Alan L. Pearlman

Two modes of radial migration in early development of the cerebral cortex.

Layer formation in the developing cerebral cortex requires the movement of neurons from their site of origin to their final laminar position. We demonstrate, using time-lapse imaging of acute cortical slices, that two distinct forms of cell movement, locomotion and somal translocation, are responsible for the radial migration of cortical neurons. These modes are distinguished by their dynamic properties and morphological features. Locomotion and translocation are not cell-type specific; although at early ages some cells may move by translocation only, locomoting cells also translocate once their leading process reaches the marginal zone. The existence of two modes of radial migration may account for the differential effects of certain genetic mutations on cortical development.

Cell lineage in the cerebral cortex of the mouse studied in vivo and in vitro with a recombinant retrovirus.

To analyze cell lineage in the murine cerebral cortex, we infected progenitor cells with a recombinant retrovirus, then used the retroviral gene product to identify the descendants of infected cells. Cortices were infected on E12-E14 either in vivo or following dissociation and culture. In both cases, nearly all clones contained either neurons or glia, but not both. Thus, neuronal and glial lineages appear to diverge early in cortical development. To analyze the distribution of clonally related cells in vivo, clonal boundaries were reconstructed from serial sections. Perinatally (E18-PN0), clonally related cells were radially arrayed as they migrated to the cortical plate. Thus, clonal cohorts traverse a similar radial path. Following migration (PN7-PN23), neuronal clones generally remained radially arrayed, while glial clones were variable in orientation, suggesting that these two cell types accumulate in different ways. Neuronal clones sometimes spanned the full thickness of the cortex. Thus, a single progenitor can contribute neurons to several laminae.

Abnormal reorganization of preplate neurons and their associated extracellular matrix: An early manifestation of altered neocortical development in the reeler mutant mouse

The formation of the distinct layers of the cerebral cortex begins when cortical plate neurons take up positions within the extracellular matrix (ECM)‐rich preplate, dividing it into the marginal zone above and the subplate below. We have analyzed this process in the reeler mutant mouse, in which cortical lamination is severely disrupted. The recent observation that the product of the reeler gene is an ECM‐like protein that is expressed by cells of the marginal zone indicates a critical role for ECM in cortical lamination. We have found that preplate cells in normal cortex that are tagged during their terminal division with bromodeoxyuridine (BrdU) are closely associated with chondroitin sulfate proteoglycans (CSPGs), which were identified by immunolabeling; this association is maintained in the marginal zone and subplate after the preplate is divided by cortical plate formation. Cortical plate cells do not aggregate within the preplate in reeler; instead, preplate cells remain as an undivided superficial layer containing abundant CSPGs, and cortical plate neurons accumulate below them. These findings indicate that preplate cells are responsible for the formation of a localized ECM, because the association of CSPGs with preplate cells is maintained even when these cells are in abnormal positions. The failure of cortical plate neurons to aggregate within the framework of the preplate and its associated ECM and to divide it is one of the earliest structural abnormalities detectable in reeler cortex, suggesting that this step is important for the subsequent formation of cortical layers. J. Comp. Neurol. 378:173–179, 1997.

Neuronal Heterotopias in the Developing Cerebral Cortex Produced by Neurotrophin-4

The marginal zone (MZ) of embryonic neocortex is crucial to its normal development. We report that neurotrophin-4 (but not NT3 or NGF), applied to embryonic rodent cortex in vitro or in vivo, produces heterotopic accumulations of neurons in the MZ. Although heterotopia production is TrkB mediated, BDNF is >10-fold less effective than NT4. Heterotopic neurons have the same birth date and phenotype as normal MZ neurons; they are not the result of NT4-induced proliferation or rescue from apoptosis. We suggest that NT4 causes excess neurons to migrate into the MZ and thus may play a role in normal MZ formation as well as in the pathogenesis of certain human cortical dysplasias.

New directions for neuronal migration

Analysis of genetic mutations that lead to abnormal migration and layer formation in the developing cerebral cortex of mice and humans has led to important new discoveries regarding the molecular mechanisms that underlie these processes. Genetic manipulation and experimental analysis have demonstrated significant tangential migrations of cortical neurons, some arriving from very distant noncortical sites.

