric/pachygyric syndromes.     

Facio-linguo-masticatory diplegia and epilepsy. Cortical dysplasia]

Neuronal migration anomalies are caused by insults occurring during the third to fifth gestational months when neuroblasts migrate from the germinal zone to the cortical plate. They lead to several cerebral malformations such as macrogyria, identified nowadays by MRI. We describe a case of bilateral parieto-rolandic macrogyria responsible for a bi-opercular syndrome resulting in a facio-linguo-masticatory diplegia associated with mental retardation and severe epilepsy.     

Classification system for malformations of cortical development: update 2001.

The many recent discoveries concerning the molecular biologic bases of malformations of cortical development and the discovery of new such malformations have rendered previous classifications out of date. A revised classification of malformations of cortical development is proposed, based on the stage of development (cell proliferation, neuronal migration, cortical organization) at which cortical development was first affected. The categories have been created based on known developmental steps, known pathologic features, known genetics (when possible), and, when necessary, neuroimaging features. In many cases, the precise developmental and genetic features are uncertain, so classification was made based on known relationships among the genetics, pathologic features, and neuroimaging features. A major change since the prior classification has been the elimination of the separation between diffuse and focal/multifocal malformations, based on the recognition that the processes involved in these processes are not fundamentally different; the difference may merely reflect mosaicism, X inactivation, the influence of modifying genes, or suboptimal imaging. Another change is the listing of fewer specific disorders to reduce the need for revisions; more detail is added in other smaller tables that list specific malformations and malformation syndromes. This classification is useful to the practicing physician in that its framework allows a better conceptual understanding of the disorders, while the component of neuroimaging characteristics allows it to be applied to all patients without necessitating brain biopsy, as in pathology-based classifications.     

Cortical malformation and pediatric epilepsy: a molecular genetic approach

Genetic malformations of the cerebral cortex are important causes of neurologic morbidity in children because they are often associated with developmental delay, motor disturbances (cerebral palsy), and epilepsy. Primary autosomal recessive microcephaly is a cortical malformation with a low incidence of epilepsy. One of its causative genes, ASPM, might play an important role in regulating proliferation of neuronal progenitor cells. Mutations in ASPM do not seem to affect later stages of cortical development, such as neuronal migration, and this might be responsible for the low epileptogenicity of this malformation. ASPM might also have played an important role in the evolutionary expansion of the human brain. Bilateral frontoparietal polymicrogyria, on the other hand, is a highly epileptogenic malformation. Its causative gene, GPR56, is also expressed in the neurogenic regions of the cortex, but its primary function might be in the determination of cell fate and/or cortical patterning. Further studies of these genes will likely lead to a better understanding of human brain development and epilepsy.     

Immunohistochemical expression of doublecortin in the human cerebrum: comparison of normal development and neuronal migration disorders

Immunohistochemical expression of the doublecortin (DCX) gene product was investigated in cerebral cortices from 33 normal developing human, aged 9 gestational weeks (GW) to 29 years, and from 26 patients with various neuronal migration disorders, aged 19 GW to 34 years. DCX immunoreactivity was detected predominantly in the fetal cerebral cortex. The neurons in the cortical plate (CP) exhibited positive labeling at 9 GW. Staining was the most marked intense at 12-20 GW, and gradually decreased thereafter, only relatively weak immunoreactivity remaining in pyramidal cells. Comparison of the immunohistochemical characteristics of DCX and those of nestin and vimentin indicated the early expression of DCX in neuroepithelial stem cells of the subventricular germinal layer, as well as in neurons of the CP. The most marked intense expression in the period of neuronal migration strongly indicated its role in neuronal migration. The abnormal distribution of DCX immunolabeling in the cerebral cortex was associated with a neuronal disarrangement in some migration disorders, such as Miller-Dieker syndrome and Fukuyama congenital muscular dystrophy. Decreased DCX immunolabeling was demonstrated in fetuses and infants with Zellweger syndrome, implicating DCX in the neuronal migration abnormality in this syndrome.     

Morphological substrates of infantile spasms: studies based on surgically resected cerebral tissue

