Introduction to Xenopus
Two species of Xenopus are commonly used by biologists, Xenopus laevis and Xenopus tropicalis. For many years Xenopus laevis was used as a biological assay to determine human pregnancy status, as female Xenopus respond to a hormone present in pregnant womens urine (chrionic gonadotropin) by laying eggs. Biologists use the same method to induce female frogs to produce eggs on demand in the laboratory. The frogs are then rested for a few months, then induced again. Very few species of frog can be induced to produce eggs in such a controlled manner, and this is one of the reasons why Xenopus is so popular with developmental and cell biologists.
Full size image If you would like to view a large image of an adult Xenopus laevis female, click on the image to the left.
Both Xenopus species are natives of Africa. Xenopus laevis has been used for many years to investigate the early period of embryonic development. Embryos develop very rapidly after fertilization, and a tadpole with a fully functional set of organs forms within a couple of days. This means that the biologist can experiment on the embryos directly following fertilization, let the embryos develop in a simple saline solution for a couple of days, then examine the tadpoles to determine if the experimental intervention had any effect. The role of genes in development can be assayed by injecting a tiny amount of an mRNA encoding the gene of interest into an early embryo, then once again allowing the embryo to grow into a tadpole. For example, the embryo on the left was injected with a gene that tells cells to form a head. The resulting tadpole had both a normal head and an additional head induced by the introduced gene. The figure on the right shows the life cycle and speed of early Xenopus laevis development.
Xenopus tropicalis has a much shorter life cycle than does Xenopus laevis, making it much more ammenable to genetics. It is also a diploid, while laevis is a tetraploid, once again, a big advantage in genetics. For more information on the advantages of Xenopus as a model for biomedical research please visit the NIH Xenopus site.