Xenopus as an Experimental Organism


The South African clawed frog, Xenopus laevis, is used extensively as a model system to study development. It is easy to maintain a breeding colony of frogs, females can be induced to lay eggs all year round and hundreds of embryos can be generated that grow synchronously from a single fertilization. Embryos are easily amenable to microsurgery at any stage of development and adult frogs survive surgeries performed under limited aseptic conditions. The ease of injecting oligonucleotides or messenger ribonucleic acid (mRNA) into Xenopus leavis embryos has contributed to the discovery of genes with key functions in development. Another singularity of Xenopus leavis is the ease to manipulate its oocytes before fertilization; this has been a powerful tool to identify the function of maternally deposited mRNAs in development. Transgenic procedures are being developed using the related species Xenopus tropicalis and this will broaden the contribution of Xenopus to the understanding of early development.

Key concepts:

  • Xenopus laevis is easy to raise and feed in large colonies in the laboratory.

  • The female Xenopus can be induced to spawn repeatedly at any season, making it possible to obtain embryos all year round.

  • Xenopus leavis eggs are quite large, and are usually laid in large numbers that are fertilized in vitro.

  • Xenopus leavis oocytes can easily be obtained and manipulated before fertilization, making it possible to study the function of maternally deposited mRNAs.

  • The Xenopus embryos develop outside the female and are amenable to manipulation at any stage of development.

  • The mechanism of tissue formation has been elucidated from research carried out on Xenopus leavis embryos.

  • Xenopus leavis is an excellent model for studying body axis formation.

  • The early development of Xenopus is very fast, making it possible to study a full developmental programme in about three days.

  • The sequencing of the diploid genome, the ease of obtaining and injecting large numbers of eggs and the relatively short lifespan makes Xenopus tropicalis a very promising model for transgenic studies of organogenesis.

  • Eggs of Xenopus leavis provide a model of choice for studying the regulation of the cell cycle.

Keywords: frog; embryology; tissue formation; gene function; cell cycle

Figure 1.

Life cycle of Xenopus laevis. The adult is known as the clawed frog because it has dark claws on its toes. The eggs (1.3 mm in diameter) develop rapidly to tailbud embryos (3 mm in length) and then to feed tadpoles. Tadpoles metamorphose into frogs in about two months. The young frogs are approximately 2 cm long and grow to 10–15 cm as an adult.

Figure 2.

Embryo surgery. Surgery is frequently done at the blastula stage, before any differentiated tissues develop. The blastula is viewed here as if it were sliced in half. This view reveals small animal cells (top), large yolky vegetal cells (bottom) and a central cavity, the blastocoel. In an explant, a piece of the embryo is cut out and cultured in the presence or absence of molecules to be tested for a role in development. The explant shown here is called the animal cap because it represents the cells on the animal side of the blastocoel. In a recombinant, two explanted pieces are put together. The recombinant shown here is an animal cap and piece of marginal zone. If the marginal zone piece is from the dorsal side, it will become the organizer and induce the formation of neural tissue from the animal cap. In a transplant, a piece is cut from embryo and used to replace part of a second embryo. The transplant shown here is similar to that used in the Spemann and Mangold experiment. Prospective organizer from the dorsal (D) side is transplanted to the ventral (V) side of a second embryo. This transplant will produce a conjoined twin (see Figure ).

Figure 3.

RNA microinjection. Injection of RNA into the early embryo can have profound effects on development. When cerberus RNA is injected, an extra head with an eye and a cement gland can be produced. When siamois RNA is injected, a conjoined twin with a complete secondary axis and a second full head can form.


Further Reading

Eisen JS and Smith JC (2008) Controlling morpholino experiments: don't stop making antisense. Development 135: 1735–1743.

Hausen P and Riebesell M (1991) The Early Development of Xenopus laevis. New York: Springer.

Heasman J (2006) Patterning the early Xenopus embryo. Development 133: 1205–1217.

Kay BK and Peng HB (eds) (1991) Xenopus laevis: Practical Uses in Cell and Molecular Biology. Methods in Cell Biology, vol. 36. San Diego, CA: Academic Press.

Nieuwkoop PD and Faber J (1994) Normal Table of Xenopus laevis (Daudin). New York: Garland Publishing.

Rubin GM (1988) Drosophila melanogaster as an experimental organism. Science 240(4858): 1453–1459.

Vize PD (1999) The Xenopus Molecular Marker Resource. WWW.Virtual Library. http://www.xenbase.org/xenbase/original/WWW/Welcome.html.

Yergeau DA and Mead PE (2007) Manipulating the Xenopus genome with transposable elements. Genome Biology 8(suppl. 1): S11.

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Tadjuidje, Emmanuel, and Heasman, Janet(Mar 2010) Xenopus as an Experimental Organism. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002030.pub2]