Verne S. Caviness

Neocortical histogenesis in normal and reeler mice: A developmental study based upon [3H]thymidine autoradiography

The relative positions of the principal neuronal classes of neocortex are inverted in the reeler mutant mouse. Neurons formed at 48-hourly intervals throughout the period of neocortical cytogenesis between E11 and E17 are labeled by [3H]thymidine. The positions of the labeled cells during and subsequent to their migrations are traced by autoradiography. Simultaneously-formed cohorts reach the neocortex at the same time in normal and reeler animals. After E13, subsequent to the appearance of the cortical plate, cohorts of migrating cells in the normal animal ascend to the interface of the cortical plate and marginal layer where they come to rest in a narrow laminar zone. In reeler, by contrast, migration is arrested in the depths of the cortex. The migrating cell appears unable to ascend through the zone occupied by the preceding cohorts. At the completion of migration neurons of both genotypes become fixed in position and undergo little subsequent shift in their relative positions in the course of future cortical growth. Despite the anomaly of migrations and the post-migratory positions of neurons in reeler, cohorts of cells formed at the same time in the two genotypes give rise to the same neuronal classes.

Identification of radial glial cells within the developing murine central nervous system:studies based upon a new immunohistochemical marker

The monoclonal antibody RC2 was generated in mouse by conventional hybridoma methodology. The antigen recognized by RC2 is robust, allowing aldehyde fixation appropriate to high resolution light and electron microscopic analyses. From the neural tube stage of fetal development the antibody delineates throughout the central nervous system a subpopulation of neuroepithelial cells which have a radial bipolar morphology. A descending process extends to the ventricular margin, and an ascending process contacts the glial limiting membrane by one or more endfeet varicosities. The persistence of these cells through the neurogenetic period allows their identification as radial glial. From as early as E9-10 the fibers appear to be organized in simple straight fascicles. Later in fetal development these fascicles show marked region-specific transformations in density and trajectory, particularly in association with cerebral corticogenesis and with cerebellar and basal ganglia development. The bipolar forms continue to stain with RC2 until they disappear in the postnatal period. Concurrently with a progressive perinatal loss of stained bipolar radial glia, RC2 identifies multipolar cell forms at various levels of the brain wall, as consistent with the transformation of radial glia into astrocytes. RC2 also recognizes monopolar cell forms in the spinal cord and the cerebellum as early as E15, and in the dentate gyrus of the hippocampal formation from the day of birth. Monopolar forms in the cerebellum are inferred to be progenitors of Bergmann glia. Although Bergmann glia are known to persist in adult life, these cells do not stain with RC2 beyond the 2nd postnatal week. The robustness of the antigen recognized by RC2 makes this probe a valuable tool to study the morphological transformations of the bipolar radial glia during their mitotic turnover. It also provides a sensitive stain for the study of the organization and the histogenetic role of the overall radial fiber system.

Obstructed neuronal migration along radial glial fibers in the neocortex of the reeler mouse: A golgi-EM analysis

The interrelationship of radial glial fibers (RGF) and young neurons migrating to the neocortex of normal and reeler mutant mice at 17 days of gestation are reconstructed from serial and from closely spaced thin sections. The glial fibers are identified unequivocally by correlated light and electron microscopy by means of the Golgi-gold toning method of Fairén and associates. The migrating cell in the normal animal is closely apposed to and coiled about the RGF throughout most of its ascent. In the terminal few microns of its movement, however, it begins rapidly to differentiate and at the same time surrenders its close attachment to the RGF. In the reeler, by contrast, the migrating cell maintains normal apposition to the RGF only until it enters the cortex. There its leading process is unable to pass between the surfaces of the RGF and those of postmigratory elements. Abnormally extensive contact between the glial fiber and the somata of postmigratory cells appears to be sustained in the mutant. The upward migration of the young neuron is terminated in the depths of the cortex and the cell soma gives rise to a profusion of small processes. This study affirms the critical role served by RGF as guides to neuronal migration and provides evidence that abnormal adhesions between postmigratory cells and the RGF obstruct neuronal migration in the reeler mouse.

MRI-Based topographic parcellation of human cerebral white matter and nuclei II. Rationale and applications with systematics of cerebral connectivity.

