Neurogenesis of heterotopic gray matter in the brain of the microcephalic mouse

Astrocytes in rare neurological conditions: Morphological and functional considerations

Astrocytes are a population of central nervous system (CNS) cells with distinctive morphological and functional characteristics that differ within specific areas of the brain and are widely distributed throughout the CNS. There are mainly two types of astrocytes, protoplasmic and fibrous, which differ in morphologic appearance and location. Astrocytes are important cells of the CNS that not only provide structural support, but also modulate synaptic activity, regulate neuroinflammatory responses, maintain the blood-brain barrier, and supply energy to neurons. As a result, astrocytic disruption can lead to widespread detrimental effects and can contribute to the pathophysiology of several neurological conditions. The characteristics of astrocytes in more common neuropathologies such as Alzheimer's and Parkinson's disease have significantly been described and continue to be widely studied. However, there still exist numerous rare neurological conditions in which astrocytic involvement is unknown and needs to be explored. Accordingly, this review will summarize functional and morphological changes of astrocytes in various rare neurological conditions based on current knowledge thus far and highlight remaining neuropathologies where astrocytic involvement has yet to be investigated.

Current Evidence for Developmental, Structural, and Functional Brain Defects following Prenatal Radiation Exposure

Ionizing radiation is omnipresent. We are continuously exposed to natural (e.g., radon and cosmic) and man-made radiation sources, including those from industry but especially from the medical sector. The increasing use of medical radiation modalities, in particular those employing low-dose radiation such as CT scans, raises concerns regarding the effects of cumulative exposure doses and the inappropriate utilization of these imaging techniques. One of the major goals in the radioprotection field is to better understand the potential health risk posed to the unborn child after radiation exposure to the pregnant mother, of which the first convincing evidence came from epidemiological studies on in utero exposed atomic bomb survivors. In the following years, animal models have proven to be an essential tool to further characterize brain developmental defects and consequent functional deficits. However, the identification of a possible dose threshold is far from complete and a sound link between early defects and persistent anomalies has not yet been established. This review provides an overview of the current knowledge on brain developmental and persistent defects resulting from in utero radiation exposure and addresses the many questions that still remain to be answered.

Mechanisms of neurotoxicity induced in the developing brain of mice and rats by DNA-damaging chemicals

It is not widely known how the developing brain responds to extrinsic damage, although the developing brain is considered to be sensitive to diverse environmental factors including DNA-damaging agents. This paper reviews the mechanisms of neurotoxicity induced in the developing brain of mice and rats by six chemicals (ethylnitrosourea, hydroxyurea, 5-azacytidine, cytosine arabinoside, 6-mercaptopurine and etoposide), which cause DNA damage in different ways, especially from the viewpoints of apoptosis and cell cycle arrest in neural progenitor cells. In addition, this paper also reviews the repair process following damage in the developing brain.

An automatic integrated approach for stained neuron detection in studying neuron migration

Neurons that come to populate the six-layered cerebral cortex are born deep within the developing brain in the surface of the embryonic cerebral ventricles. It is very important to detect these neurons for studying histogenesis of the brain and abnormal migration that had been linked to cognitive deficits, mental retardation, and motor disorders. The visualization of labeled cells in brain sections was performed by immunocytochemical examination and its image data were documented to microscopic pictures. Based on the fact, automatic accurate neurons labeling is prerequisite instead of time-consuming manual labeling. In this article, a fully automated image processing approach is proposed to detect all the stained neurons in microscopic images. First of all, dark stained neurons are achieved by thresholding in blue channel of image. And then a modified fuzzy c-means clustering method, called alternative fuzzy c-means is applied to achieve higher classification accuracy in extracting constraint factor. Finally, watershed based on gradient vector flow is employed to the constraint factor image to segment all the neurons, including clustered neurons. The results demonstrate that the proposed method can be a useful tool in neuron image analysis.

