The subrhinal paleocortex in the hedgehog tenrec: a multiarchitectonic characterization and an analysis of its connections with the olfactory bulb

In the Madagascan hedgehog tenrec, Echinops telfairi, the entire paleocorti cal region (PCx) subjacent to the rhinal indentation is composed of three l ayers and occupies up to two thirds of the lateral hemisphere. A clear diff erentiation of PCx into its presumed constituents, the piriform cortex and the entorhinal cortex, as seen in other mammals, has not been obtained so f ar. To gain insight into location and intrinsic organization of these areas in a basal placental mammal we investigated the tenrec's PCx using cyto-, myelo- and chemoarchitectural criteria (zinc, acetylcholinesterase, NADPh-d iaphorase, Wisteria floribunda agglutinin, parvalbumin, calbindin, calretin in) and analysed its connections with the olfactory bulb. The layers 2 and 3 of the tenrec's PCx differed from the corresponding layers in the rat. Th e layer 2 showed a complex distribution of corticobulbar cells but could no t be subdivided , in contrast to layer 3. Additional cell groups in the dep th of PCx were tentatively compared with subdivisions of the endopiriform r egion. The architectural and connectional features varied clearly along the rostrocaudal and dorso-ventral extents of PCx, and gave hints for the pres ence of different paleocortical subdivisions. With the possible exception o f an area located at the most caudal tip of the dorsomedial hemisphere, how ever, no conclusive evidence was obtained for the presence of a multilayere d, entorhinal region. The bulbar projections to the PCx were very extensive and almost exclusively ipsilateral. The laterality of the projection is si milar to that in higher mammals, but differs from that in the erinaceous he dgehog.