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Introduction to Xenopus laevis as a Molecular
and Histological Model for Genetic Studies
This issue of Microscopy Research and Technique
focuses on the contribution of the African frog Xenopus
laevis to cellular biology.
Several unique properties of Xenopus laevis early
development have brought this amphibian into laboratories where biologists found the animal easy to rear
without specific requirements.
It was mainly the eggs that took all the attention.
These cells of about 1.2 mm diameter can be observed
with minimal equipment. Furthermore, Xenopus early
development is a very spectacular biological event to
follow because cell divisions proceed externally and are
very rapid (every 30 minutes). In 1956, Nieuwkoop and
Faber established the morphological details of oogenesis, maturation, and embryogenesis of Xenopus laevis,
including the precise timing of development. This normal table of Xenopus laevis development remains a
reference for the laboratories working on Xenopus.
Biochemists were attracted by Xenopus in the very early
days because the egg is an enormous stockpile of cellular
proteins; to give an estimate, the amount of protein stored
in an egg is the equivalent to that of 105 to 106 somatic cells.
Because the eggs are easy to manipulate, experiments
based on microinjections have been very abundant. The
success of this approach became obvious when it was found
that early development of Xenopus proceeds without detectable transcription. It was then possible to destroy an
endogenous mRNA by antisense oligonucleotides or to
introduce a modified mRNA. A step forward was achieved
when Xenopus eggs became a source of cell free extracts
that could be manipulated in vitro. A specific protein can be
removed from the extract by immunoprecipitation and
replaced by a recombinant mutated protein to assess its
function. Depending on the conditions used to prepared
these extracts, it is then possible to specifically study
cellular events such as DNA replication, DNA repair,
nuclear membrane breakdown, chromosome condensation,
spindle assembly, etc.
Cellular biologists and especially histologists have
been frustrated for a long time because the Xenopus egg
is not a transparent cell. Unlike cultured cells, it is
rather difficult to visualize internal structures and
molecules. Only chemical treatments that remove the
pigmentation or the use of eggs from albinos animals
allowed in vivo observations. This inconvenience is now
overcome by the use of confocal microscopes.
Genetics have also been poorly developed in Xenopus
laevis for several obvious reasons. First, its genome has
been duplicated during evolution and contains four
alleles of each gene. Second, it takes a very long time to
obtain an adult Xenopus compared to other animals like
the Drosophila, for example. Although techniques such
as nuclear transplantation and transgenicity have been
developed to allow genetic approaches, they are mainly
devoted to cell lineage studies.
In the first articles of this issue of Microscopy Research and Technique, the reader will mostly find studies
focused on the organization of the cytoskeleton, which
represent one of the fields in which Xenopus early developr 1999 WILEY-LISS, INC.
ment, Xenopus egg extracts, Xenopus cultured cells have
largely contributed, and still contribute to our knowledge.
The first paper from Dave Gard describes the organization of the Xenopus oocyte cytoskeleton during oogenesis. A
very beautiful and impressive gallery of confocal images
and 3-D reconstitutions of microtubule, actin, and cytokeratin networks illustrates a very comprehensive review of our
current knowledge of these organization. The following
report by Patrick Chang and collaborators complements
the first review by considering the mechanisms of determinant localization, anchorage, and redistribution, which
involves interactions with the different cytoskeleton networks. Ultrastructural views of isolated cortices using a
rapid-freeze deep-etch method are shown.
The next paper shows an example of micro-injection
experiments in Xenopus eggs. Nadezhdina and collaborators micro-injected xenogenic centrioles in activated oocyte
and show electron microscopic pictures of centrioles taken
after the first division. Centrioles are the major microtubule organizing center of the cell. Microtubule dynamics
have been extensively studied in Xenopus laevis egg extracts, Shirasu and collaborators present a review focusing
on proteins that regulate microtubule behavior.
Various cellular mechanisms have been analyzed in
Xenopus egg extracts. Hutchison and collaborators
have investigated nuclear envelop assembly and in
particular the role of lamina. De Smedt and collaborators have developed a new in vitro assay to study
membrane vesicles and associated proteins. They present here a new mode of regulation of p34cdc2 kinase.
A number of proteins have been identified during
studies of Xenopus laevis early development. In order to
clarify their role in somatic cells, various Xenopus
cultured cells have been developed. Uzbekov and collaborators report a detailed characterization of the XL2 line
cell cycle together with a synchronization method. Michael
Klymkowsky used the Xenopus A6 cell line to follow the
localization of tagged proteins after transfection. Interestingly, he reports that the nature of the tag has to be
carefully taken into account especially for GFP.
I hope that these articles will give a current view of the
possibilities of investigation offered by the animal model
Xenopus laevis.
I thank all the authors for their contributions, the
people from the Department of Developmental Biology
and Genetics in Rennes for their help, and all the
reviewers for their constructive critiques and suggestions. We are all grateful to Dr John E. Johnson, Jr., for
giving us the opportunity to publish our work in
Microscopy Research and Technique.
Department of Developmental Biology et Genetics
Cell Cycle Group
University School of Medicine
35043 Rennes Cedex, France
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