Phylogenic Studies on the Olfactory System in Vertebrates

Abstract

The olfactory receptor organs and their primary centers are classified into several types. The olfactory receptor organs are divided into fish-type OE, mammal-type OE, MCE, LCE, recess epithelium, mammal-type VNO, snake-type VNO and SO. The fish-type OE is observed in flatfish and lungfish. The mammal-type OE is dominant and observed in amphibians, reptiles, birds and mammals. The MCE and LCE are unique to Xenopus and turtles, respectively. The SO is observed only in mammals. The recess epithelium is unique to lungfish. The mammal-type VNO is dominant and observed in amphibians, lizards and mammals. The snake-type VNO is unique to snakes. The VNO itself is absent in turtles and birds. The primary olfactory centers are divided into mammal-type MOB, mammal-type AOB and fish-type MOB. The mammal-type MOB is observed in amphibians, reptiles, birds and mammals. The mammal-type AOB is observed in amphibians, lizards, snakes and mammals. The fish-type MOB is unique to fish. The fish-type OE seems to be derived from the putative primitive OE. The mammal-type OE, MCE, LCE and recess epithelium seem to be descendants of the fish-type 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 mammal-type MOB first appears in amphibians and succeeds to reptiles, birds and mammals. The fish-type MOB may be the ancestor of the mammal-type MOB, while the mammal-type AOB may be the remnant of the fish-type MOB.