Cortical connections of the claustrum and subjacent cell groups in the hedgehog tenrec

Cyto- and chemoarchitectural findings have recently suggested that in the h edgehog tenrec, the claustrum is not located below but between the layers o f the rhinal/insular cortex (Kunzle and Radtke-Schuller 2000b). The present connectional study confirms this unusual position. Tracer injections were made into various isocortical and allocortical regions. They showed that th e tenrec's dorsal claustrum was reciprocally and bilaterally connected with the neocortex. The ventral claustrum was connected with mainly the ipsilat eral paleocortex, additionally with the ventromedial frontal cortex and pos sibly the subiculum. A sparsely labeled cell group separated the claustrum from the labeled cells located in the depth of the RCx and the adjacent pal eo- and neocortices. On the basis of the linear arrangement of these latter cells immediately adjacent to the subcortical white matter, and the restri ction of their labeling to the ipsilateral side, one might interpret prelim inarily these cells as layer 6B cells or persisting subplate neurons. Their cortical projections showed a similar topographic organization as the clau stro-cortical projections. The unusual features described in tenrec were di scussed with respect to similar organizations in other mammals with poorly differentiated brains and compared with embryonic brains of mammals with mo re differentiated brains.