Extensive surgical resections of neocortical cerebral tissue (including hemispherectomies) from 13 infants and children with infantile spasms showed that 12 of 13 specimens contained either malformative and dysplastic lesions of the cortex and white matter (sometimes with associated hamartomatous proliferation of globular cells), or destructive lesions possibly acquired as a result of anoxic-ischemic injury, or a combination of the two. In brain tissue from 4 patients, coarse neuronal cytoplasmic fibrils resembling neurofibrillary tangles were seen in areas of dysplastic brain on silver-stained (Bielschowsky technique) sections. Immunohistochemical (immunoperoxidase) study of cortical lesions containing globular cells employing primary antibodies to glial fibrillary acidic protein and synaptophysin as markers of astrocytic and neuronal differentiation, respectively, revealed that many cells showed astrocytic and/or neuronal features, suggesting the local proliferation of primitve or multipotential neuroectodermal cells as one substrate for this seizure disorder. Morphological abnormalities of a severe degree and wide extent in the resected tissue (e.g., in one patient with hemimegalencephaly) often showed features to suggest that they may represent variants of tuberous sclerosis. These most likely result from abnormal movement and/or local proliferation of neuroectodermal precursors that have migrated from the germinal matrix to the cortical mantle. Cellular, molecular and neurophysiological study of these abnormalities is likely to yield information about basic molecular mechanisms of brain malformation and injury important in the pathogenesis of infantile spasms and other forms of focal or generalized epilepsy.     

Disruption of neurogenesis in the subventricular zone of adult mice,and in human cortical neuronal precursor cells in culture,by amyloid beta-peptide: implications for the pathogenesis of Alzheimer's disease

The adult mammalian brain contains populations of stem cells that can proliferate and then differentiate into neurons or glia. The highest concentration of such neural progenitor cells (NPC) is located in the subventricular zone (SVZ) and these cells can produce new olfactory bulb and cerebral cortical neurons. NPC may provide a cellular reservoir for replacement of cells lost during normal cell turnover and after brain injury. However, neurogenesis does not compensate for neuronal loss in age-related neurodegenerative disorders such as Alzheimer's disease (AD), suggesting the possibility that impaired neurogenesis contributes to the pathogenesis of such disorders. We now report that amyloid beta-peptide (Abeta), a self-aggregating neurotoxic protein thought to cause AD, can impair neurogenesis in the SVZ/cerebral cortex of adult mice and in human cortical NPC in culture. The proliferation and migration of NPC in the SVZ of amyloid precursor protein (APP) mutant mice, and in mice receiving an intraventricular infusion of Abeta, were greatly decreased compared to control mice. Studies of NPC neurosphere cultures derived from human embryonic cerebral cortex showed that Abeta can suppress NPC proliferation and differentiation, and can induce apoptosis. The adverse effects of Abeta on neurogenesis were associated with a disruption of calcium regulation in the NPC. Our data show that Abeta can impair cortical neurogenesis, and suggest that this adverse effect of Abeta contributes to the depletion of neurons and the resulting olfactory and cognitive deficits in AD.     

A developmental and genetic classification for malformations of cortical development

Increasing recognition of malformations of cortical development and continuing improvements in imaging techniques, molecular biologic techniques, and knowledge of mechanisms of brain development have resulted in continual improvement of the understanding of these disorders. The authors propose a revised classification based on the stage of development (cell proliferation, neuronal migration, cortical organization) at which cortical development was first affected. The categories are based on known developmental steps, known pathologic features, known genetics (when possible), and, when necessary, neuroimaging features. In those cases in which the precise developmental and genetic features are uncertain, classification is based on known relationships among the genetics, pathologic features, and neuroimaging features. The major change since the prior classification has been a shift to using genotype, rather than phenotype, as the basis for classifying disorders wherever the genotype-phenotype relationship is adequately understood. Other substantial changes include more detailed classification of congenital microcephalies, particularly those in which the genes have been mapped or identified, and revised classification of congenital muscular dystrophies and polymicrogyrias. Information on genetic testing is also included. This classification allows a better conceptual understanding of the disorders, and the use of neuroimaging characteristics allows it to be applied to all patients without necessitating brain biopsy, as in pathology-based classifications.     

Glutamate acting at NMDA receptors stimulates embryonic cortical neuronal migration

During cortical development, embryonic neurons migrate from germinal zones near the ventricle into the cortical plate, where they organize into layers. Mechanisms that direct neuronal migration may include molecules that act as chemoattractants. In rats, GABA, which localizes near the target destination for migrating cortical neurons, stimulates embryonic neuronal migration in vitro. In mice, glutamate is highly localized near the target destinations for migrating cortical neurons. Glutamate-induced migration of murine embryonic cortical cells was evaluated in cell dissociates and cortical slice cultures. In dissociates, the chemotropic effects of glutamate were 10-fold greater than the effects of GABA, demonstrating that for murine cortical cells, glutamate is a more potent chemoattractant than GABA. Thus, cortical chemoattractants appear to differ between species. Micromolar glutamate stimulated neuronal chemotaxis that was mimicked by microM NMDA but not by other ionotropic glutamate receptor agonists (AMPA, kainate, quisqualate). Responding cells were primarily derived from immature cortical regions [ventricular zone (vz)/subventricular zone (svz)]. Bromodeoxyuridine (BrdU) pulse labeling of cortical slices cultured in NMDA antagonists (microM MK801 or APV) revealed that antagonist exposure blocked the migration of BrdU-positive cells from the vz/svz into the cortical plate. PCR confirmed the presence of NMDA receptor expression in vz/svz cells, whereas electrophysiology and Ca2+ imaging demonstrated that vz/svz cells exhibited physiological responses to NMDA. These studies indicate that, in mice, glutamate may serve as a chemoattractant for neurons in the developing cortex, signaling cells to migrate into the cortical plate via NMDA receptor activation.     

