Introduction
This year, we would like to introduce you to 'frogs as laboratory animal models', which have achieved great things in basic biomedical research. In the following sections, you will learn more about two commonly used species: the African clawed frog (Xenopus laevis) and the Western/Tropical clawed frog (Xenopus tropicalis). The name of the subgenus Xenopus originates from the Greek words 'xenos' meaning 'stranger' and 'pous' meaning 'foot', alluding to the unusual appearance of their hind limbs, three of which have claws and two of which are clawless. The term 'laevis' means 'smooth', and 'tropicalis' means 'tropical'.
Below, we have compiled information showing why and in which areas frogs are used in research, their properties, and the milestones achieved with them.
Why are frogs used in research?
Frogs belong to the class Amphibia and are more closely related to humans than fish is. They live in both water and on land, and they have similar internal organs (e.g. lungs, kidneys, thymus and spleen) and skeletal structures (e.g. collarbone).
As well as the frogs themselves, their eggs are also used in research. Depending on species, age, and frequency of hormone treatment, clawed frogs can spawn 300–9,000 eggs at a time. These eggs can be used, for example, in a broad range of toxicity studies. As the eggs can be fertilised and develop into adults outside the body, embryonic development can be easily observed and studied. 1
In which areas of biomedical research do frogs play a role? 2
The biomedical research areas in which frogs are used as model laboratory animals are:
- Research of biology fundamentals such as cell division, DNA repair, cell nuclear reprogramming (modification of cell types by interfering with DNA) and stem cells, development of organs and embryos
- Use of fundamental knowledge in disease-related processes such as immune and inflammatory reactions, tissue regeneration, toxicity reactions and drug development
- Understanding the role of genes in health and disease, e.g. neurological disorders, cancer, diabetes and kidney disease
- Simulate human diseases such as tumor growth, pneumonia, blood-brain membrane dysfunction, heart disease
Profile - African vs. Tropical clawed frogs
African clawed frog (Xenopus laevis)
- Adult animals can reach a body length of up to 10-12 cm (females) or 7-8 cm (males)
- They become adults in about 12 months
- They can produce 300 - 1000 eggs per spawning, which are up to 1.3 mm in diameter
- Their genetic material has four copies of each gene
Western/tropical clawed frog (Xenopus tropicalis)
- Adult animals can grow up to 5 - 6 cm (females) and 4 cm (males) in length
- They become adults in about 6 months
- They can produce 1000 - 9000 eggs per spawning, which have a diameter of 0.8 mm
- Their genetic material has two copies of each gene
Natural habitats, social structure and dietary preferences of Xenopus
- X. laevis: Originates from southern Africa; X. tropicalis: Originates from western Africa
- Completely aquatic
- Muddy and turbid ponds and lakes
- There is no hierarchy between individuals but they can fight for food.
- Carnivores that feed on insects, larvae and small fish
Anatomy and physiology
- The rib cage is not made of bone
- Hind limbs and jaws have strong muscles
- Relatively simple nervous system
- 7 sensory organs: eyes, ears, nose, tongue, lateral line organ for water currents and vibrations, vomeronasal organ for pheromones
- The skin has a very important function for respiration and protection against microbes
Reproduction & Development
- Reproduction is possible all year round
- Males make mating calls
- Rapid development to the tadpole stage (X. laevis 4 days and X. tropicalis 3 days) and is temperature-dependent
- Most organs are already present in the tadpole stage
- Metamorphosis is controlled by thyroid hormones, whereby the limb buds develop and the swimming body is lost
Please follow the links to the illustrations and photos of the developing clawed frogs:
Husbandry, artificial habitat and feeding
- Static or flow-through water tanks
- Adult African clawed frogs require lower water temperatures (17°C - 20°C), while tadpoles and younger frogs can live at water temperatures between 22°C and 25°C. Both the adults and the tadpoles of the tropical clawed frog prefer higher water temperatures of 23°C to 27°C.
- The water tanks contain enrichment objects such as tubes, leaves and dark hiding places.
