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

Abstract

The cyto- and chemoarchitecture of the anterior olfactory nucleus and piriform cortex of the short-beaked echidna and platypus were studied to determine: (1) if these areas contain chemically distinct subdivisions, and (2) if the chemoarchitecture of those cortical olfactory regions differs from therians. Nissl and myelin staining were applied in conjunction with enzyme reactivity for NADPH diaphorase and acetylcholinesterase, and immunoreactivity for calcium-binding proteins (parvalbumin, calbindin and calretinin) and tyrosine hydroxylase. Golgi impregnations were also available for the echidna. In the echidna, the anterior olfactory nucleus is negligible in extent and merges at very rostral levels with a four-layered piriform cortex. Several rostrocaudally running subregions of the echidna piriform lobe could be identified on the basis of Nissl staining and calcium-binding protein immunoreactivity. Laminar-specific differences in calcium-binding protein immunoreactivity and NADPH-d-reactive neuron distribution were also noted. Neuron types identified in echidna piriform cortex included pyramidal neurons predominating in layers II and III and non-pyramidal neurons (e.g., multipolar profusely spiny and neurogliaform cells) in deeper layers. Horizontal cells were identified in both superficial and deep layers. By contrast, the platypus had a distinct anterior olfactory nucleus and a three-layered piriform cortex with no evidence of chemically distinct subregions within the piriform cortex. Volume of the paleocortex of the echidna was comparable to prosimians of similar body weight and, in absolute volume, exceeded that for eutherian insectivores such as T. ecaudatus and E. europaeus. The piriform cortex of the echidna shows evidence of regional differentiation, which in turn suggests highly specialized olfactory function.

Development of the Olfactory Pathways in Platypus and Echidna

“…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]. By contrast, the platypus has a distinct anterior olfactory nucleus and 3-layered piriform cortex with no evidence of chemically distinct sub-regions [Ashwell and Phillips, 2006]. The volume of the paleocortex of the echidna is comparable to prosimians of similar body weight and exceeds that for many eutherian olfactory specialists.…”

“…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]. By contrast, the platypus has a distinct anterior olfactory nucleus and 3-layered piriform cortex with no evidence of chemically distinct sub-regions [Ashwell and Phillips, 2006].…”

“…Measurements of structures were made with the aid of ImageJ 1.37v software. All olfactory system structures were identified by reference to published accounts of the anatomy of the olfactory pathways in these two species [Ashwell, 2006a, b;Ashwell and Phillips, 2006], marsupials [Brunjes et al, 1992;Malun and Brunjes, 1996;Chuah et al, 1997;Ashwell et al, 2008] and a reference work on the anatomy of olfactory regions in the rat [Shipley et al, 2004].…”

“…The two lineages of monotremes have very different olfactory pathways in the adult [Ashwell, 2006a, b;Ashwell and Phillips, 2006]. The nuclear and pallial components of the main olfactory system of the platypus are relatively small and simple in structure and the major central component of the accessory pathway (i.e.…”

Characterization of NADPH Diaphorase- and Doublecortin-Positive Neurons in the Lizard Hippocampal Formation

“…Nitric oxide (NO) has been studied in a variety of systems and in virtually all groups of animals [Sandell, 1986;Johansson and Carlberg, 1994;Dávila et al, 1995;Martínez, 1995;Wallhäusser-Franke et al, 1995;Muñoz et al, 1996;Franca et al, 2000;Ashwell and Phillips, 2006]. It modulates the activity of endotheliocytes, macrophages, muscle cells, and neurons, playing roles in blood pressure control [Palmer et al, 1988], inflammatory responses [Nathan and Hibbs, 1991], muscle relaxation [Bolotina et al, 1994], and neurotransmission [Arnold et al, 1977], respectively.…”