Human disorders of cortical development: from past to present

Epilepsy and mental retardation, originally of unknown cause, are now known to result from many defects including cortical malformations, neuronal circuitry disorders and perturbations of neuronal communication and synapse function. Genetic approaches in combination with MRI and related imaging techniques continually allow a re-evaluation and better classification of these disorders. Here we review our current understanding of some of the primary defects involved, with insight from recent molecular biology advances, the study of mouse models and the results of neuropathology analyses. Through these studies the molecular determinants involved in the control of neuron number, neuronal migration, generation of cortical laminations and convolutions, integrity of the basement membrane at the pial surface, and the establishment of neuronal circuitry are being elucidated. We have attempted to integrate these results with the available data concerning, in particular, human brain development, and to emphasize the limitations in some cases of extrapolating from rodent models. Taking such species differences into account is clearly critical for understanding the pathophysiological mechanisms associated with these disorders.     

Development and developmental disorders of the human cerebellum

The human cerebellum develops over a long time, extending from the early embryonic period until the first postnatal years. This protracted development makes the cerebellum vulnerable to a broad spectrum of developmental disorders. The development of the cerebellum occurs in four basic steps: 1) characterization of the cerebellar territory at the midbrain-hindbrain boundary; 2) formation of two compartments for cell proliferation: first, the Purkinje cells and the deep cerebellar nuclei arise from the ventricular zone of the metencephalic alar plate; second, granule cell precursors are formed from a second compartment of proliferation, i. e. the upper rhombic lip; 3) inward migration of the granule cells: granule precursor cells form the external granular layer, from which (and continuing into the first postnatal year), granule cells migrate inwards to their definite position in the internal granular layer, and 4) formation of cerebellar circuitry and further differentiation. The precerebellar nuclei, i. e. the pontine nuclei and the inferior olive, arise from the lower rhombic lip. Developmental disorders of the cerebellum are often accompanied by malformations of the precerebellar nuclei. In this review the development of the cerebellum and some of its more frequent developmental disorders, such as the Dandy-Walker and related midline malformations, and the pontocerebellar hypoplasias, are discussed.     

Single cause, polymorphic neuronal migration disorders: an animal model

Injury to the developing cortical plate can result in a variety of neuronal migration disorders. The results are reported of experimental research aimed at determining whether these different types of neocortical malformations are the consequence of comparable injury of varying intensity. Freezing probes were placed on the skulls of 44 newborn rats (age equivalent to 4 to 5 months of gestation in humans) and induced either one or two freezing injuries of durations ranging from 2 to 20 seconds. A variety of cortical malformations including minor laminar dysplasias, molecular layer ectopias, microgyria, and porencephalic cysts were seen in the brains of these animals when they were examined on postnatal day (P)2, P21, and P60. The severity of the malformation was directly related to the strength (number of hits and duration) of the freezing injury. These results suggest that a single etiologic event of varying severity during neuronal migration to the neocortex can induce widely disparate malformations of the cortex.     

Bilateral periventricular nodular heterotopia and lissencephaly in an infant with unbalanced t(12;17)(q24.31; p13.3) translocation

Periventricular nodular heterotopia and Miller-Dieker syndrome are two different disorders of brain development. Miller-Dieker syndrome exhibits classical lissencephaly and is related to defects in the lissencephaly gene (LIS1). Periventricular nodular heterotopia is characterized by aggregates of grey matter adjacent to the lateral ventricle and is mainly linked to mutations in the Filamin A (FLNA) gene. We describe a male infant presenting with facial dysmorphisms resembling those of Miller-Dieker syndrome, neuromotor delay, and drug - resistant infantile spasms. Magnetic resonance imaging of the brain showed periventricular nodular heterotopia overlaid by classical lissencephaly with complete agyria. Cytogenetic and molecular investigations detected a maternally inherited unbalanced translocation involving chromosome arms 17p and 12q. This resulted in partial monosomy of 17p13.3-->pter and partial trisomy of 12q24.3-->qter. No mutation was found in the FLNA gene. The patient died at the age of 22 months from respiratory insufficiency during an infection of the lower respiratory tract. Our observation extends the list of the overlying cortical malformations associated with periventricular nodular heterotopia. It remains to be established whether this peculiar neuronal migration disorder represents a phenotype totally linked to 17q13.3 deletion or results from a combination of gene defects at 17q13.3 and 12q24.3.