Olfactory bulb and retrobulbar regions in the hedgehog tenrec: Organization and interconnections

Abstract

The Madagascan lesser hedgehog tenrec (Echinops telfairi) is a terrestrial, nocturnal insectivore with a low encephalization index and a huge olfactory bulb. To gain insight into the organization and evolution of olfactory regions in placental mammals, the cytoarchitecture (Nissl), neurochemical attributes [zinc and acetylcholinesterase stain, nicotinamide adenine dinucleotide phosphate (NADPh)-diaphorase, and calcium-binding proteins], and interconnections (injections of wheat germ agglutinin-horseradish peroxidase and biotinylated dextran amine) of tenrec bulbar and retrobulbar regions were examined. The tenrec has a well-laminated main olfactory bulb, and modified (atypical) glomeruli are found that, to date, have been demonstrated only in murine rodents. Compared with the main olfactory bulb, the accessory bulb is relatively small, with clearly different staining characteristics, particularly with respect to NADPh-diaphorase, anticalbindin, and anticalretinin. External and central anterior olfactory nuclei also show characteristic cytoarchitectural and chemoarchitectural features. The medial olfactory peduncle seems to differ considerably from that in rodents. A small taenial structure can be separated from the hippocampal continuation. This taenia tecti presumably corresponds to the superior part of the tenia tecti in rodents, but no homologue of the rodent's prominent inferior taenia tecti could be found. The connections of bulbar and retrobulbar regions are similar to those seen in other mammals. Interbulbar projection systems connect the two olfactory bulbs through an external (topographic) and central (nontopographic) anterior nucleus; however, the topographic arrangement of the intrabulbar association system seems to differ from that seen in rodents. A reciprocity of direct olfactory bulb connections with the frontal (sulcal/orbital) cortex was found in the tenrec that has not been reported so far in other species.

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

“…The material and methods used are largely the same as described previously (Radtke-Schuller and Künzle 2000). In short, the lesser hedgehog tenrecs (Echinops telfairi, Et), obtained from our breeding colony (Künzle 1998), were anesthetized with tribromoethanol (1.0 ml/ 100 gm, i.p.)…”

“…The erinaceous hedgehog has a higher encephalization index than the tenrec and clearly shows a multilayered entorhinal cortex with a medial and a lateral subdivision (Schwerdtferger et al 1985;Stephan et al 1991). In contrast to other mammals it has a prominent projection from the piriform cortex to the contralateral olfactory bulb (de Carlos et al 1989) and, unlike tenrec and rat, it does not show "modified glomeruli" (Crespo et al 1999;Radtke-Schuller and Künzle 2000; for other differences see Künzle 1995).…”

“…The bulbar injection fields were described in detail previously by Radtke-Schuller and Künzle (2000). In four cases the injection fields appeared restricted to the OfB, while in another four cases the external and partly the central portion of the anterior olfactory region were also injected with tracer.…”

Transposition and Intermingling of Gαi2 and Gαo Afferences into Single Vomeronasal Glomeruli in the Madagascan Lesser Tenrec Echinops telfairi

“…As previously described [36], the olfactory bulbs of the tenrec are relatively large structures located at the rostralmost telencephalon (Figure 1). They contain a prominent olfactory ventricle (OV), surrounded by small and dense periventricular cells that may represent newly born neurons [37].…”

“…The anterior olfactory nuclear complex (AON), composed of an external (AONe) and a central (AONc) portion [36], partially encircles the deep MOB from its lateral aspect and can be observed lying below the AOB (Figure 1A). The AONe is a narrow band of densely packed cells, caudal to AOB granular cells, while the AONc prolongs caudally into the frontal cortex (FrCx), in a transition zone that has been referred to as the sulcal cortex [36]. There is a clear boundary between the MOB and AOB (dotted line; Figure 1B, 1C and 1D), marked by an abrupt discontinuity of MOB granular cells.…”