Development of the Corticothalamic Projections

Abstract

In this review we discuss recent advances in the understanding of corticothalamic axon guidance; patterning of the early telencephalon, the sequence and choreography of the development of projections from subplate, layers 5 and 6. These cortical subpopulations display different axonal outgrowth kinetics and innervate distinct thalamic nuclei in a temporal pattern determined by cortical layer identity and subclass specificity. Guidance by molecular cues, structural cues, and activity-dependent mechanisms contribute to this development. There is a substantial rearrangement of the corticofugal connectivity outside the thalamus at the border of and within the reticular thalamic nucleus, a region that shares some of the characteristics of the cortical subplate during development. The early transient circuits are not well understood, nor the extent to which this developmental pattern may be driven by peripheral sensory activity. We hypothesize that transient circuits during embryonic and early postnatal development are critical in the matching of the cortical and thalamic representations and forming the cortical circuits in the mature brain.

Structural and functional brain rewiring clarifies preserved interhemispheric transfer in humans born without the corpus callosum

“…Growth cones at the tip of elongating axons are sensitive to different signals that attract or guide them to the midplane before crossing, and repel them as long as they reach the other hemisphere (25). These signals are often similar to those produced in other strategic points in the developing brain, such as the dorsal midplane for callosal and hippocampal axons, the ventral midplane for anterior commissure fibers, and the internal capsule for different descending and ascending pathways (26). In addition, callosal fibers tend to fasciculate onto themselves and other fiber tracts, driven by a Wnt receptor expressed ubiquitously in the brain, and guided by interactive contacts with adhesion molecules on their surface (27).…”

Development, evolution and pathology of neocortical subplate neurons

“…Over the next 4 days, this nucleus becomes filled with eGFP-expressing fibres, and they coalesce into a barreloid pattern. Despite close spatial proximity, the dorsal lateral geniculate nucleus does not become filled by eGFP-expressing fibres until P10-P14 in normally developing mice [76][77][78] . This temporal pattern of axon extension is dependent on spontaneous retinal activity 78 because the eyes are closed at this time.…”

“…Subplate cells have subcortical and cortical projections, the latter being both ipsi-and contralateral 1,68 . Subplate cells are the first cortical cells to send subcortical projections across the pallial-subpallial boundary in mice 74,75 , but other cortical cell populations innervate the subplate-targeted thalamic nuclei first 76 . In the Golli-τ-eGFP (GTE) transgenic mouse, both subplate and layer 6a axons and dendrites are labelled with enhanced green fluorescent protein (eGFP) 77 .…”

Developmental mechanisms channeling cortical evolution

“…These observations suggest the interesting possibility that these regions selforganize later in development than the seed regions, a possibility under current investigation. Such a conjecture has recently been explored for the formation of the V1-V2 boundary [63].…”