Chemoarchitecture of the Monotreme Olfactory Bulb

Abstract

The cyto- and chemoarchitecture of the olfactory bulb of two monotremes (shortbeaked echidna and platypus) was studied to determine if there are any chemoarchitectural differences from therian mammals. Nissl staining in conjunction with enzyme reactivity for NADPH diaphorase, and immunoreactivity for calcium binding proteins (parvalbumin, calbindin and calretinin), neuropeptide Y, tyrosine hydroxylase and non-phosphorylated neurofilament protein (SMI-32 antibody) were applied to the echidna. Material from platypus bulb was Nissl stained, immunoreacted for calretinin, or stained for NADPH diaphorase. In contrast to eutherians, no immunoreactivity for either the SMI-32 antibody or calretinin was found in the mitral or dispersed tufted cells of the monotremes and very few parvalbumin or calbindin immunoreactive neurons were found in the bulb of the echidna. On the other hand, immunoreactivity for tyrosine hydroxylase in the echidna was similar in distribution to that seen in therians, and periglomerular and granule cells showed similar patterns of calretinin immunoreactivity to eutherians. Multipolar neuropeptide Y immunoreactive neurons were confined to the deep granule cell layer and underlying white matter of the echidna bulb and NADPH diaphorase reactivity was found in occasional granule cells, fusiform and multipolar cells of the inner plexiform and granule cell layers, as well as underlying white matter. Unlike eutherians, no NPY immunoreactive or NADPH diaphorase reactive neurons were seen in the glomerular layer. The bulb of the echidna was comparable in volume to prosimians of similar body weight, and its constituent layers were highly folded. In conclusion, the monotreme olfactory bulb does not show any significant chemoarchitectural dissimilarities from eutheria, despite differences in mitral/tufted cell distribution.

Cyto- and Chemoarchitecture of the Monotreme Olfactory Tubercle

“…As previously reported [Ashwell, 2006], the olfactory bulb of the echidna is comparable in volume to prosimians of similar body weight and comparable in absolute terms to the volume of the olfactory bulbs of the largest eutherian insectivores. Furthermore, the layers of the olfactory bulb of the echidna are highly folded to an extent which might increase its functional capacity of the olfactory bulb beyond that of eutherian insectivores.…”

“…Both of these monotremes show unusual cytoarchitecture of the olfactory bulb, with presumptive projection cell somata spread throughout the external plexiform layer [Switzer and Johnson, 1977]. In a previous study [Ashwell, 2006], I presented data concerning the cyto-and chemoarchitecture of the olfactory bulb of the echidna and platypus using enzyme-and immunohistochemistry, but very little is known about the organization of more central olfactory structures such as the olfactory tubercle, piriform and entorhinal cortex in either monotreme apart from studies originating in the early 20th century [Hines, 1929;Abbie, 1940]. In previous studies on the cerebral cortex and claustrum of the echidna [Abbie, 1940;Hassiotis et al, 2004;Ashwell et al, 2004], the olfactory tubercle of the echidna has been noted in passing, but those studies did not fully describe this region.…”

The Anterior Olfactory Nucleus and Piriform Cortex of the Echidna and Platypus

“…The platypus is a semi-aquatic mammal hunting benthic invertebrates in fresh-water streams, dams and lakes and uses electroreception as its principal sense for prey location [Scheich et al, 1986;Gregory et al, 1987;Iggo et al, 1992;Manger and Pettigrew, 1996;Proske et al, 1998]. Although the accessory olfactory bulb of the platypus is substantial in size, the main part of the olfactory bulb [Ashwell, 2006a] and the piriform cortex (present study) are very small.…”

“…Although the sample size is only 4 animals, there appears to be some correlation between olfactory bulb and paleocortex volume as echidna 96/01, which had by far the largest paleocortex volume estimate, was previously found to have an olfactory bulb volume of 635 mm 3 compared to 243 mm 3 , 227 mm 3 , 211 mm 3 for the other three echidnas [Ashwell, 2006a]. Figure 14 shows values for the volume of the paleocortex of the four echidnas plotted alongside estimates for the volume of the paleocortex calculated from the data of Stephan and co-workers [Stephan et al, 1970;Stephan and Andy, 1977].…”

“…Both monotremes have unusual cytoarchitecture of the olfactory bulb, with the usual monolayer of mitral cell somata seen in eutherians replaced by a broad spread of large perikarya in the external plexiform layer [Switzer and Johnson, 1977]. The olfactory bulb of the echidna is large and folded [Ashwell, 2006a] and yet the olfactory tubercle of the echidna is poorly laminated and lacks some of the chemoarchitectural features of this region seen in therians [Ashwell, 2006b]. In previous studies on the cerebral cortex and claustrum of the echidna [Abbie, 1940;Hassiotis et al, 2004;Ashwell et al, 2004], the piriform cortex of the echidna has been noted in passing as extraordinarily extensive with potentially a large number of subregions, but those studies did not have suffi cient space to fully describe this region.…”