Beyond the mouse: non-rodent animal models for study of early mammalian development and biomedical research.

The preimplantation development of mammals generally follows the same plan. It starts with the formation of a totipotent zygote, and through consecutive cleavage divisions and differentiation events leads to blastocyst formation. However, the intervening events may differ between species. The regulation of these processes has been extensively studied in the mouse, which displays some unique features among eutherian mammals. Farm animals such as pigs, cattle, sheep and rabbits share several similarities with one another, and with the human developmental plan. These include the timing of epigenetic reprogramming, the moment of embryonic genome activation and the developmental time-frame. Recently, efficient techniques for genetic modification have been established for large domestic animals. Genome sequences and gene manipulation tools are now available for cattle, pigs, sheep and goats, and a larger number of genetically engineered livestock is now accessible for biomedical research. Yet, these animals still make up less than 0.5% of animals in research, mainly due to our inadequate knowledge of the processes responsible for pluripotency maintenance (to date no stable naïve embryonic stem cell lines have been established) and early development. In this review, we highlight our present knowledge of the key preimplantation events in the 3 non-rodent species which present the highest potential for biomedical research related to early embryonic development: cattle, which offer an excellent model to study human in vitro embryo development, pigs which emerge as models to study the long-term effects of gene-based therapies and rabbits, which in many aspects of embryology resemble the human.