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

Abstract

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 ¢eld and are postulated as sources of some cortical neuron types (and even of whole cortical areas). There is equal need to explain relevant developmental evolutionary changes in the dorsal thalamus, the major source of a¡erent inputs 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-cortical, nuclear structure. Nuclear pallial elements are markedly voluminous in reptiles and birds, where they build the dorsal ventricular ridge, or hypopallium, which has been recently divided molecularly and structurally into a lateral pallium and a ventral pallium. A¡erent pallial connections are often simpli¢ed as consisting of thalamic ¢bres that project either to focal cell aggregates in the ventral pallium (predominant in reptiles and birds) or to corticoid areas in the dorsal pallium (predominant in mammals). Karten's hypothesis, put forward in 1969, on the formation of some isocortical areas postulates an embryonic translocation into the nascent isocortex of the ventropallial thalamorecipient foci and respective downstream ventropallial target populations, as speci¢c layer IV, layers II^III, or layers V^VI neuron populations. This view is considered critically in the light of various recent data, contrasting with the alternative possibility of a parallel, separate evolution of the di¡erent pallial parts. The new scenario reveals as well a separately evolving tiered structure of the dorsal thalamus, some of whose parts receive input from midbrain sensory centres (collothalamic 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 collothalamic pathways, which target ventropallial foci, are largely conserved in mammals, while some emergent cortical connections can be established by means of new collaterals in some of these pathways. The lemnothalamic pathways, which typically target ancestrally the dorsopallial isocortex, show parallel increments of relative size and structural diversi¢cation of both the thalamic cell populations and the cortical recipient areas. The evolving lemnothalamic pathways may interact developmentally with collothalamic corticopetal collaterals in the modality-speci¢c invasion of the emergent new areas of isocortex.

An intrinsic mechanism of corticogenesis from embryonic stem cells

“…This surprising finding indicates that a specific cortical area can differentiate without any influence from the brain, through an intrinsic pathway that may correspond to an ancestral developmental program of the mammalian forebrain 43,44 .…”

Dynamic mapping of human cortical development during childhood through early adulthood

“…Phylogenetically, some of the oldest cortical regions lie on the inferior brain surface in the medial aspect of the temporal lobe (the posterior part of the piriform cortex and the entorhinal cortex, for example) or on the inferior and medial aspect of the frontal lobe near the caudal end of the olfactory sulcus (anterior piriform cortex and the orbital periallocortex) (35)(36)(37). The maturation process in the vicinity of these areas appeared to have started early (ontogenetically) already by the age of 4 years, as seen by the linear or flat plots (Fig.…”

Expression analysis of Sulf1 in the chick forebrain at early and late stages of development

“…This large nuclear complex can be subdivided into 5 main histogenetic areas Puelles, 2001b;Martínez-de-la-Torre et al, 2002;García-López et al, 2004). These areas are the habenular/subhabenular complex or epithalamus, the dorsal tier group, the intermediate tier group, the ventral tier group, and the anteroventral group Puelles, 2001b;Martinez-de-la-Torre et al, 2002;Puelles et al, 2007).…”

“…Ventral to the subhabenular nuclei appears the dorsal tier domain, which produces a voluminous mass of derivates in its mantle layer that represent the largest components of the avian thalamus Puelles, 2001b;Puelles et al, 2007). High levels of Sulf1 expression was detected in several of the nuclear components in the dorsal tier: Puelles et al, 2007).…”