- The bottom of the tanks is darkened to minimise the natural stress response to predators from below.
- Adult clawed frogs of all species are fed with special food pellets. Tadpoles and younger frogs mainly eat special pellets, but their diet can be enriched with shredded muscle meat such as beef heart, bloodworms, algae powder, etc.
The most commonly used techniques in biomedical research with frogs
- In vitro fertilization and embryo formation for developmental studies
- Removal of unfertilized eggs from the ovaries for cell nuclear studies
- Injection of molecules to modify DNA in eggs and early embryos to create genetically modified animals or to study the function of membrane channels
- Removal or displacement of tissue for developmental studies
Milestones in biomedicine with clawed frogs
For almost a century, we have been getting answers to essential biological processes thanks to clawed frogs. In the timeline below, you can discover the most important groundbreaking developments that have shaped biomedical research. 3,4,5,6,7,8,9
For the interactive version, please click here:Milestones with clawed frogs
To download the PDF version, please click here:
Number of clawed frogs used in research 2022
According to the latest report by the Bf3R - German Center for the Protection of Laboratory Animals at the Federal Institute for Risk Assessment, clawed frogs account for 0.45% of the total number (1.73 million animals) of laboratory animals used in Germany in 2022. 40% of these frogs were used for basic research and 58% for regulatory purposes and routine production. The remainder was used for translational and applied research, higher education and vocational skills training.
Research with clawed frogs in North Rhine-Westphalia
RWTH Aachen
Establishment of the body condition score for adult female Xenopus laevis
Ruhr-University Bochum
Discovery of triazole-bridged aryl adamantane analogs as an intriguing class of multifunctional agents for treatment of Alzheimer's disease
University of Bonn
Chromosome alignment maintenance requires the MAP RECQL4, mutated in the Rothmund-Thomson syndrome
University of Duisburg-Essen
Red fluorescent Xenopus laevis: a new tool for grafting analysis
Der Krallenfrosch Xenopus laevis als Labortier: Biologie, Haltung, Zucht und experimentelle Nutzung
Growth of Xenopus laevis under different laboratory rearing conditions
Preference of Xenopus laevis for different housing conditions
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Side view of a clawed frog observed in the study “Establishment of the body condition score for adult female Xenopus laevis”. -
Seen from above, a clawed frog observed in the study “Establishment of the body condition score for adult female Xenopus laevis”
References
- Kashiwagi, K., Kashiwagi, A., Kurabayashi, A., Hanada, H., Nakajima, K., Okada, M., Takase, M., & Yaoita, Y. (2010). Xenopus tropicalis: an ideal experimental animal in amphibia. Experimental animals, 59(4), 395–405.
- Moody, S.A., Sater, A.K. (2016). Xenopus Community White Paper https://www.xenbase.org/xenbase/static/community/xenopuswhitepaper/2016/2016-XenopusWhitePaper-Final.pdf
- Sander, K., & Faessler, P. E. (2001). Introducing the Spemann-Mangold organizer: experiments and insights that generated a key concept in developmental biology. The International journal of developmental biology, 45(1), 1–11.
- Gurdon, J. B., & Hopwood, N. (2000). The introduction of Xenopus laevis into developmental biology: of empire, pregnancy testing and ribosomal genes. The International journal of developmental biology, 44(1), 43–50.
- GURDON J. B. (1962). The developmental capacity of nuclei taken from intestinal epithelium cells of feeding tadpoles. Journal of embryology and experimental morphology, 10, 622–640.
- Jackson P. K. (2008). The hunt for cyclin. Cell, 134(2), 199–202.
- Preston, G. M., Carroll, T. P., Guggino, W. B., & Agre, P. (1992). Appearance of water channels in Xenopus oocytes expressing red cell CHIP28 protein. Science (New York, N.Y.), 256(5055), 385–387.
Acknowledgements
We would like to thank the animal welfare officers and the managers of laboratory animal facilities at the University of Bonn for their cooperation in this campaign. We are also very grateful to the Department of University Communication for their support.