The model animal
The African clawed frog (Xenopus laevis) is an amphibian of the order Anura. It originates from the Sub-Saharan Africa, but as an invasive species it now occupies freshwater areas all over the world. It lives in warm, stagnant grassland ponds as well as in streams in arid and semi-arid regions. The adult frog is a scavenger and eats living, dead, or dying arthropods and other pieces of organic waste, whereas the larvae feed on small organic particles, such as algae. To locate food it uses predominantly its acute sense of smell. It spends most of its time underwater and comes to surface to breathe. Respiration is predominantly through its well developed lungs, there is little cutaneous respiration.Xenopus laevis is an excellent model for olfactory system research.
Xenopus laevis is an excellent model animal to study the olfactory system. The husbandry and breeding of this species is relatively easy and, as it is a poikilothermal animal, all experiments can be carried out at room temperature. Larval Xenopus laevis do not have a lamina cribrosa and therefore the olfactory mucosa is not separated from the olfactory bulb by a bony structure. This is a great advantage for the preparation of acute slices of the olfactory epithelium and the olfactory bulb and of so called nose-brain preparations. Also, amphibians represent an evolutionary intermediary between fishes and mammalian species with respect to life style, anatomical specialisation of their olfactory organ, and evolution of olfactory receptor gene families. Amphibians are, therefore, particularly suited to investigate questions regarding the evolution of the vertebrate olfactory system.
The olfactory system of Xenopus laevis
The nose of the adult Xenopus laevis is made up of three interconnected chambers that form three different epithelia for the detection of different classes of odorants. The largest chamber forms the principal cavity ("air nose"), which is permanently filled with air and can be closed underwater by a membrane. The other two chambers, the lateral olfactory cavity ("water nose") and the vomeronasal organ are permanently filled with water. The olfactory epithelium of the principal cavity serves for the detection of air-borne, the olfactory epithelium of the lateral cavity detects water-borne stimuli. The vomeronasal organ is implicated with the detection of pheromones. Premetamorphotic animals posses two separate chambers, the principal cavity, which is the larval ("water nose") and the vomeronasal organ.
All of the three epithelia, like in other vertebrates, consist of three main cell types: olfactory receptor neurons which transmit the olfactory information from the nose to the olfactory bulb in the brain via the olfactory nerve, supporting cells which share common properties with glial and epithelial cells, and basal cells including olfactory stem cells which maintain the regenerative capacity of the olfactory epithelium. As other vertebrates, Xenopus laevis possesses at least two types of olfactory receptor neurons, one type having cilia and the other type having microvilli. Axon terminals of olfactory receptor neurons synapse directly onto second-order neurons in the olfactory bulb forming spheroidal structures called glomeruli. Thereby, olfactory receptor neurons from the principal and the lateral cavity project to glomeruli in the main olfactory bulb, whereas receptor cells from the vomeronasal system project to glomeruli in the accessory olfactory bulb.The second-order neurons in the olfactory bulb are called mitral cells. These cells output the olfactory information to higher olfactory centers. In addition to mitral cells, numerous interneuron types exist in the olfactory bulb, including periglomerular cells which synapse within and between glomeruli, and granule cells which synapse with mitral cells. These interneurons modulate the olfactory information before it leaves the olfactory bulb (for more information see Manzini and Schild, 2010).