Functional Architecture of the Vomeronasal Organ of the Frog (Genus Rana)

Abstract

Abstract The vomeronasal organ in the frog, genus Rana, is composed of three interconnected cavities; superior, middle and inferior, which are separated from and anterior to the principal olfactory cavity. The superior cavity is found just underneath the external naris and forms a vestibule both for the principal olfactory organ and the vomeronasal organ. The vomeronasal sensory epithelium is located in the medial region of the inferior cavity and contains ciliated cells and microvillous receptor cells. Inspection of microscopic sections of frogs that had been swimming in fluorescent colorants revealed fluorescence on the surface of the vomeronasal organ, but not on that of the olfactory organ. Observations in vivo show that water enters via the external naris by two fissures, one on each side of the movable nasal lid, passes the middle cavity to flow via the sensory epithelium of the inferior cavity. The design of the frog nose makes it possible for this amphibious animal to sample the chemical composition of its environment; above water the frog can inhale air and expose its olfactory organ to volatile substances; in water the vomeronasal organ samples water‐borne substances. These new findings are discussed in relation to the air/water interface and the position of the amphibians in the evolution of terrestrial vertebrates.