Robin S. Fisher

Fiber composition of the human corpus callosum

The densities of fibers of different sizes were calculated in ten regions of the corpus callosum of twenty human brains (ten females, ten males). Light microscopic examination revealed a consistent pattern of regional differentiation of fiber types in the corpus callosum. Thin fibers are most dense in the anterior corpus callosum (genu), and decrease in density posteriorly towards the posterior midbody, where they reach a minimum. Towards the posterior corpus callosum (splenium), the density of thin fibers increases again, but in the posterior pole of the callosum the density decreases locally. Large-diameter fibers show a pattern complementary to that of thin fibers, having a peak of density in the posterior midbody and a local increase of density in the posterior pole of the corpus callosum. Across subjects, the overall density of callosal fibers had no significant correlation with callosal area and an increased callosal area indicated an increased total number of fibers crossing through. Considering different fiber sizes, this was only true for small diameter fibers, whose large majority is believed to interconnect association cortex. No sex differences in fiber composition of the corpus callosum were found.

Individual differences in brain asymmetries and fiber composition in the human corpus callosum.

There have been several recent reports concerning individual differences in the gross morphometry of the human corpus callosum. However, no studies exist on individual differences in the fiber composition of the corpus callosum. Here we report for the first time the relation of fiber composition in specific callosal segments (as seen in light microscopy) to anatomical asymmetries in language-gifted cortex, as a function of sex. We found a significant negative correlation between Sylvian fissure asymmetries and the total numbers of fibers in the isthmus of males, and in the anterior splenium of females. In addition, a population of relatively large fibers (between 1 micron and 3 microns in diameter) in the isthmus showed a strong negative correlation with perisylvian asymmetries only in males. These findings suggest a sex-dependent, pathway-specific decrease in interhemispheric connectivity with increasing lateralization.

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.

Infant rats: Sensorimotor ontogeny and effects of substantia nigra destruction

The ontogeny of sensorimotor behaviors of albino rats were evaluated from birth through adulthood (Experiment 1). Sensorimotor behaviors (e.g., visual and tactile orientation, forelimb and hindlimb hopping, righting reflexes) achieved mature (adultlike) characteristics at various ages during ontogeny and a rostral-caudal developmental pattern was revealed. In Experiment 2, the substantia nigra was bilaterally or unilaterally destroyed in rats at 10 or 25 days of age and the ontogeny of sensorimotor and regulatory (feeding, drinking, body weight regulation) behaviors were evaluated. Bilateral destruction of the substantia nigra, zona compacta, at 10 and 25 days of age resulted in transient cessation of suckling and/or feeding and drinking followed by recovery. Male brain-damaged rats had reduced body weight through 150-170 days of age. Specific feeding and drinking tests revealed the presence of residual regulatory deficits which seemed permanent. Sensorimotor testing revealed transient dysfunction for a variety of sensorimotor behaviors, with eventual recovery of normal sensorimotor capacity. The results are related to sensorimotor ontogeny and recovery from infant brain damage.

Neuropathologic findings in surgically treated hemimegalencephaly: immunohistochemical, morphometric, and ultrastructural study

SummaryTissues from three cases of hemimegalencephaly (HME) causing intractable seizures treated by cortical resection were studied using immunohistochemical, ultrastructural, and morphometric techniques. Severe cortical dysplasia was seen in all cases and included lesions best characterized as hemilissencephaly and polymicrogyria. Blurring of the cortex-white matter junction, the presence of large neuronal heterotopias, and neuronal cytomegaly were frequent observations. Immunohistochemical analysis demonstrated cellular colocalization of astrocytic markers glial fibrillary acidic protein and vimentin in one case of hemilissencephaly. Morphometric data showed significant increases over controls in neuronal profile area in all cases of HME. Neuronal cell density was increased significantly above controls in one of the cases. The study shows that HME results from severe cortical dysplasia which may be caused by multiple insults, manifest in one of several ways, and reflects abnormal or altered signals that regulate cortical morphogenesis.

Visualization of corticofugal projections during early cortical development in a τ-GFP-transgenic mouse

The first postmitotic neurons in the developing neocortex establish the preplate layer. These early‐born neurons have a significant influence on the circuitry of the developing cortex. However, the exact timing and trajectory of their projections, between cortical hemispheres and intra‐ and extra‐cortical regions, remain unresolved. Here, we describe the creation of a transgenic mouse using a 1.3 kb golli promoter element of the myelin basic protein gene to target expression of a τ–green fluorescent protein (GFP) fusion protein in the cell bodies and processes of pioneer cortical neurons. During embryonic and early neonatal development, the timing and patterning of process extension from these neurons was examined. Analysis of τ‐GFP fluorescent fibers revealed that progression of early labeled projections was interrupted unexpectedly by transient pauses at the corticostriatal and telencephalic–diencephalic boundaries before invading the thalamus just prior to birth. After birth the pioneering projections differentially invaded the thalamus, excluding some nuclei, e.g. medial and lateral geniculate, until postnatal days 10–14. Early labeled projections were also found to cross to the contralateral hemisphere as well as to the superior colliculus. These results indicate that early corticothalamic projections appear to pause before invading specific subcortical regions during development, that there is developmental regulation of innervation of individual thalamic nuclei, and that these early‐generated neurons also establish early projections to commissural and subcortical targets.

