Cyto- and Chemoarchitecture of the Monotreme Olfactory Tubercle

Abstract

This study was undertaken to determine whether the olfactory tubercles of two monotremes (platypus and echidna) showed cyto- or chemoarchitectural differences from the tubercles of therian mammals. Nissl staining was applied in conjunction with enzyme reactivity for NADPH diaphorase and acetylcholinesterase, and immunoreactivity for calcium binding proteins (parvalbumin, calbindin and calretinin) and tyrosine hydroxylase (echidna only). Golgi impregnations of the tubercle were also available for the echidna. The olfactory tubercle is a poorly laminated structure in the echidna, despite the pronounced development of other components of the echidna olfactory system, and the dense cell layer of the olfactory tubercle was found to be discontinuous and irregular. Granule cell clusters (islands of Calleja) were present, but were small, poorly defined and did not show the intense NADPH diaphorase activity seen in marsupial and placental mammals. A putative small island of Calleja magna was seen in only one echidna out of four. In Golgi impregnations of the echidna olfactory tubercle, the most abundant neuron type was a medium-sized densely spined neuron similar to that seen in the olfactory tubercle of some therians. Large spine-poor neurons were also seen in the polymorphic layer. In the platypus, the olfactory tubercle was very small but showed more pronounced lamination than the echidna, although no granule cell clusters were seen. In both monotremes, the development of the olfactory tubercle was poor relative to other components of the olfactory system (bulb and piriform cortex). The small olfactory tubercle region in the platypus is consistent with poor olfaction in that aquatic mammal, but the tubercle in the echidna is more like that of a microsmatic mammal than other placentals occupying a similar niche (e.g., insectivores).

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

“…The piriform cortex could be distinguished from adjacent olfactory tubercle, insular cortex and entorhinal cortex by differences in cytoarchitecture. Cytoarchitectural features of insular and entorhinal cortex in the echidna have been published previously [Hassiotis et al, 2004], and the olfactory tubercle cytoarchitecture has been described recently [Ashwell, 2006b]. Measurements of the dimensions of brains processed for sectioning as described above showed that fi xation, cryoprotection in sucrose, sectioning and staining result in 8.14% linear shrinkage in each dimension for Nissl-stained sections, equivalent to 22.5% shrinkage in volume.…”

“…This animal (05/01) had been obtained postmortem from Taronga Zoo, Sydney. The details of impregnation and sectioning have been described previously [Ashwell, 2006b].…”

“…Nissl-stained sections were viewed at a magnifi cation of 10.4 ! with a computer-linked microscope running the Magellan 5.2 program and the perimeter of the piriform lobe and olfactory tubercle [Ashwell, 2006b] outlined. This program automatically measures the area of the piriform cortex and olfactory tubercle and these values were summed and multiplied by the intersection interval to obtain an estimate of paleocortex volume by Cavalieri's basic estimator.…”

“…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.…”

Development of the Olfactory Pathways in Platypus and Echidna

“…In the echidna, the accessory OB occupies only a small proportion of the total bulb volume [Ashwell, 2006a]; in the platypus, it is not only as large as its main bulb, but is also substantially larger than the accessory OB of echidnas of 6-to 8-fold greater body weight. The olfactory tubercle is small and simple in both monotremes [Ashwell, 2006b], but there are profound differences between the two species in other components of the olfactory system. In the echidna, the anterior olfactory nucleus is negligible in extent and merges at very rostral levels with a large, chemically differentiated, 5-layered piriform cortex [Ashwell and Phillips, 2006].…”