Should we redefine the classic lateral pallium?
Three main pillars support the neuromeric or prosomeric vertebrate brain model. First, it provides us with the true longitudinal brain axis following both the hindbrain and the forebrain flexures. This axis can be demonstrated directly by visualization of the activity of longitudinally expressed genes such as sonic hedgehog and islet1 (Ericson et al., 1995) and reveals the dorsoventral axis at every anteroposterior level.
Second, beyond rhombomeres (hindbrain segments), which quickly became generally accepted (for example, because of cell lineage restriction within rhombomeres; Fraser et al., 1990; Lumsden, 2004), molecular gene expression studies demonstrated prosomeres (forebrain segments) in the posterior forebrain (pretectum, thalamus, and prethalamus). Thus the neuromeric model integrates approaches to divide the central nervous system of vertebrates into transverse and longitudinal zones and suggests a coherent definition of a vertebrate brain bauplan by proposing a matrix of transverse zones (neuromeres) along the anteroposterior axis that each contain an element of all longitudinal zones. Such neuromeres or transverse brain units represent a highly useful topological framework for comparing and interpreting morphological observations across vertebrate taxa.
A third essential element in the understanding of the vertebrate brain bauplan is the concept of histogenetic units (Puelles and Medina, 2002), that is, proliferative zones arranged at the ventricular side of the neuroepithelial wall of the neural tube from which the adult structures arise through differential gene activity within those units. This brings us to the focus of the present commentary, which is the recognition of the same histogenetic units throughout telencephalic evolution.
The telencephalon, among all other brain regions, evokes more controversial and contentious debate regarding its developmental and, in particular, its comparative interpretation. A great advance in recent times is the recognition of a quadripartite pallium (medial, dorsal, lateral, and ventral pallium representing, respectively, the hippocampus, isocortex, olfactory cortex, and pallial amygdala in mammals; Fig. 1A) and in particular the novel description of the ventral pallium, which contributes to the generation of many pallial amygdalar nuclei (Striedter, 1997; Puelles et al., 1999; Medina et al., 2004; Martínez‐García et al., 2009). Equally importantly, it was realized that the ventral pallium is represented by the nidopallium in birds (Bruce and Neary, 1995; Striedter, 1997; Smith‐Fernandez et al., 1998, Puelles et al., 1999; Medina et al., 2004; for the sake of simplicity referred to as the Puelles school below). These detailed comparative studies were based both on distinct patterns of gene expression during embryonic development and on patterns of connectivity in adult brains. Another general conclusion in these studies was that the lateral pallium (primarily defined as including the olfactory or piriform cortex of mammals) corresponds to the mesopallium of birds.
This view remains controversial. Continuing reports suggest a homology of the dorsal ventricular ridge (DVR: nido‐ and mesopallium) with the mammalian isocortex based on a general conception proposed earlier and first by Harvey Karten (1969). Recent reports include pallial cell‐specific gene expression studies (Dugas‐Ford et al., 2012) as well as connectivity studies (Wang et al., 2010), recently reviewed by Karten (2013). Extensive developmental and gene expression studies (Chen et al., 2013; Jarvis et al.