Mouse exo utero development system: protocol and troubleshooting.

The exo utero development system allows us to manipulate or operate on live embryos of mice or rats at mid- to late gestation stages, from late organogenetic to histogenetic periods, and keep them alive in situ until the analysis of their effects at a desired time point. We can examine the effects of injecting bioactive molecules or cells into targeted parts of a live embryo, destroying specific embryonic regions, or performing fetal surgery. This system is far simpler and more time- and cost-effective for in vivo functional analyses than establishing genetically modified mouse lines and provides a fine-tuned experimental design for developmental scientists. To promote use of the mouse exo utero development system, we elaborate on the technical procedures, discuss critical points for troubleshooting the system, and illustrate some apparatuses essential for fetal microinjection.