Layering defect in p35 deficiency is linked to improper neuronal-glial interaction in radial migration

Several genes essential for neocortical layering have been identified in recent years, but their precise roles in this process remain to be elucidated. Mice deficient in p35—an activator of cyclin-dependent kinase 5 (Cdk5)—are characterized by a neocortex that has inverted layering. To decipher the physiological mechanisms that underlie this defect, we compared time-lapse recordings between p35−/− and wild-type cortical slices. In the p35−/− neocortex, the classic modes of radial migration—somal translocation and locomotion—were largely replaced by a distinct mode of migration: branched migration. Branched migration is cell-autonomous, associated with impaired neuronal-glial interaction and rare in neurons of scrambler mice, which are deficient in Dab1. Hence, our findings suggest that inside-out layering requires distinct functions of Reelin and p35/Cdk5 signaling, with the latter being important for proper glia-guided migration.

Concentration of membrane antigens by forward transport and trapping in neuronal growth cones

Formation of the nervous system requires that neuronal growth cones follow specific paths and then stop at recognition signals, sensed at the growth cones leading edge. We used antibody-coated gold particles viewed by video-enhanced differential interference contrast microscopy to observe the distribution and movement of two cell surface molecules, N-CAM and the 2A1 antigen, on growth cones of cultured cortical neurons. Gold particles are occasionally transported forward at 1-2 microns/s to the leading edge where they are trapped but continue to move. Concentration at the edge persists after cytochalasin D treatment or ATP depletion, but active movements to and along edges cease. We also observed a novel outward movement of small cytoplasmic aggregates at 1.8 microns/s in filopodia. We suggest that active forward transport and trapping involve reversible attachment of antigens to and transport along cytoskeletal elements localized to edges of growth cones.

Single unit receptive fields and the cellular layers of the pigeon optic tectum

Abstract In the pigeon optic tectum 370 single units were studied with metal microelectrodes and glass micropipettes. Efforts to localize the cells studied included small electrolytic lesions with reconstruction of the electrode tract, and iontophoretic intracellular injections of Procion yellow. Some properties of the receptive fields of the cells studied vary with depth in the tectum. From the more superficial to the deeper layers the size of the receptive field increases, the shape often becomes more irregular, the response to stationary stimuli disappears, and the effect of the inhibitory surround diminishes. Other receptive field properties such as directional selectivity, the null directions, and the range of the stimulus speeds to which cells respond, are similar in all layers. In keeping with the extensive overlap of axons and dendrites of tectal cells between layers, the transition in receptive field properties from tectal surface to depth is gradual. There are no sharp dichotomies in receptive field properties corresponding to the well-defined layers formed by the nuclei of tectal neurons.

A psychophysical investigation of spatial vision in the normal and reeler mutant mouse.

Abstract Spatial vision in the normal mouse (C57B1/6J) and the reeler ( rlrl ) mutant was studied by observing optokinetic nystagmus (OKN) in response to 2 superimposed vertical square wave gratings moving in opposite directions. One grating (standard) had a fixed spatial frequency and variable contrast. The spatial frequency of the 2nd (test) grating could be set to each of 6 values with the contrast held constant. We have defined a “grating sensitivity” function for the mouse by determining the standard grating contrast necessary to make the standard and test gratings equally effective in eliciting OKN. Under these test conditions, grating sensitivity was highest for a test frequency of 0.125 c/deg, and the high frequency limit of mouse grating sensitivity was reached at 0.50 c/deg. There were no differences between the normal mouse and the reeler mutant with respect to grating sensitivity.

Retinopic organization of the striate cortex (area 17) in the reeler mutant mouse.

Abstract The retinotopic projection of the visual field onto the primary visual cortex (area 17) of the reeler mutant mouse was studied with extracellular unit recordings. The map is organized in a manner similar to that of the normal mouse, indicating that the absence of normal cortical lamination in the reeler does not prevent the thalamocortical afferents from establishing their normal retinotopic order.