Distribution of perihippocampo-hippocampal projection neurons in the lesser hedgehog tenrec

The entorhinal cortex in the Madagascan lesser hedgehog tenrec is thought to be part of the three-layered subrhinal paleocortex (PCx) but cyto- and chemoarchitectural studies have failed so far to identify the area. To reach this goal tracer injections were made into the tenrec's hippocampus. Retrogradely labeled cells were found in dorsal portion of the posterior PCx, the adjacent rhinal cortex (RCx) and the so-called area XCx. The main paleocortical portion in the ventral PCx, however, remained unlabeled with the exception of a caudal region possibly equivalent to the amygdalo-piriform transition area. The labeled neurons showed a bilaminar distribution with the cells in the layer 2A giving rise to fibers to predominantly the dentate area and the cells in the layer 3A mainly projecting to the cornu ammonis and the subiculum. The latter regions, in addition, gave rise to a feedback projection to the layer 3B of especially the caudal RCx and the XCx. The analysis of the terminal projections, however, was hampered by the fact, that under certain conditions retrogradely transported biotinylated dextran was also transported in anterograde direction via collaterals of the entorhino-dentate fibers. The findings are compared with equivalent regions in more differentiated mammals particularly with regard to the perirhinal area showing little if any connections with the dentate gyrus.