Abstract
The amygdala in mammals plays a key role in emotional processing and learning, being subdivided in pallial and subpallial derivatives. Recently, the cortical ring model and the pallial amygdalar radial model (Puelles et al. 2019; Garcia-Calero et al. 2020) described the pallial amygdala as an histogenetic field external to the allocortical ring, and subdivided it in five major radial domains called lateral, basal, anterior, posterior and retroendopiriform units. The anterior radial unit, whose cells typically express the Lhx9 gene (see molecular profile in Garcia-Calero et al. 2020), is located next to the pallial/subpallial boundary. This radial domain shows massive radial translocation and accumulation of its derivatives into its intermediate and superficial strata, with only a glial palisade representing its final periventricular domain. To better understand the development of this singular radial domain, not described previously, we followed the expression of Lhx9 during mouse amygdalar development in the context of the postulated radial subdivisions of the pallial amygdala and other telencephalic developmental features.
Highlights
We examined the changes in Lhx9 expression during amygdalar development (Figs. 1, 2) within the conceptual context of our recently proposed radial model of the pallial amygdala, wherein 5 amygdalar radial units were defined (Table 1; Garcia-Calero et al 2020)
We illustrated the development of other amygdalar regions found labelled by Lhx9 signal outside the anterior radial unit, such as the posterior radial unit (AHi/posteromedial corticoid nucleus (PMCo)), the periventricular retroendopiriform nucleus (REP) lying lateral to the BLP nucleus, the bed nucleus of the accessory olfactory tract (BAOT), and parts of the anterior and medial subpallial amygdala
We will discuss below the relationships of Lhx9 transcripts with other molecular markers potentially distinguishing a ventral pallium-like sector in the pallial amygdala and consider in the context of our recent amygdalar radial model the conventional notion of a migratory cortical origin of diverse amygdalar pallial sectors (Medina et al 2004; Deussing and Wurst 2007)
Summary
The telencephalic amygdala of mammals is a mixed pallial/ subpallial nuclear complex located at the tip of the temporal lobe (Burdach 1819–1822; Johnston 1923; Loo 1930, 1931; De Olmos et al 1985, 2004; Alheid et al 1995; Swanson and Petrovich 1998; Martínez-García et al 2012; OluchaBordonau et al 2015, Medina et al 2017). Our early results at E12.5 corroborate the previously described complete ventriculo-pial radial distribution of the Lhx9-labelled domain, identified by us as the primordium of the anterior amygdalar radial unit, whereas no such signal was found at the neighbouring lateral and basal radial units, which contrasted by expressing instead selectively the Enc gene marker This already raised questions about the apparent heterogeneity of amygdalar components thought to be ventropallial (Puelles et al 2016a). The present results, together with correlative Enc and Tbr data, plus a re-evaluation of the Dbx1-derived progeny data of Puelles et al (2016a), corroborate the molecular singularity of the mode of development of each of the diverse amygdalar radial units (Garcia-Calero et al 2020), all of which seem in retrospect to derive from Dbx1-positive neuroepithelium This leads us to discuss the issue whether it is helpful to extrapolate cortical pallial sectors into the separate pallial amygdalar field, concluding that it may be advantageous not to do so. We discuss minimally the apparent functional role of the anterior amygdalar unit within the amygdalar system
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