Pallial patterning and the origin of the isocortex

Juan F Montiel,Francisco Aboitiz

doi:10.3389/fnins.2015.00377

Abstract

Together with a complex variety of behavioral, physiological, morphological, and neurobiological innovations, mammals are characterized by the development of an extensive isocortex (also called neocortex) that is both laminated and radially organized, as opposed to the brain of birds and reptiles. In this article, we will advance a developmental hypothesis in which the mechanisms of evolutionary brain growth remain partly conserved across amniotes (mammals, reptiles and birds), all based on Pax6 signaling or related morphogens. Despite this conservatism, only in mammals there is an additional upregulation of dorsal and anterior signaling centers (the cortical hem and the anterior forebrain, respectively) that promoted a laminar and a columnar structure into the neocortex. It is possible that independently, some birds also developed an upregulated dorsal pallium.

Highlights

According to a developmental perspective, brain homologies across vertebrates are supported by anatomical topographic correspondence and embryonic expression of homeobox and homeoboxlike genes
This was achieved by virtue of a strong upregulation of dorsal patterning centers like the hem, in combination with the expansion of Pax6 expression that generated a subventricular zone (SVZ) that amplified the proliferation of neuronal progenitors
We have reviewed developmental evidence supporting the concept that the origin of the mammalian brain relies on the amplification of several morphogenetic centers that participate in patterning the dorsal cerebral hemisphere or pallium

Summary

INTRODUCTION

According to a developmental perspective, brain homologies across vertebrates are supported by anatomical topographic correspondence and embryonic expression of homeobox and homeoboxlike genes. Other avian components have increased in complexity, (iii) the hyperpallium (derived from the dorsal pallium and receiving lemnothalamic visual and somatosensory input), and (iv) the arcopallium (posterior DVR of reptiles, derived from the ventral pallium and subpallium, and comparable to some parts of the mammalian amygdala) All these structures have a morphology that is radically different from that of the isocortex of mammals, as they show no evident signs of laminar or radial organization ( there are important similarities in sensory connectivity and internal circuitry (Jarvis et al, 2005, 2013; Wang et al, 2010; Karten, 2013; Ahumada-Galleguillos et al, 2015; Calabrese and Woolley, 2015). The cortical hem, the antihem and the anterior forebrain (but the cortical hem), are the sites of generation of Cajal-Retzius cells that secrete the glycoprotein reelin, required for proper neocortical lamination and dendritic growth of pyramidal neurons (Nomura et al, 2008; Meyer, 2010; Kupferman et al, 2014; Martínez-Cerdeño and Noctor, 2014)

A UNIFYING HYPOTHESIS

SUMMARY