Histogenetic processes leading to the laminated neocortex: migration is only a part of the story.

The principal events of neocortical histogenesis were anticipated by work published prior to the 20th century. These were neuronal proliferation and migration and the complex events of cortical pattern formation leading to a laminated architecture where each layer is dominated by a principal neuronal class. Work that has followed has extended the knowledge of the workings of the proliferative epithelium, cellular mechanisms of migration and events through which cells are winnowed and then differentiate once their postmigratory positions are established. Work yet ahead will emphasize mechanisms that coordinate the molecular events that integrate proliferation and cell class specification in relation to the final neocortical neural system map.

Numbers, time and neocortical neuronogenesis: a general developmental and evolutionary model.

The number of neurons in the neocortex is the product of the size of the preneuronogenetic founder population, that is, the number of proliferative cells that are present at the onset of neuronogenesis, and neuronogenetic amplification occurring as neurons are being produced. The amount of neuronogenetic amplification is determined by changes in the output fraction, Q, from 0 to 1, over a fixed number of cell cycles. Greater neuronogenetic amplification would occur across species if the number of cell cycles during which Q < 0.5 increased. Since neither the length of the cell cycle nor the length of the neuronogenetic interval, that is, time per se, influence neuron number directly, it is speculated that changes in these parameters are essential to neuronal diversity.

Cell birth, cell death, cell diversity and DNA breaks: how do they all fit together?

Substantial death of migrating and differentiating neurons occurs within the developing CNS of mice that are deficient in genes required for repair of double-stranded DNA breaks. These findings suggest that large-scale, yet previously unrecognized, double-stranded DNA breaks occur normally in early postmitotic and differentiating neurons. Moreover, they imply that cell death occurs if the breaks are not repaired. The cause and natural function of such breaks remains a mystery; however, their occurrence has significant implications. They might be detected by histological methods that are sensitive to DNA fragmentation and mistakenly interpreted to indicate cell death when no relationship exists. In a broader context, there is now renewed speculation that DNA recombination might be occurring during neuronal development, similar to DNA recombination in developing lymphocytes. If this is true, the target gene(s) of recombination and their significance remain to be determined.

Brain asymmetries in autism and developmental language disorder: a nested whole-brain analysis.

We report a whole-brain MRI morphometric survey of asymmetry in children with high-functioning autism and with developmental language disorder (DLD). Subjects included 46 boys of normal intelligence aged 5.7-11.3 years (16 autistic, 15 DLD, 15 controls). Imaging analysis included grey-white segmentation and cortical parcellation. Asymmetry was assessed at a series of nested levels. We found that asymmetries were masked with larger units of analysis but progressively more apparent with smaller units, and that within the cerebral cortex the differences were greatest in higher-order association cortex. The larger units of analysis, including the cerebral hemispheres, the major grey and white matter structures and the cortical lobes, showed no asymmetries in autism or DLD and few asymmetries in controls. However, at the level of cortical parcellation units, autism and DLD showed more asymmetry than controls. They had a greater aggregate volume of significantly asymmetrical cortical parcellation units (leftward plus rightward), as well as a substantially larger aggregate volume of right-asymmetrical cortex in DLD and autism than in controls; this rightward bias was more pronounced in autism than in DLD. DLD, but not autism, showed a small but significant loss of leftward asymmetry compared with controls. Right : left ratios were reversed, autism and DLD having twice as much right- as left-asymmetrical cortex, while the reverse was found in the control sample. Asymmetry differences between groups were most significant in the higher-order association areas. Autism and DLD were much more similar to each other in patterns of asymmetry throughout the cerebral cortex than either was to controls; this similarity suggests systematic and related alterations rather than random neural systems alterations. We review these findings in relation to previously reported volumetric features in these two samples of brains, including increased total brain and white matter volumes and lack of increase in the size of the corpus callosum. Larger brain volume has previously been associated with increased lateralization. The sizeable right-asymmetry increase reported here may be a consequence of early abnormal brain growth trajectories in these disorders, while higher-order association areas may be most vulnerable to connectivity abnormalities associated with white matter increases.

Cortical abnormalities in schizophrenia identified by structural magnetic resonance imaging.

