Embryos of the amphibian Xenopus laevis have been used as a model system for the analysis of developmental mechanisms since the 1950s. As described by Gurdon (1), one of the reasons for the popularity of Xenopus is that it is easy to obtain large numbers of embryos. This is illustrated by the fact that until the mid-1950s, Xenopus was used as a pregnancy test in humans: injection of urine from a pregnant woman into the dorsal lymph sac of a female Xenopus causes the frog to lay eggs. This simple assay can readily be applied only to Xenopus, for as Gurdon points out, to persuade Rana species to lay eggs, it is necessary to inject homogenized pituitary glands, a procedure that, furthermore, only works at certain times of the year. Thus, Xenopus is the easiest amphib'ian species from which to obtain embryos, especially as it is now possible to buy human chorionic gonadotrophin from Sigma (St. Louis, MO), thus circumventing the requirement, in a busy developmental biology laboratory, for a constant supply of pregnant colleagues.

However, there are other reasons for working on Xenopus. The animal is completely aquatic, and therefore much easier to keep than other amphibians, which have a tendency to hop away. It is also rather robust, and only rarely succumbs to disease or infection. The early embryo is relatively large in size and, like all amphibian embryos, is readily accessible to the investigator because it develops outside the mother. Dissection and microinjection are therefore easily performed, and because each embryonic cell is provided with yolk reserves to serve as an energy source, tissues can be isolated and cultured for several days in simple salt solutions without the need for poorly characterized serum components. Finally, development is rapid. Together, these virtues have made the Xenopus embryo a favorite of vertebrate developmental biologists.

From: Methods in Molecular Biology, Vol. 97: Molecular Embryology: Methods and Protocols Edited by: P. T. Sharpe and I. Mason © Humana Press Inc., Totowa, NJ

This chapter provides a brief overview of Xenopus embryogenesis from egg to the formation of the definitive body plan. It concentrates on the stages that are most relevant to the molecular embryologist; other, more detailed, descriptions have recently been published in refs. 2 and 3. In particular, the movements of gastrulation have been extensively described by Keller and colleagues (3-5). The staging series is that of Nieuwkoop and Faber (6).

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