The impact of gene expression analysis on evolving views of avian brain organization

Recent studies have presented data on adult and developing avian brain organization. Jarvis et al. ([2013] J Comp Neurol. 521:3614-3665) identify four pallial and two subpallial gene expression domains and demonstrate that the mesopallium and adjoining divisions of the hyperpallium (hyperpallium intercalatum and hyperpallium densocellulare), have very similar gene expression profiles to each other, distinct from those of the nidopallium, the arcopallium, and the more distant divisions of the hyperpallium (hyperpallium apicale). The study proposes an update of the current nomenclature (Jarvis et al. [2005] Nat Rev Neurosci. 6:151-159). The authors perform densitometric quantifications of the in situ expression of 50 selected genes, use correlations of distances between vectors that represent these gene expression patterns within the 23 avian brain regions of their study, and group them according to similarity in their expression profiles. The generated cluster tree further supports their argument for a new terminology. The authors hypothesize that the mesopallium and adjoining divisions of the hyperpallium have a common developmental origin, and in the accompanying paper (Chen et al. [2013] J Comp Neurol. 521:3666-3701) show that these structures/subdivisions initially form continuous gene expression domains. With subsequent development these domains fold into distinct subdivisions in the dorsal and ventral avian pallium, forming mirror images to each other. Jarvis et al. ([2013] J Comp Neurol. 521:3614-3665) also demonstrate interesting principles of the functional organization of the avian brain by showing that specific sensory stimulation or motor behavior elicits gene expression in functional units perpendicular to the axis of the gene expression reversal and compare their arrangements and cell types with mammalian cortical columns.