BACKGROUND: Relatively few magnetic resonance imaging studies of schizophrenia have investigated the entire cerebral cortex. Most focus on only a few areas within a lobe or an entire lobe. To assess expected regional alterations in cortical volumes, we used a new method to segment the entire neocortex into 48 topographically defined brain regions. We hypothesized, based on previous empirical and theoretical work, that dorsolateral prefrontal and paralimbic cortices would be significantly volumetrically reduced in patients with schizophrenia compared with normal controls. METHODS: Twenty-nine patients with DSM-III-R schizophrenia were systematically sampled from 3 public outpatient service networks in the Boston, Mass, area. Healthy subjects, recruited from catchment areas from which the patients were drawn, were screened for psychopathologic disorders and proportionately matched to patients by age, sex, ethnicity, parental socioeconomic status, reading ability, and handedness. Analyses of covariance of the volumes of brain regions, adjusted for age- and sex-corrected head size, were used to compare patients and controls. RESULTS: The greatest volumetric reductions and largest effect sizes were in the middle frontal gyrus and paralimbic brain regions, such as the frontomedial and frontoorbital cortices, anterior cingulate and paracingulate gyri, and the insula. In addition, the supramarginal gyrus, which is densely connected to prefrontal and cingulate cortices, was also significantly reduced in patients. Patients also had subtle volumetric increases in other cortical areas with strong reciprocal connections to the paralimbic areas that were volumetrically reduced. CONCLUSION: Findings using our methods have implications for understanding brain abnormalities in schizophrenia and suggest the importance of the paralimbic areas and their connections with prefrontal brain regions.

Cerebral structural abnormalities in obsessive-compulsive disorder. A quantitative morphometric magnetic resonance imaging study.

BACKGROUND: A previous pilot study of only posterior brain regions found lower white-matter volume in patients with obsessive-compulsive disorder than in normal control subjects. We used new cohorts of patients and matched normal control subjects to study whole-brain volume differences between these groups with magnetic resonance imaging-based morphometry. METHODS: Ten female patients with obsessive-compulsive disorder and 10 female control subjects, matched for handedness, age, weight, education, and verbal IQ, underwent magnetic resonance imaging with a 3-dimensional volumetric protocol. Scans were blindly normalized and segmented by means of well-characterized semiautomated intensity contour mapping and differential intensity contour algorithms. Brain structures investigated included the cerebral hemispheres, cerebral cortex, diencephalon, caudate, putamen, globus pallidus, hippocampus amygdala, third and fourth ventricles, corpus callosum, operculum, cerebellum, and brain stem. Anterior to posterior neocortical regions, including precallosum, anterior pericallosum, posterior pericallosum, and retrocallosum, with adjacent white matter were also measured. Volumes found different between groups were correlated with Yale-Brown Obsessive Compulsive Scale score and Rey-Osterieth Complex Figure Test measures. RESULTS: Confirming results of our earlier pilot study and expanding the findings to the whole brain, patients with obsessive-compulsive disorder had significantly less total white matter but, in addition, significantly greater total cortex and opercular volumes. Severity of obsessive-compulsive disorder and nonverbal immediate memory correlated with opercular volume. CONCLUSIONS: Replication of volumetric white-matter differences suggests a widely distributed structural brain abnormality in obsessive-compulsive disorder. Whereas determining the etiogenesis may require research at a microscopic level, understanding its functional significance can be further explored via functional neuroimaging and neuropsychological studies.

The Golgi rapid method in clinical neuropathology: the morphologic consequences of suboptimal fixation.

Cytologic changes in neurons of the neocortex of mice consequent to suboptimal fixation have been investigated systematically in Golgi-rapid preparations. With few exceptions, there is no alteration in cellular morphology if the brain is refrigerated after death, and fixed by immersion within 3 hours. With latencies of fixation of 6 hours or more, autolytic changes supervene which modify the general histologic appearance and the morphology of individual cells. In general, the degree of tissue and cellular change is proportional to the latency between death and tissue fixation. Similar alterations in cellular morphology and general tissue appearance are found in Golgi-rapid impregnations of human brains obtained at autopsy. However, the degree of tissue autolysis in the human specimens bears a less predictable relationship to the latency of fixation after death. The duration of preterminal metabolic encephalopathy appears to be equally decisive as a determinant of tissue preservation.

