The Mouse as a Developmental Model

Paul T. Sharpe

The laboratory mouse, Mus musculus, is the developmental biologist's mammal of choice for studies of development. Its embryology and genetics have been extensively studied for over 100 years. However, it is the advent of in vivo gene manipulation in the last few years that has established the mouse as probably the single most powerful animal system in vertebrate biology.

Mouse developmental biology, as it exists today, has its origins in genetics and embryology. These days it is hard to separate mouse development from mouse genetics, the two having become intertwined as genetics provides increasingly more powerful tools for studying development.

By the early 1980s, progress in understanding mouse embryology had started to stagnate. Molecular biology had provided the tools to clone genes, but identification of important genes in mouse development from the estimated 80,000100,000 genes in the genome seemed almost intractable. One breakthrough that was to change the course of developmental studies came in 1984 with the discovery of the homeobox in flies, and the realization that mice and other vertebrates had the same genes (1-3). It seems hard to believe now, when evolutionary conservation of genes and functions between in vertebrates and mammals is the norm, that in the 1980s many developmental biologists believed there would be no such homologies, and even if certain gene (protein) sequences, such as the homeobox, were conserved, it was believed they would have completely different functions in mammals. Evolution of developmental mechanisms has provided developmental biologists with their most powerful tool: conservation of gene function.

The ability to clone potentially important developmental genes by screening mouse libraries with Drosophila gene probes, together with the advent of in situ hybridization to study their spatial expression in embryos, provided the

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

impetus for the explosion in mouse developmental studies over the last 15 years. Armed with potentially interesting developmental genes, mouse embry-ologists were able to begin to utilize transgenesis to investigate their functions and regulation. The first transgenic mice were produced using pronuclear injection in 1980/1981 in the laboratories of Frank Ruddle at Yale (4) and Frank Costantini and Elizabeth Lacy in Oxford (5) and the methodology they described is now widely used. The limitations of studying developmental gene function using transgenic animals produced by pronuclear injection soon became apparent, since it allowed only gain-of-function gene manipulations, which were not always informative. Concurrent with the progress in identifying important regulatory genes in mouse development, groups working on pluripotentiality of mouse embryo cells produced the first embryonic stem cells (ES cells) (6,7). The isolation of ES cells and the subsequent development of gene targeting thus came at the perfect time. Mouse developmental biologists had the genes and their expression patterns, but could only surmise the functions. Gene knockout provided the missing tool in the bag, enabling gene function to be assayed directly in vivo. Although perhaps not fully appreciated at the time, these two strands were to come together in the most dramatic way to provide the basis for the understanding mouse development and to start to approach that of Droso-phila. ES cells also provided a way of identifying important developmental genes based on function rather than homology by gene trapping, and large-scale gene traps have been undertaken in several laboratories (Chapter 8).

The prospects of using large-scale mutagenesis to identify mouse developmental genes, as used in Drosophila and more recently zebrafish, was not considered viable for many years. Although mouse developmental mutants, generated by traditional mutagenesis methods, have over the years proved a valuable resource, the advent of targeted mutation techniques greatly reduced the need for traditionally generated mutants. However, more recently, the possibility of large-scale mutagen-esis screens for developmental genes has been revisited by several groups using N-ethyl-N-nitrosourea (ENU) to generate point mutations. Such screens, in conjunction with the mouse genome mapping and sequencing projects, could provide an important contribution to mouse developmental biology in the future.

Basic development of mouse embryos have been more than adequately described in several landmark texts, such as The Mouse by Roberts Rugh (8), The House Mouse by Karl Theiler (9), and The Atlas of Mouse Development by M. H. Kaufman (10), and therefore we did not consider it necessary for this to be duplicated in this text. The chapters in this section on the mouse as a developmental model provide the background and detail to some of the latest manipulation techniques. We recommend that readers consult Manipulating the Mouse Embryo by Hogan et al. (11), which provides considerable detail on the production of transgenic mice.

References

1. McGinnis, W., Levine, M. S., Hafen, E., Kuroiwa, A., and Gehring, W. J. (1984) A conserved DNA sequence in homeotic genes of the Drosophila Antennapedia and bithorax complexes. Nature 308, 428-433.

2. Scott, M. P. and Weiner, A. J. (1984) Structural relationships among genes that control development: sequence homology between Antennapedia, Ultrabithorax and fushi tarazu loci of Drosophila. Proc. Natl. Acad. Sci. USA 81, 4115-4119.

3. Carrasco, A. E., McGinnis, W., Gehring, W. J., and De Robertis, E. M. (1984) Cloning of a Xenopus leavis gene expressed during early embryogenesis that codes for a peptide region homologous to the Drosophila homeotic genes. Cell 37, 409-414.

4. Gordon, J. W., Scangos, G. A., Plotkin, D. J., Barbosa, J. A., and Ruddle, F. H. (1980) Genetic transformation of mouse embryos by microinjection of purified DNA. Proc. Natl. Acad. Sci. USA 77, 7380-7384.

5. Costantini, F. and Lacy, E. (1981) Introduction of a rabbit P-globin gene into the mouse germ line. Nature 294, 92-94.

6. Evans, M. J. and Kaufman, M. H. (1981) Establishment in culture of pluripotential cells from mouse embryos. Nature 292, 154-156.

7. Martin, G. (1981) Isolation of a pluripotent cell-line from early mouse embryos cultured in medium conditioned by teratocarcinoma stem cells. Proc. Natl. Acad. Sci. USA 78, 7634-7638.

8. Rugh, R. (1990) The Mouse: Its Reproduction and Development. Oxford Science Publications, Oxford, UK.

9. Theiler, K. (1989) The House Mouse: Atlas of Embryonic Development. SpringerVerlag, New York.

10. Kaufman, M. H. (1992) The Atlas of Mouse Development. Academic, London, UK.

11. Hogan, B., Beddington, R., Costantini, F., and Lacy, E. (1994) Manipulating the Mouse Embryo. A Laboratory Manual. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY.

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