Phylogenic Studies on the Olfactory System in Vertebrates

Abstract

The olfactory receptor organs and their primary centers are classified into several types. The receptor organs are divided into fish-type olfactory epithelium (OE), mammal-type OE, middle chamber epithelium (MCE), lower chamber epithelium (LCE), recess epithelium, septal olfactory organ of Masera (SO), mammal-type vomeronasal organ (VNO) and snake-type VNO. The fish-type OE is observed in flatfish and lungfish, while the mammal-type OE is observed in amphibians, reptiles, birds and mammals. The MCE and LCE are unique to Xenopus and turtles, respectively. The recess epithelium is unique to lungfish. The SO is observed only in mammals. The mammal-type VNO is widely observed in amphibians, lizards and mammals, while the snake-type VNO is unique to snakes. The VNO itself is absent in turtles and birds. The mammal-type OE, MCE, LCE and recess epithelium seem to be descendants of the fish-type OE that is derived from the putative primitive OE. The VNO may be derived from the recess epithelium or fish-type OE and differentiate into the mammal-type VNO and snake-type VNO. The primary olfactory centers are divided into mammal-type main olfactory bulbs (MOB), fish-type MOB and mammal-type accessory olfactory bulbs (AOB). The mammal-type MOB first appears in amphibians and succeeds to reptiles, birds and mammals. The fish-type MOB, which is unique to fish, may be the ancestor of the mammal-type MOB. The mammal-type AOB is observed in amphibians, lizards, snakes and mammals and may be the remnant of the fish-type MOB.

Immunohistochemical analysis for G protein in the olfactory organs of soft-shelled turtle, <i>Pelodiscus sinensis</i>

“…Most vertebrates in higher order than amphibians possess two distinct olfactory organs, the OE and the VNO. In most cases, ciliated receptor cells are clustered in the OE and microvillous receptor cells in the VNO [ 30 , 31 ]. As in other tetrapods, it has been thought that vomeronasal receptor cells of turtles are generally microvillous based on the electron microscopic studies of gopher tortoises, box turtles [ 7 ], Reeve’s turtles [ 11 , 32 , 34 ] and red-eared sliders [ 19 ].…”

“…Many vertebrates have two distinct olfactory systems, the main olfactory system and the vomeronasal system. The olfactory cells and the vomeronasal receptor cells are olfactory receptor cells distributed in the olfactory epithelium (OE) and the vomeronasal organ (VNO), respectively, and separately project their axons into the main and accessory olfactory bulbs (OB) [ 17 , 30 , 31 , 33 ]. The olfactory cells and vomeronasal receptor cells of mammals bear cilia and microvilli on the tip of their dendrites, respectively.…”

Histomorphology of Vomeronasal Organ in Scincella tsinlingensis

“…On the contrary, VNS is often specialized for the reception of nonvolatile chemicals with high molecular weight (Filoramo & Schwenk, 2009). The vomeronasal organ (VNO) may be derived from the recess epithelium or fishtype olfactory epithelium, which can be differentiated into the mammal-type VNO and snake-type VNO (Taniguchi & Taniguchi, 2014). In most tetrapods, the paired and blindending tubes are observed in ventral nasal septum (Saito et al, 2003;Aland et al), which plays pivotal roles in interindividual interactions and environmental recognition (Brykczynska et al, 2013).…”

Number of olfactory receptor neurons in the Chinese soft-shelled turtle

“…In many vertebrates, there are two types of olfactory organs: the olfactory epithelium and the vomeronasal organ [ 9 ]. In mammals, the olfactory epithelium and the vomeronasal organ are anatomically separated from each other.…”