Structural and chemical alterations in the cerebral maldevelopment of fetal cerebro-hepato-renal (Zellweger) syndrome.

The cerebra of four abortuses (estimated gestational age 14-22 weeks), diagnosed as cerebro-hepato-renal (Zellweger) syndrome in utero, were examined morphologically with light microscopic, immunocytochemical and ultrastructural techniques and biochemically with gas liquid chromatographic assays for cholesterol ester fatty acids and plasmalogens. Centrosylvian architectonic abnormalities consisting, in part, of thin cortical plates and broad subcortical heterotopic zones were found in all abortuses. Astrocytes, neuroblasts, immature neurons and radial glia contained abnormal pleomorphic cytosomes, presumably of variable lipid composition. The same areas exhibited increases in cholesterol ester very long chain fatty acids and decreased plasmalogens. A pathogenetic hypothesis, proposing that regional tissue constraints act in concert with a peroxisomal-derived biochemical abnormality to impede centrosylvian neuronal migration, is discussed.

Anatomy of stroke, Part II: volumetric characteristics with implications for the local architecture of the cerebral perfusion system.

BACKGROUND AND PURPOSE: The margin of a stroke is assumed to approximate a trace of the isobar of the perfusion threshold for infarction at the time that infarction occurred. Working from this hypothesis, we have analyzed stroke topography and volume in MR images obtained at a time remote from the stroke event. We have derived parameters from these images that may give information on local perfusion competence and microvascular architecture because they influenced the contour of stroke at the time infarction occurred. METHODS: MR images were obtained months after presumed embolic middle cerebral artery stroke in 21 subjects. Volumetric analyses of image data were undertaken with respect to the tissue shape of stroke and scaling ratios of anatomic partitions involved in stroke. RESULTS: For stroke confined to a single volume, the 3-dimensional form conforms to a parabola in which the height-to-width ratios are variable. The ratio for cortex is greater than that for underlying white matter. Scaling ratios indicate a close correlation between volume of cortex and radiata destroyed and total volume of stroke, but the relative proportions vary as a function of location within the M4 territory. CONCLUSIONS: Scaling ratios for cortex and radiata to stroke volume are consistent with vascular studies that depict a modular microvascular perfusion architecture for the cortex and underlying white matter. The stroke descriptors are inferred to be related to the competence of collateral perfusion at the time that stroke occurred. This inference may be tested by serial volumetric analysis of the perfusion-diffusion examination mismatch immediately and over the longer-term evolution of stroke.

Anatomy of stroke, Part I: an MRI-based topographic and volumetric System of analysis.

BACKGROUND AND PURPOSE: The clinical diagnosis and treatment of stroke, as well as investigations into the underlying pathophysiology of the disease, hinge on inferences from the anatomy of the stroke lesion. We describe an MRI-based system of topographic and volumetric analysis that considers distribution of infarct with respect to neuroanatomic structures, superficial and deep perfusion compartments, and gray and white matter tissue types. METHODS: MRI-based 3-dimensional topographic and volumetric analysis of presumed MCA embolic stroke was performed months after the acute event in 21 subjects ranging in age from 34 to 75 years. RESULTS: The topography of infarction was greatly variable, with virtually all regions of the MCA territory involved in at least 1 stroke in the series. In 14, there was involvement of the M1 as well as the M2 through M4 territories; in 6, there was involvement of only the M2 through M4 territories; and in 2, there was involvement of only the M1 territory. The volumes varied from 3.1 to 256 cm3, corresponding approximately to a range of 1% to 90% of the total MCA territory. CONCLUSIONS: The system of topographic and volumetric analysis is generally applicable to all strokes in the forebrain where the infarct is visualized in MRI, independent of vascular territory, clinical correlates, and interval between stroke and MRI. The results emphasize the variety of topographic patterns and lesion volumes of strokes. Intended long-range applications include correlation of outcome of stroke with predictions from acute-phase diffusion- and perfusion-weighted imaging and investigations of the potential benefit of therapeutic agents.