We describe a system for parcellation of the human cerebral white matter and nuclei, based upon magnetic resonance images. An algorithm for subdivision of the cerebral central white matter according to topographic criteria is developed in the companion manuscript. In the present paper we provide a rationale for this system of parcellation of the central white matter and we extend the system of cerebral parcellation to include principal subcortical gray structures such as the thalamus and the basal ganglia. The volumetric measures of the subcortical gray and white matter parcellation units in 20 young adult brains are computed and reported here as well. In addition, with the comprehensive system for cerebral gray and white matter structure parcellation as reference, we formulate a systematics of forebrain connectivity. The degree to which functionally specific brain areas correspond to topographically specific areas is an open empirical issue. The resolution of this issue requires the development of topographically specific anatomic analyses, such as presented in the current system, and the application of such systems to a comprehensive set of functional-anatomic correlation studies in order to establish the degree of structural-functional correspondence. This system is expected to be applied in both cognitive and clinical neuroscience as an MRI-based topographic systematics of human forebrain anatomy with normative volumetric reference and also as a system of reference for the anatomic organization of specific neural systems as disrupted by focal lesions in lesion-deficit correlations.

Interkinetic and Migratory Behavior of a Cohort of Neocortical Neurons Arising in the Early Embryonic Murine Cerebral Wall

Neocortical neuronogenesis occurs in the pseudostratified ventricular epithelium (PVE) where nuclei of proliferative cells undergo interkinetic nuclear movement. A fraction of daughter cells exits the cell cycle as neurons (the quiescent, or Q, fraction), whereas a complementary fraction remains in the cell cycle (the proliferative, or P, fraction). By means of sequential thymidine and bromodeoxyuridine injections in mouse on embryonic day 14, we have monitored the proliferative and postmitotic migratory behaviors of 1 and 2 hr cohorts of PVE cells defined by the injection protocols. Soon after mitosis, the Q fraction partitions into a rapidly exiting (up to 50 μm/hr) subpopulation (Qr) and a more slowly exiting (6 μm/hr) subpopulation (Qs). Qr andQs are separated as two distributions on exit from the ventricular zone with an interpeak distance of ∼40 μm. Cells in Qr andQs migrate through the intermediate zone with no significant change in the interpeak distance, suggesting that they migrate at approximately the same velocities. The rate of migration increases with ascent through the intermediate zone (average 2–6.4 μm/hr) slowing only transiently on entry into the developing cortex. Within the cortex, Qr andQs merge to form a single distribution most concentrated over layer V.

MRI-based surface-assisted parcellation of human cerebellar cortex: an anatomically specified method with estimate of reliability

We revisit here a surface assisted parcellation (SAP) system of the human cerebellar cortex originally described in Makris, N., Hodge, S.M., Haselgrove, C., Kennedy, D.N., Dale, A., Fischl, B., Rosen, B.R., Harris, G., Caviness, V.S., Jr., Schmahmann, J.D., 2003. Human cerebellum: surface-assisted cortical parcellation and volumetry with magnetic resonance imaging. J Cogn Neurosci 15, 584-599. This system preserves the topographic and morphologic uniqueness of the individual cerebellum and allows for volumetric analysis and representation of multimodal structural and functional data on the cerebellar cortex. This methodology integrates features of automated routines of the program FreeSurfer as well as semi-automated and manual procedures of the program Cardviews to create 64 cerebellar parcellation units based on fissure information and anatomical landmarks of the cerebellar surface. Using this technique, we undertook the parcellation of ten cerebella by two independent raters. The reliability of the resulting parcellation units (64 total) was high, with an average Intraclass Correlation Coefficient (ICC) of 0.724 in the vermis and 0.853 in the hemispheres. Clusters of parcellation units were then created, based on lobar and connectivity data and functional hypotheses. These 36 clusters, when treated as anatomical units, had an average ICC of 0.933. Whereas the individual units provide a high level of detail and anatomical specificity, the clusters add flexibility to the analysis by providing higher reliability.

Axon strata of the cerebral wall in embryonic mice

The stratification of principal fiber systems affiliated with the developing neocortex has been analyzed by means of HRP tracing methods, monoamine histofluorescence and silver impregnations in mouse embryos ranging from the 15th to 16th embryonic day (E15/16) to the end of gestation (E19 = the day of birth). As early as E15/16 a fiber stratum divides the subplate and marks the inferior boundary of the developing cortex. Axons coursing in this fiber plane, termed the external sagittal stratum (ESS), include components of at least 5 identifiable systems: thalamocortical, corticothalamic, ipsilateral corticocortical, callosal and monoaminergic. The neocortical afferents of extrinsic origin, i.e., the thalamocortical, callosal and monoaminergic systems, cross the intermediate zone from their separate directions and converge upon the ESS. After a variable course through this stratum, single fibers ascend from their parent fascicles to ramify densely in the cortical subplate (CSB). Fibers of each of the extrinsic afferent systems mingle with each other and with locally arising axons within the CSB. Axons of the monoaminergic projection as well as fibers of the thalamic projection cross the cortical plate to ramify in the marginal zone. Other axons apparently of local intracortical origin course tangentially through the cortical plate. Otherwise, the cortical plate is devoid of proliferating axons at this early developmental stage. The set of observations illustrates the existence of sharply defined boundaries between axon-rich and axon-poor strata of the developing neocortex. These boundaries also compartmentalize postmigratory neurons with respect to their state of differentiation.

