Multiarchitectonic characterization of insular, perirhinal and related regions in a basal mammal, Echinops telfairi

Abstract

The rhinal cortex was investigated in the Madagascan lesser hedgehog tenrec, a basal placental mammal. This region parallels the rhinal indentation and presumably contains the equivalents of the insular and perirhinal cortices. Using cyto- and myeloarchitectural, enzyme- and immunohistochemical criteria as well as data on the connections with the olfactory bulb, the rhinal cortex was subdivided tentatively along its rostrocaudal and dorsoventral planes. An area caudally adjacent to the rhinal cortex received a prominent input from the olfactory bulb and was also preliminarily characterized in this study. Because previous studies in insectivores remained controversial with regard to the identification of the claustrum, special attention was paid to the laminar organization of the rhinal cortex and its deep cell groups. The tenrec's claustrum was identified and delineated cytoarchitecturally and by its negative acetylcholinesterase stain. Latexin, a molecular marker for characterizing infragranular and claustral cells, also helped to differentiate the claustrum from the cell groups subjacent to it. Thus, the data indicate that in poorly differentiated mammals the claustrum occupies an intermediate deep position within the width of the rhinal cortex, i.e., it is separated from the subcortical white matter by additional, still unidentified, cell groups.

Pallio-Pallial Tangential Migrations and Growth Signaling: New Scenario for Cortical Evolution?

“…The latter's 6-layered structure [Brodmann, 1909] first evolved in early mammals before subsequent surface expansion occurred (some insectivores and afrotherians continue to have very little neocortex) [Künzle and Radtke-Schuller, 2000]. A fully 6-layered neocortex is reportedly present and probably expanded independently in monotremes [Krubitzer, 1998[Krubitzer, , 2009Hassiotis et al, 2005].…”

“…They occupy a deep position corresponding to layer VIb, with some dispersion into the overlying layers (mainly layers VIa and V), and are described as 'modified pyramids'. One clear-cut example is represented by Nurr1 -expressing cells [Arimatsu et al, 2003], a subgroup of which also express latexin, a carboxypeptidase A inhibitor [Arimatsu et al, , 1999a[Arimatsu et al, , b, 2009Hatanaka et al 1994;Künzle and Radtke-Schuller, 2000;Liu et al, 2000;Takiguchi-Hayashi, 2001;Arimatsu and Ishida, 2002]. Latexin-positive neurons seem to be involved in ipsilateral cortico-cortical associative feedback connectivity [Bai et al, 2004], which possibly reveals a pallio-pallial hodological property also shared by claustro-cortical connections [Crick and Koch, 2005].…”

Comments on the Updated Tetrapartite Pallium Model in the Mouse and Chick, Featuring a Homologous Claustro-Insular Complex

“…The mammalian insula is currently understood as enclosing multiple sensorimotor and associative subareas [review in Nieuwenhuys, 2014; see also Burwell et al, 1995;Künzle and Radtke-Schuller, 2000]; as commented above, these connections and functions might bear comparison with the avian mesopallial data of Atoji and Wild [2012]. Moreover, the disgranular insular cortex receives gustatory and visceral input from the parabrachial nucleus, as well as some direct olfactory projections [Shipley and Geinisman, 1984;Lundy and Norgren, 2004;Whitehead, 2012].…”

Neocortical connections with perihippocampal and periamygdalar regions in the hedgehog tenrec

“…1 shows the medial (A,C), dorsal (A,C) and lateral (B) views of the Echinops' cerebral hemisphere, its neocortical, rhinal and paleocortical subdivisions as well as the cortical zones used for the description of the results in absence of distinct cytoarchitectural boundaries. These zones are defined on the basis of proportional coordinates (ppc) taken from the reference distance between the rostral pole of the cerebral hemisphere (ppc 0) and the point where the hemispheres diverge from each other (ppc 1.0; for further details see Künzle and Radtke-Schuller 2000c). References to these coordinates, however, should not be interpreted too rigorously; they represent approximate levels that vary due to individual variations and differences in the plane of sectioning of this small brain (average length of the reference distance is 5.4 mm).…”

“…Subsequent tracer studies, however, showed (Künzle 2002) that the area projecting to the dentate gyrus only involves a minor portion of the dorsal PCx but extends considerably into the rhinal cortex (RCx) and the caudally adjacent area XCx ( Fig. 1B; see also Künzle and Radtke-Schuller 2000c). It is known that the presumed equivalents of RCx and XCx in other species, the perirhinal and postrhinal cortices project to the hippocampus (Kosel et al 1983;Suzuki and Amaral 1990;Naber et al 1999), but never to the extent and the intensity as shown from the tenrec's RCx/XCx and only to the cornu ammonis and the subiculum, and not to the dentate area.…”