Reduced basal ganglia volumes in trichotillomania measured via morphometric magnetic resonance imaging.

A morphometric magnetic resonance imaging (MRI) study compared volumes of brain structures in 10 female subjects with trichotillomania (repetitive hair-pulling) versus 10 normal controls matched for sex, age, handedness, and education. Three-dimensional MRI scans were blindly normalized and segmented using well-characterized semiautomated intensity and differential contour algorithms by signal intensity-frequency histograms. Consistent with one a priori hypothesis, left putamen volume was found to be significantly smaller in trichotillomania subjects as compared with normal matched controls. This is the first report of a structural brain abnormality in trichotillomania. Results are discussed in terms of putative relationships between trichotillomania, Tourette's syndrome, and obsessive-compulsive disorder.

Thalamic and amygdala-hippocampal volume reductions in first-degree relatives of patients with schizophrenia: an MRI-based morphometric analysis.

BACKGROUND: Schizophrenia is characterized by subcortical and cortical brain abnormalities. Evidence indicates that some nonpsychotic relatives of schizophrenic patients manifest biobehavioral abnormalities, including brain abnormalities. The goal of this study was to determine whether amygdala-hippocampal and thalamic abnormalities are present in relatives of schizophrenic patients. METHODS: Subjects were 28 nonpsychotic, and nonschizotypal, first-degree adult relatives of schizophrenics and 26 normal control subjects. Sixty contiguous 3 mm coronal, T1-weighted 3D magnetic resonance images of the brain were acquired on a 1.5 Tesla magnet. Cortical and subcortical gray and white matter and cerebrospinal fluid (CSF) were segmented using a semi-automated intensity contour mapping algorithm. Analyses of covariance of the volumes of brain regions, controlling for expected intellectual (i.e., reading) ability and diagnosis, were used to compare groups. RESULTS: The main findings were that relatives had significant volume reductions bilaterally in the amygdala-hippocampal region and thalamus compared to control subjects. Marginal differences were noted in the pallidum, putamen, cerebellum, and third and fourth ventricles. CONCLUSIONS: Results support the hypothesis that core components of the vulnerability to schizophrenia include structural abnormalities in the thalamus and amygdala-hippocampus. These findings require further work to determine if the abnormalities are an expression of the genetic liability to schizophrenia.

Patterns of cell and fiber distribution in the neocortex of the reeler mutant mouse.

In the neocortex of the reeler mutant mouse, there is inversion in the normal relative positions of polymorphic and pyramidal cells and of large with respect to medium-sized and small pyramidal cells. Granule cells are concentrated at a near-normal mid-cortical level in the mutant. As in the normal animal, and despite cell malposition in reeler, the principal tangential fiber system lies in the zone of polymorphic cells. Large fiber fascicles, known from experimental studies to be principally thalamo-cortical afferents, enter the tangential fiber system in the polymorphic cell zone of both reeler and normal neocortex. In the mutant these fascicles must traverse the full width of the cortex to reach this fiber system in its superficial location. In both normal and mutant animals single fibers, again principally thalamo-cortical afferents, pass from the principal tangertial fiber system to ramify in a fiber feltwork in the zone of granule cells. In the mutant these descend whereas in the normal animal they ascend. Also, as in the normal mouse, single fibers pass radially between all levels of the mutant cortex and the central white matter. Regional variations in the character, the pattern of distribution and the relative prominence of homologous cell and fiber elements are closely parallel in reeler and normal. This suggests that cell differentiation and the tangenital organization of reeler neocortex are normal despite cell malposition in the mutant.

Architectonic map of neocortex of the normal mouse.

The neocortex of the normal mouse has been subdivided into architectonic fields on the basis of its cellular and fiber patterns. The map of medial, retrohippocampal, frontal and insular regions is little different from that of Brodmann as modified in minor ways by Krieg. The map of parietal, occipital and temporal regions follows closely the major rearrangements introduced to Brodmann's map by Krieg. Krieg's map has been modified to give individual status to the barrel fields and to disignate occipital fields around the full circumference of field 17, and temporal fields circumferentially around field 41.