Heterologous synapses upon purkinje cells in the cerebellum of the reeler mutant mouse: An experimental light and electron microscopic study

The projections of the spinal cord upon the cerebellum of normal and Reeler mutant mice were compared by light and electron microscopic methods after hemicordotomy. In both genotypes this afferent system projects to the cerebellar cortex and to the roof nuclei. In the Reeler, there is an additional projection among the Purkinje cells and interneurons of the central cerebellar mass. In both normal and Reeler cerebellar cortex this mossy fiber system terminates as large glomeruli. In Reeler the spinal projection also gives rise to a smaller terminal which is distributed both to the cortex and the central cerebellar mass. In both genotypes the dendrites of granule cells and the somata and dendrites of Golgi cells are synaptic targets of the glomeruli of the cortical projection. In Reeler both the glomeruli and smaller terminals also form heterologous synaptic contacts with dendrite spines of heterotopic intracortical and subcortical Purkinje cells. In both genotypes the synapses are exclusively type I. A second class of heterologous synapse, a type I junction between axons of Golgi cells and Purkinje cell spines, is also recognized in electron micrographs. The present study is the first unequivocal demonstration by experimental hodologic method of heterologous synaptic junctions in the mammalian central nervous system. The existence of such junctions in the cytoarchitectonically anomalous cerebellum of this mutant emphasizes the critical role played by the cellular environment in shaping neural circuits in the developing nervous system.

Distribution of glucuronic acid-and-sulfate-containing glycoproteins in the central nervous system of the adult mouse

The distribution of glucuronic acid-and-sulfate-containing carbohydrate (GSC) epitope recognized by two monoclonal antibodies, HNK-1 and 4F4, was studied by immunocytochemistry in adult mouse brain. Both antibodies recognized proteins ranging in molecular weight from 60 to above 250 kDa in Western blot but no glycolipid was recognized in the adult brain. With both light and electron microscopic study, two patterns of staining are observed: diffuse neuropil staining, and individual neuronal somata staining. The diffuse neuropil staining is concentrated in discrete anatomically defined areas. At the EM level, this immunoreactivity is associated with numerous dendrites or astrocytic processes. At cell somata, most of neurons are stained only at Golgi apparatus (type 2); however, a distinct population of cells showed membranous staining (type 1) as well. Type 1 membranous immunoreactivity is observed only in membrane adjacent to astrocytic processes. In the cerebral cortex, type 1 neurons are found in layers III and V-VIa of somatosensory cortex, but only in layers V-VIa in most other cortical fields. Other areas containing type 1 neurons include the globus pallidus, the thalamic reticular nucleus, the hippocampus, the deep cerebellar nuclei, and a majority of the primary sensory and motor nuclei in the brainstem. The subpopulation of type 1 neurons show an overlap in distribution and morphology with some GABA-containing cells.

Barrels in somatosensory cortex of normal and reeler mutant mice

The pattern of projection of the ventrobasal thalamic complex (VB) upon the neocortical barrel field of normal and reeler mutant mice was determined by orthograde degeneration experiments and by Timms histochemical method. In both reeler and normal mice the thalamic terminals are concentrated at the midcortical zone dominated by granule cells and the adjacent zone dominated by medium-sized pyramidal cells. The medium-sized pyramids are principally supragranular in normal cortex but infragranular in the reeler. Throughout the barrel fields of reeler and normal mice thalamic terminals are organized as a mosaic of radially oriented columns coextensive in the tangential plane with barrels whose cross-sectional areas and shapes are similar in both genotypes. The radial extent of the columns in reeler is 2-3 times that in normal cortex. These observations suggest that the tangential organization of the thalamocortical projection is normal in reeler and that thalamic terminals are distributed among the same neuronal classes in normal and reeler mice despite cell malposition in the mutant.