Thoughts on the development, structure and evolution of the mammalian and avian telencephalic pallium

Various lines of evidence suggest that the development and evolution of the mammalian isocortex cannot be easily explained without an understanding of correlative changes in surrounding areas of the telencephalic pallium and subpallium. These are close neighbours in a common morphogenetic field and are postulated as sources of sonic cortical neuron types (and even of whole cortical areas). There is equal need to explain relevant developmental evo lutionary changes in the dorsal thalamus, the major source of afferent inpu ts to the telencephalon (to both the pallium and subpallium). The mammalian isocortex evolved within an initially small dorsal part of the pallium of vertebrates, surrounded by other pallial parts, including some with a non-c ortical, nuclear structure. Nuclear pallial elements are markedly voluminou s in reptiles and birds, where they build the dorsal ventricular ridge, or hypopallium, which has been recently divided molecularly and structurally i nto a lateral pallium and a ventral pallium. Afferent pallial connections a re often simplified as consisting of thalamic fibres that project either to focal (cell aggregates in the ventral pallium ( predominant in reptiles an d birds) or to corticoid areas in the dorsal pallium (predominant in mammal s). Karten's hypothesis, put Forward in 1969, on the formation of some isoc ortical areas postulates an embryonic translocation into the nascent isocor tex of the ventropallial thalamorecipient foci and respective downstream ve ntropallial target populations, as specific layer IV, layers II-III, or lay ers V-VI neuron populations. This view is considered critically in the ligh t of various recent data, contrasting with the alternative possibility of a parallel, separate evolution of the different pallial parts. The new scena rio reveals as well a separately evolving tiered structure of the dorsal th alamus, sonic of whose parts receive input from midbrain sensory centres (c ollothalamic nuclei), whereas other parts receive oligosynaptic 'lemniscal' connections bypassing the midbrain (lemnothalamic nuclei). An ampler look into known hodological patterns from this viewpoint suggests that ancient c ollothalamic pathways, which target ventropallial foci, are largely conserv ed in mammals, while sonic emergent cortical connections can be established by means of new collaterals in sonic of these pathways. The lemnothalamic pathways, which typically target ancestrally the dorsopallial isocortex, sh ow parallel increments of relative size and structural diversification of b oth the thalamic cell populations and the cortical recipient areas. The evo lving lemnothalamic pathways may interact development ally with collothalam ic corticopetal collaterals in the modality-specific invasion of the emerge nt new areas of isocortex.