Tangential organization of thalamic projections to the neocortex in the mouse.

Using the anterograde degeneration technique, we examine the tangential organization of a thalamofugal axon population (class I of Frost and Caviness, '80) whose terminations are preferentially distributed to the middle tier (located in layers III and/or IV) of three radially separated tiers of thalamic projections to the neocortex. Less extensive data are also presented on the tangential organization of thalamofugal axon populations (class II of Frost and Caviness, '80) that do not terminate preferentially in the middle tier, but that are otherwise heterogeneous with respect to their radial pattern of intracortical termination. The projections of class I axons are distributed to all neocortical fields with the possible exception of fields 13,25, and 35. The class I projections to a given cortical field (with the possible exception of the cortex of the second somatosensory representation) originate in only one thalamic nucleus. The class I projections of an individual thalamic nucleus form a cortical representation of the nucleus that constitutes a "first order line-to-line" (topologic) transformation of the nuclear volume. The ensemble of class I projections forms a cortical representation of the corresponding thalamic regions that constitutes a "second order line-to-line" (non-topologic) transformation of the thalamic volume. Class II axons project to all neocortical fields. Classs II and class I projections contrast in that the class II projections of multiple thalamic nuclei overlap in the tangential plane of any given sector of the cortex. While the class II projections of the intralaminar nuclei and the widely projecting ventromedial nucleus are known to be topologically organized, the tangential organization of class II projections arising in other nuclei is incompletely understood.

Radial organization of thalamic projections to the neocortex in the mouse.

The intracortical distributions of the thalamic projections to a large number of neocortical fields are studied by the anterograde degeneration methods in the mouse. The basic radial distribution of terminating thalamofugal axons is uniform throughout the mouse cortex and is essentially the same as that encountered in other mammalian species. Terminating axons are concentrated in three tiers: an outer tier in layer I, a middle tier in layers IV and/or III, and an inner tier in layer VI. In most fields, terminating axons also extend, to some extent, into layer V. Variations are encountered from field to field, particularly in the density and degree of divergence of projections and in the radial extent of individual tiers with respect to cytoarchitectonic layers. In accord with other studies, the thalamic projections to each field appear to be composed of two general axon classes. Class I axons terminate densely in the middle tier, seem to be of large caliber, and often have collaterals to the other tiers. Class II axons do not terminate densely in the middle tier and seem to be of small caliber. Terminating class II axons may be distributed to one or more tiers and may be concentrated in the inner and/or outer tiers. The thalamic projection to each field has its origin in multiple nuclei. All thalamic nuclei projecting to the neocortex appear to have class II projections and many also have class I projections. Patterns of degeneration in the cortex associated with lesions in different positions in many nuclei suggest that thalamic relay neurons are organized along "lines of projection"--neurons in the same line projecting to the same tangentially restricted cortical region. The neurons of origin of class I and class II axons are intermixed along the lines of projection.

Interhemispheric neocortical connections of the corpus callosum in the reeler mutant mouse: a study based on anterograde and retrograde methods.

The tangential organization of the callosal system of interhemispheric connections, as judged by the distribution of axon terminals as well as by the distribution of cells of origin of callosal axons, is normal in the reeler mutant mouse. As in the normal animal connections between the two cerebral hemispheres are homotopic. In the reeler, as in the normal animal, medium-sized pyramidal cells are, numerically speaking, the principal cells of origin of the callosal system. These lie superficially in the cortex of the normal animal but deep within the cortex of reeler. Callosal terminals are most densely concentrated at the cortical level of the small and medium-sized pyramids in both reeler and normal animals. It is probable, therefore, that the same classes of neurons are interconnected by the callosal system in the normal and reeler mouse despite malposition of neurons in reeler. The patterns of intracortical distribution of terminals of callosal axons is evidently governed by the positions of their target cells.

Interhemispheric neocortical connections of the corpus callosum in the normal mouse: a study based on anterograde and retrograde methods.

