Laminar expression of ephrin-A2 in primary somatosensory cortex of postnatal rats

The enigmatic fetal subplate compartment forms an early tangential cortical nexus and provides the framework for construction of cortical connectivity

The most prominent transient compartment of the primate fetal cortex is the deep, cell-sparse, synapse-containing subplate compartment (SPC). The developmental role of the SPC and its extraordinary size in humans remain enigmatic. This paper evaluates evidence on the development and connectivity of the SPC and discusses its role in the pathogenesis of neurodevelopmental disorders. A synthesis of data shows that the subplate becomes a prominent compartment by its expansion from the deep cortical plate (CP), appearing well-delineated on MR scans and forming a tangential nexus across the hemisphere, consisting of an extracellular matrix, randomly distributed postmigratory neurons, multiple branches of thalamic and long corticocortical axons. The SPC generates early spontaneous non-synaptic and synaptic activity and mediates cortical response upon thalamic stimulation. The subplate nexus provides large-scale interareal connectivity possibly underlying fMR resting-state activity, before corticocortical pathways are established. In late fetal phase, when synapses appear within the CP, transient the SPC coexists with permanent circuitry. The histogenetic role of the SPC is to provide interactive milieu and capacity for guidance, sorting, "waiting" and target selection of thalamocortical and corticocortical pathways. The new evolutionary role of the SPC and its remnant white matter neurons is linked to the increasing number of associative pathways in the human neocortex. These roles attributed to the SPC are regulated using a spatiotemporal gene expression during critical periods, when pathogenic factors may disturb vulnerable circuitry of the SPC, causing neurodevelopmental cognitive circuitry disorders.

EphA7 expression identifies a unique neuronal compartment in the rat striatum

Prior studies have identified two anatomically and neurochemically distinct cellular compartments within the mammalian striatum, termed striosomes and matrix, which express μ-opioid receptors (μOR) and EphA4, respectively. Here we identify and characterize an additional compartment in the rat striatum composed of neurons that express EphA7. In situ hybridization and immunohistochemical data indicate that neurons expressing EphA7 mRNA and protein are arranged in a banded "matrisome-like" pattern confined to the matrix in the dorsal striatum. Within the ventral striatum, EphA7-positive (+) neurons have a less organized mosaic pattern that partially overlaps areas expressing μOR. Immunolabeling data demonstrate that EphA7+ striatofugal axons form distinct fascicles leaving the striatum. Within the globus pallidus, EphA7+ axons terminate primarily within ventromedial areas of the nucleus and along its striatal border. EphA7+ axons avoid regions containing dopamine neurons within the substantia nigra and preferentially innervate areas near the rostral and caudal margins of the nucleus. Within both nuclei, EphA7+ axons have similar but more restricted terminal fields than the entire population of EphA4+ matrix axons, indicating that EphA7+ axons comprise a subpopulation of matrix axons. Ligand binding data demonstrate that ephrin-A5 selectively binds areas of the striatum, globus pallidus, and substantia nigra containing EphA7+ neurons and axons, but not areas expressing only EphA4. Our findings demonstrate that EphA7 expression identifies a novel "matrisome" compartment within the matrix that binds ephrin-A5 and possesses unique axonal projections. Our findings also suggest that EphA7 and ephrin-A5 may participate in the formation of this matrisome subcompartment and its striatofugal projections.