Neuronal cytoskeletal abnormalities in human cerebral cortical dysplasia

The cytoskeletal abnormalities of cortical neurons in human cerebral cortical dysplasia were compared by immunohistochemical methods to the neurofibrillary tangles of Alzheimers disease (AD). Surgical specimens from cortical resections performed for the treatment of intractable childhood seizures as well as autopsied samples from AD patients were analyzed with different antibodies directed against high- or medium-molecular mass neurofilament epitopes, phosphorylated or non-phosphorylated forms of neurofilaments, ubiquitin, the microtubule-associated protein tau, and paired helical filaments (PHF), a defining feature of AD tangles. A strong abnormal increase in immunoreactivity to the high and medium molecular mass neurofilament epitopes was seen in hypertrophic neurons of cortical dysplasia. These neurofilamentous accumulations of cortical dysplasia as well as AD tangles also displayed immunoreactivity with antibodies also displayed immunoreactivity with antibodies against phosphorylated and non-phosphorylated neurofilament epitopes, tau and ubiquitin. Only the AD tangles, however, were immunoreactive to the antiserum to PHF. These results replicate and extend our previous findings that the neurofibrillary accumulations in cerebral cortical dysplasia share some common antigens with the neurofibrillary tangles of AD but do not demonstrate immunoreactivity to PHF antiserum. The results also suggest that the cytoskeletal abnormalities observed in neurons of cortical dysplasia may result in part from alterations in the level of expression, in phosphorylation state or in transport of cytoskeletal components.

D2 dopamine receptor distribution in the rodent CNS using anti-peptide antisera.

D2 dopamine receptors were identified immunohistochemically in rodent tissues using anti-peptide antisera to distinguish regional and cellular staining patterns. These subtype selective polyclonal antibodies were directed against both extracellular and intracellular regions of the native protein and showed that the D2 dopamine receptors are widely distributed within the nervous system. The highest expression of D2-like dopamine receptor immunoreactivity was visualized in the forebrain and components of the basal ganglia, supportive of previous investigations of the D2 dopamine receptor distribution using in vitro autoradiographic ligand binding or in situ hybridization for its messenger RNA. The anti-peptide antisera could detect the dopamine receptor in both perfusion-fixed and fresh-frozen tissue preparations. The reactive cells and their processes could be distinguished using experimental incubations from 1:8,000 (in immunofluorescence processing) to 1:80,000 (in immunoperoxidase processing) in the most reactive nervous system region, the neostriatum. The antisera are selectively directed against extracellular or intracellular epitopes in both the long and short isoforms of the D2 dopamine receptor, and should prove useful in subsequent studies of the subcellular distribution of this receptor in particular, and the dopamine system in general.

The GABAergic striatonigral neurons of the cat: demonstration by double peroxidase labeling.

GABAergic striatonigral neurons were demonstrated in the adult cat by the specific double peroxidase labeling of a transmitter marker with an agranular appearance (GAD, the synthetic enzyme of GABA) and a connectivity marker with a granular appearance (WGA-HRP). Each marker was associated with different organelles confined to the perikaryal cytoplasm of neurons. GABAergic striatonigral neurons were of medium size, high frequency and wide location in the rostral caudate nucleus and putamen based on correlative light and electron microscopic identification. These cells had somatic and/or proximal dendritic spines and folded nuclear envelopes in some cases. They received GABAergic axosomatic and axodendritic inputs with symmetric synaptic specializations. They were also contacted by axosomatic, axodendritic and axospinous terminals with asymmetric synaptic specializations. These results indicate that the GABAergic striatonigral neurons are, for the most part, medium spiny cells that also emit intrastriatal axonal collaterals. Their intra- and extrastriatal axons mediate inhibitory postsynaptic influences on their targets. Their degeneration might contribute to the GABAergic deficits found in the basal ganglia in Huntingtons disease.

Interhemispheric organization of corticocaudate projections in the cat: A retrograde double-labelling study

The organization of interhemispheric corticocaudate projections in the cat was assessed by the retrograde axonal transport of different marker substances. These double-labelling methods indicated the existence of three types of neocortical efferents terminating in the caudate nucleus (Cd): (1) uncrossed axons projecting only to the ipsilateral Cd; (2) crossed axons projecting strictly to the contralateral Cd; and, (3) divergent axons projecting to both Cds. In all cases, the corticocaudate projections originated from small-to-medium sized pyramidal neurons situated in layers III, IV and V of the rostral neocortex. Interhemispheric inputs to the Cd may account, to some extent, for the functional coupling of the right and left basal ganglia.