Expression of layer-specific markers in the adult neocortex of BCNU-Treated rat, a model of cortical dysplasia

The experimental model of cortical dysplasia (CD) obtained by administering carmustine (1-3-bis-chloroethyl-nitrosurea [BCNU]) in pregnant rat uterus mimics the histopathological abnormalities observed in human CD patients: altered cortical layering, and presence of heterotopia and dysmorphic/heterotopic neurons. To investigate further the cortical layering disruption and the neuronal composition of heterotopia in BCNU-exposed cortex, we analyzed the expression pattern of the transcription factors Nurr1, Er81, Ror-beta, and Cux2 (respectively specific markers of layers VI, V, IV and superficial layers) in the cortical areas of BCNU-treated rats by means of in situ hybridization, and compared the findings with those observed in adult control rats. Combining in situ hybridization and immunohistochemistry we also investigated the origin of dysmorphic or heterotopic neurons. The main results of the present study are (i) the analysis of cortical layer thickness revealed that the cortical thinning in the BCNU model was prevalently restricted to the superficial layers; (ii) in cortical and periventricular heterotopia, the prevalent presence of superficial layer neurons in the internal areas, and deeper layer neurons in a more peripheral region, demonstrated a rudimentary pattern of laminar organization in nodule formation; and (iii) the Er81 signal in the dysmorphic and heterotopic pyramidal neurons located in layers I/II showed that they belong to layer V. These results shed light on the disorganization of the laminar architecture of the BCNU model by providing correlations with normal cortical layering and revealing the ontogenesis of heterotopia and heterotopic/dysmorphic neurons. They also provide strong evidence of the usefulness of layer-specific markers in investigating the neuropathology of CD, thus opening up the possibility of expanding their application to human neuropathology.

Spatial learning and expression of neural cell adhesion molecule L1 in rats X-irradiated prenatally

The present study was designed to present evidence to clarify the relationships between learning ability, neuronal cell adhesion molecule L1 expression and hippocampal structural changes in the rat model received X-irradiation at an embryonic stage (E15). Water maze task indicated that all of the irradiated rats failed to learn the task in the whole training procedure. Their latency to the platform and swimming distance were significant differences from those sham-treated controls. Histological studies showed that the hippocampal ectopias induced by X-rays in the CA1 were involved in the spatial learning impairment, in which they hampered normal processes in learning development and transmission of information. Number, size and positions of the ectopias in the dorsal parts of the hippocampus were confirmed to be related to degrees of spatial learning impairment. On the other hand, L1 expression in the hippocampus was examined with Western blot analysis. The results indicated a lower content of L1 in the irradiated rats. A decrease in L1 might be one of reasons to cause disorganization of the septohippocampal pathways. These findings suggest some mechanisms of spatial learning impairment can be attributed to the formation of the hippocampal ectopias and redaction of L1 following prenatal exposure to X-irradiation.

Multiple defects in the formation of rat cortical axonal pathways following prenatal X-ray irradiation

Prenatal X-ray irradiation is known to result in severe defects of neuronal migration and laminar formation in the cerebral cortex. We examined the formation of cortical afferent and efferent pathways in rats that had been exposed to X-ray irradiation (1.0 Gy) at embryonic day 14 (E14), by birthdating with bromodeoxyuridine (BrdU) and axonal labeling with 1-1'-dioctodecyl-3,3,3',3'- tetramethyl-indocarbocyanine perchlorate (DiI), in addition to immunohistochemical staining for various axonal markers including neurofilament, and cell adhesion molecules L1 and TAG-1. The results obtained were as follows. (i) The neuroepithelium formed germinal rosettes and concavities in the cortical anlage from 2 days after irradiation. Neurons generated in the neuroepithelium accumulated to form subcortical heterotopia and obstructed pathway formation in the intermediate zone, resulting in an aberrant trajectory of TAG-1-immunoreactive cortical efferent axons. (ii) In rats exposed to X-ray irradiation at E14, cystic cavities were formed in the cortex-striatum boundary region between E15 and E17, probably because of delayed cell death of neurons generated at E14. These cavities transiently interrupted both cortical afferent (L1-positive) and efferent (TAG-1-positive) axons. (iii) X-ray irradiation at E14 partially destroyed subplate neurons (transient targets of thalamic afferent axons) and disturbed the arrangement of the subplate layer. This resulted in a misrouting of neurofilament- and L1-immunoreactive thalamocortical axons that obliquely traversed the cortical plate to run up to the superficial layer. The present study demonstrates for the first time that X-ray irradiation during initial cortical development causes multiple defects in the formation of cortical afferent and efferent pathways.