Interhemispheric neocortical connections are widely distributed through the corpus callosum in the mouse. Callosal connections are present in all cytoarchitectonic fields except field 25. The distal extemity representations of SmI, and MsI the representation of the mystacial vibrissae in SmI, and the more peripheral field representation of VI are relatively acallosal. Dense projections lie in the midline or truncal representations of SmI, MsI, SmII, at the vertical meridian representations bordering field 17, and medial to the AI representation. The radial distribution of terminals is bimodal in most cytoarchitectonic fields. It is unimodal in the supracallosal segment of field 29b and fields 49 and 27, trimodal in fields 13 and 35. The cells of origin of callosal fibers appear to have the same topographic pattern of distribution as the callosal terminals, observing the same steep and gradual density gradients. No cells giving rise to callosal axons are identified in the acallosal regions of fields 2 and 17. Further, superficial focal lesions in cortical areas which receive callosal connections give rise only to homotopic contralateral degeneration. Acallosal areas of 17 and 2 give rise to no callosal connections. The cells of origin of callosal connections are located at all laminar levels of the cortex and include pyramidal and polymorphic cells but not the granule cells of layer IV.

Thalamocortical connections in newborn mice.

Thalamocortical axons reach the developing neocortex and become distributed within the cortical subplate during the third week of gestation. The present study is an analysis of the organization of connections that link thalamus and cortical subplate (corresponding to future layers V and VI) at birth. This age antedates the ascent of thalamic axons to contact cells of the supragranular layers, their principal targets in the adult cortex. At birth thalamic nuclear subdivisions are explicit; field-characteristic cytoarchitectonic features, relating principally to the infragranular layers, delineate the majority of neocortical fields. The projection of principal relay nuclei upon the majority of fields of the cerebral convexity has been mapped by means of retrograde transport of HRP. Nucleus-to-field interrelationships as well as topologic order of the overall thalamic projection prove to be identical to that in the adult animal. The neonatal projection appears to be somewhat more divergent than that of the adult.

Thalamocortical projections in the reeler mutant mouse.

The normal radial distribution of the different neocortical cell classes is inverted in the reeler mutant mouse. The organization of the thalamocortical projection in adult reelers has been investigated by using anterograde degeneration techniques. Thalamocortical axons follow anomalous trajectories to their target cytoarchitectonic fields in the mutant. After leaving the internal capsule, the axons ascend in sigmoid-shaped fascicles to enter a fiber stratum near the cortical surface. The axons course through this superficial stratum until they reach their target fields and then descend to terminate in deeper cortical planes. In the normal animal, by contrast, the axons course tangentially at the interface of the cortex with the subcortical white matter; upon reaching their target fields, they ascend to terminate more superficially in the cortex. Thus, in both genotypes the tangential portions of the axon trajectories pass through the polymorphic cell population. Both with respect to degree of divergence and the radial distribution of terminals within the different cytoarchitectonic fields, the thalamocortical projection in reeler, like that in the normal animal, appears to be composed of two distinct axon classes. The "class I" axons, the less-divergent system which terminates densely in two or three tiers within the cortex, are the subject of the present analysis. The "class I" projections form an orderly cortical representation of the thalamus: Despite distortions in the topography of the reeler thalamus and cortex, both the nucleus to field relationships and the detailed topologic organization of the projection appear to be normal. The terminals of class I projections are distributed in radially segregated tiers that resemble the tiered pattern of termination as in normal mice, and there are field-specific variations in the tiers that, like those of normal mice, are systematically related to variations of cortical cytoarchitecture. Thus, similar mechanisms, which may depend in part on the interaction of ingrowing axons with specific postsynaptic cell surfaces, must determine the restricted radial distribution of thalamocortical projections in both genotypes. However, there is an abnormal mix of somata and dendrites at all radial levels of the mutant cortex, suggesting that the spatial domain of termination of thalamocortical axons may be governed to some extent by factors other than the distribution of specific prospective postsynaptic cell surfaces. The reeler mutation alters the radial but not the tangential structure of the neocortex, suggesting that these two aspects of cortical organization develop under the control of independent mechanisms. The present results suggest that despite the anomalies of radial position, normal relationships between thalamocortical afferents and distinct classes of postsynaptic elements with characteristic radial distributions are largely conserved in reeler. Certain types of aberrant connections are highly probable, however.