Figure 4.

Scheme of thalamocortical axon trajectories from thalamic nuclei through the subpallium/ventral telencephalon to arrive to distinct neocortical areas.

Upper right panles: Schematic diagram illustrating multiple carbocyanine dye placements in the cerebral cortex positioned along an anterioposterior axis revealed the arrangements of backfilled dorsal thalamic neurons in a mediolateral fashion. The schematic panels indicate the appropriate sections with labelling. The right hemisphere is enlarged to illustrate some of the moleculas mechanisms that are involved in the guidance fo the thalamic axons across the thalamic eminence, corridor and subpallium to reach the appropriate regions in the cortex.

TCAs from different nuclei in the thalamus (VA/VL: ventroanterior/ventrolateral nuclei, VB: ventrobasal complex, dLGN: dorsal lateral geniculate nucleus), emerge at the thalamic eminence en route to the neocortex, and are sorted within a corridor of Islet1-positive cells in the subpallium/ventral telencephalon along the rostral to caudal axis (E13.5-E15.5 in the mouse). Within the corridor, TCAs expressing different combinations of axon guidance cue receptors (listed in the box within the dorsal thalamus) are guided by gradients of repellent and attractant cues (EphrinA5, Netrin1, Semaphorin3A, 3F, Slit1), influenced by Neuregulin-1 and serotonin (5-HT, 5-hydroxy tryptamine). In the ventral pallium, with the exception of Slit1, similar gradients are present. The thalamic axons target cortical areas that will contribute putative primary motor cortex (M1), somatosensory cortex (S1), and visual cortex (V1), but their entry to the cortex is regulated by subplate. The some of the various grandients in subplate and cortical plate are listed (Fgf8, Sp8, COUPTF1, Pax6, Emx2). Within the neocortex additional molecular cues and activity- dependent mechanisms promote the final synaptic targeting of TCAs. This simple initial topography can be considerably rearranged at the time of entry to the cortical plate.