Scientific Understanding of Consciousness
Cortical Columns in Embryo Development
Nature 461, 524-528 (24 September 2009)
Integration of neuronal clones in the radial cortical columns by EphA and ephrin-A signalling
Masaaki Torii1,2, Kazue Hashimoto-Torii1, Pat Levitt2 & Pasko Rakic1
Department of Neurobiology and Kavli Institute for Neuroscience, Yale University School of Medicine, 333 Cedar Street, New Haven, Connecticut 06510, USA
Zilkha Neurogenetic Institute and Department of Cell and Neurobiology, Keck School of Medicine of USC, Los Angeles, California 90089, USA
Cerebral cortex is a laminated sheet of neurons composed of arrays of intersecting radial columns. During development, excitatory projection neurons originating from the proliferative units at the ventricular surface of the embryonic cerebral vesicles migrate along elongated radial glial fibres to form a cellular infrastructure of radial (vertical) ontogenetic columns in the overlaying cortical plate. However, a subpopulation of these clonally related neurons also undergoes a short lateral shift and transfers from their parental to the neighbouring radial glial fibres, and intermixes with neurons originating from neighbouring proliferative units. This columnar organization acts as the primary information processing unit in the cortex.
Here we show that an Eph receptor A (EphA) and ephrin A (Efna) signalling-dependent shift in the allocation of clonally related neurons is essential for the proper assembly of cortical columns.
Functional columns in the neocortex emerge developmentally from a proliferative zone protomap, and consist of several classes of interconnected neurons. In the present study, we have explored the possible roles of A-type Eph receptors and their ligands, ephrin-As, in the integration of neurons in radial columns because they have been implicated in controlling cell positioning in a variety of developmental contexts by sorting cell types, restricting their intermingling, or regulating their migration. In fact, several Epha receptors and Efna ligands were detected in the subventricular zone (SVZ) and/or the intermediate zone, through which neurons that originated in the ventricular zone migrate to the cortical plate.
To examine the roles of EphA receptors and ephrin-A ligands in neuronal migration in the neocortex, we analysed the triple knockout (TKO) mous for Efna1, Efna3 and Efna5 that account for almost all ephrin-A genes in the developing neocortex.
To gain insight into the functional organization of the TKO neocortex, corticotectal projection neurons (CTPN) and callosal projection neurons (CPN) were retrogradely labelled at P2 and observed at P4.
To address the mechanism of lateral dispersion, we performed cell ablation experiments, in which the diphtheria-toxin-A-chain fragment (DTA) and an enhanced yellow fluorescent protein (EYFP) reporter plasmids were electroporated into the cortex at E13.5, and the effects on the migrating neurons and upper layer neurons were examined at E15.5 and P0.
Because EphAs and ephrin-As can act as both receptors and ligands to mediate forward and reverse signalling, and both are required for cell sorting, we electroporated ephrin-A2, -A5, and the truncated forms of EphA7 and EphA4, together with the EYFP plasmid to determine which signalling is involved in columnar segregation. Our analyses indicate that EphA and ephrin-A signalling regulates lateral neuronal dispersion and intermingling during the multipolar stage of radial migration, and that this mechanism is required to generate cortical columns with appropriate cellular components.
It is well established that progenitor cells have heterogeneous molecular properties, proliferative capacity and lineage commitment. These adaptive processes may be essential to achieve the modular function of cortical columns. Appropriate radial and tangential distribution of clonally related neurons may be essential for proper synaptic development and function of the radial units.
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