Hemorrhagic hydrocephalus (hhy): a novel mutation on mouse chromosome 12

A novel mouse hemorrhagic hydrocephalus mutation (hhy) inherited in an autosomal recessive manner on chromosome 12 has been found at the Osaka Prefecture University. The hhy homozygous mutant had dilated lateral ventricles and a communicating aqueduct, with no histological abnormalities either in the subarachnoid space or in the choroid plexus. Multiple hemorrhages in the meninges and throughout the brain parenchyma of the mutant were relevant to advanced stages of hydrocephalus. Subcortical heterotopia was detected unexceptionally in the mutants. Thus, the hhy mutation is characterized by three different abnormalities, i.e. hydrocephalus, intracranial hemorrhage and subcortical heterotopia.

Distribution of Cortical Interneurons in Grey Matter Heterotopia in Patients with Epilepsy

PURPOSE

Grey matter heterotopia are well-defined malformations of the cortex often associated with severe epilepsy. Defects have been identified in genes, including DCX and FLN1, that influence radial migration of postmitotic cells from the ventricular zone to the cortical plate. A proportion of cortical gamma-aminobutyric acid (GABA)-containing interneurons may arise from the ganglionic eminence of the ventral telencephalon. We aimed to identify the subtypes and localisation of interneurons within grey matter heterotopia relative to cortex.

METHODS

By using quantitative immunohistochemistry, we studied the density and distribution of interneurons within six cases of grey matter heterotopia in postmortem tissue from patients with epilepsy.

RESULTS

In many cases, a suggestion of focal rudimentary laminar arrangement and small reelin-positive cells was identified within the heterotopia. Immunohistochemistry for glutamic acid decarboxylase(65/57), parvalbumin, calbindin, and calretinin showed inhibitory neurons of all subtypes represented within the heterotopia, and of normal morphology. The mean densities of interneurons were overall similar to those of the overlying cortex, but the interneurons showed less organisation and were more randomly orientated compared with cortex.

CONCLUSIONS

Interneurons within heterotopia probably arise from the ventricular zone, but their abnormal local organization may influence the epileptogenicity of these lesions.

Experimental model for irradiating a restricted region of the rat brain using heavy-ion beams

Heavy-ion beams have the feature to administer a large radiation dose in the vicinity of the endpoint in the beam range, its irradiation system and biophysical characteristics are different from ordinary irradiation instruments like X-rays or gamma-rays. In order to get clarify characteristic effects of heavy-ion beams on the brain, we have developed an experimental system for irradiating a restricted region of the rat brain using heavy-ion beams. The left cerebral hemispheres of the adult rat brain were irradiated at dose of 50 Gy charged carbon particles (290 MeV/nucleon; 5 mm spread-out Bragg peak). After irradiation, the characteristics of the heavy-ion beams and the animal model were studied. Histological examination and measurement showed that extensive necrosis was observed between 2.5 mm and 7.5 mm depth from the surface of the rat head, suggesting a relatively high dose and uniform dose was delivered among designed depths and the spread-out Bragg peak used here successfully and satisfactorily retained its high-dose localization in the defined region. We believe that our experimental model for irradiating a restricted region of the rat brain using heavy-ion beams is a good model for analyzing regional radiation susceptibility of the brain.

Types and three-dimensional distribution of neuronal ectopias in the brain of mice prenatally subjected to X-irradiation

The types and three-dimensional distribution of neocortical ectopias following prenatal exposure to X-irradiation were studied by a histological examination and computer reconstruction techniques. Pregnant ICR mice were subjected to X-irradiation at a dose of 1.5 Gy on embryonic day 13. The brains from 30-day-old mice were serially sectioned on the frontal plane at 15 microns, stained with HE and observed with a microscope. The image data for the sections were input to a computer, and then reconstructed to three-dimensional brain structures using the Magellan 3.6 program. Sectional images were then drawn on a computer display at 240 microns intervals, and the positions of the different types of neocortical ectopias were marked using color coding. Three types of neocortical ectopias were recognized in the irradiated brains. Neocortical Lay I ectopias were identified as small patches in the caudal occipital cortex, and were located more laterally in the neocortex in caudal sections than in the rostral sections. Periventricular ectopias were located more rostrally than Lay I ectopias, and were found from the most caudal extent of the presumed motor cortex to the most caudal extent of the lateral ventricle. Hippocampal ectopias appeared as continuous linear bands, and were frequently associated with the anterior parts of the periventricular ectopias.