A radial histogenetic model of the mouse pallial amygdala

Abstract

Conventional anatomic models of the rodent (mammalian) amygdala are based on section planes oblique to its intrinsic radial glial organization. As a result, we still lack a model of amygdalar histogenesis in terms of radial units (progenitor domains and related radial migration and layering patterns). A radial model of the mouse pallial amygdala is first offered here, based on three logical steps: (1) analysis of amygdalar radial structure in variously discriminative genoarchitectonic material, using an optimal ad hoc section plane; (2) testing preliminary models with experiments labelling at the brain surface single packets of radial glia processes, to be followed into the ventricular surface across intervening predicted elements; (3) selection of 81 differential amygdalar gene markers and checking planar and radial aspects of their distribution across the model elements. This approach shows that subtle changes to the conventional schema of the amygdala allow a radial histogenetic model to be recognized, which is consistent with molecularly coded differential identities of its units and strata. It is expected that this model will help both causal studies of amygdalar developmental patterning and comparative evolutionary studies. It also may have potential impact on hodological and functional studies.

Histogenetic Radial Models as Aids to Understanding Complex Brain Structures: The Amygdalar Radial Model as a Recent Example

“…The radial AHi/PMCo complex is located at the caudomedial region of the telencephalic temporal pole, protruding medialwards at the brain surface behind the medial amygdala (rostrally to the ventral end of the dentate gyrus and other parts of the hippocampus), and caudalwards with the prominent PMCo portion. According to our results, it is properly the posterior radial unit of the pallial amygdala (Garcia-Calero et al, 2020). What is identified as AHi in the literature is the periventricular stratum of this unit, which appears related radially to the intermediate and superficial PMCo corticoid mass, as was recognized already in some works (AHi, PMCo;Figures 8C-F;e.g., McDonald, 2003;Martínez-García et al, 2012;Olucha-Bordonau et al, 2015).…”

“…Topographical amygdalar models Figure 7; Garcia-Calero et al, 2020). McDonald (2003 observed different neurogenetic patterns between some pallial amygdalar areas (with an outside-in pattern) and surrounding cortical structures such as piriform or hippocampal areas (with an inside-out neurogenetic pattern).…”

“…It can be safely said that current literature ignores the number and type of radial histogenetic units that shape up the pallial amygdala of mammals and non-mammals. Recently we elaborated a radial model of the mouse pallial amygdala, aiming to start to unravel the conceptual difficulties implied in understanding causally such a complex nuclear structure (Garcia-Calero et al, 2020 ). In both classic and modern descriptions of the rodent pallial amygdala (e.g., Johnston, 1923 ; Krettek and Price, 1978 ; De Olmos et al, 1985 , 2004 ; Alheid et al, 1995 ; Paxinos and Franklin, 2007 , 2013 ), individual nuclei are distinguished and named by their apparent relative topographic position in a coronal section series.…”

“…The conceptual approach at the basis of the mouse amygdalar radial model (Garcia-Calero et al, 2020 ) would be useful for examining other brain regions. Such an approach was applied, perhaps somewhat unwittingly (because developmental notions were not foremost in the relevant adult neuroanatomic scenario), when Heimer and colleagues started examining the radial relationships of subpallial masses with the respective ventricular and pial surfaces (Heimer et al, 1982 , 1991 , 1995 , 1997 , 1999 ; Heimer and Alheid, 1991 ; reviewed in Heimer et al, 2008 ).…”

“…One recent example in this scenario is the recently proposed radial model of the mouse pallial amygdala . This is theoretically valid generally in mammals (Garcia-Calero et al, 2020 ), and subdivides the nuclear complex of the pallial amygdala into five main radial units. The approach applies a novel ad hoc amygdalar section plane, given the observed obliquity of the amygdalar radial glial framework.…”