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

The rhinal cortex was investigated in the Madagascan lesser hedgehog tenrec , a basal placental mammal. This region parallels the rhinal indentation an d presumably contains the equivalents of the insular and perirhinal cortice s. Using cyto- and myeloarchitectural, enzyme- and immunohistochemical crit eria 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 gro ups. The tenrec's claustrum was identified and delineated cytoarchitectural ly and by its negative acetylcholinesterase stain. Latexin, a molecular mar ker for characterizing infragranular and claustral cells, also helped to di fferentiate the claustrum from the cell groups subjacent to it. Thus, the d ata indicate that in poorly differentiated mammals the claustrum occupies a n intermediate deep position within the width of the rhinal cortex, i.e., i t is separated from the subcortical white matter by additional, still unide ntified, cell groups.