Preimplantation mouse development. (A) Oogenesis and early development. Maternal RNA and proteins accumulate within individual oocytes during a ~2 week growth phase, but many are degraded during meiotic maturation and ovulation. The zona pellucida surrounds growing oocytes and ovulated eggs (green) and is modified following fertilization (red) to prevent polyspermy and to protect the embryo as it passes through the oviduct. At embryonic day 0.5 (E0.5), 1-cell embryos can be recovered from the oviduct and, by the 2-cell stage (E1.5), there is robust embryonic genome activation. Following the third cell division, embryos undergo a Ca2+-mediated compaction to form morulae (E2.5), and develop a fluid-filled blastocoel cavity from the 32-cell stage to become blastocysts (E3.5) that implant in the uterus wall (E4.5). (B) Late preimplantation development. At four cells, blastomeres with distinct cell boundaries are encased within the protective confines of the zona pellucida. Individual cells remain pluripotent. Following the 8-cell stage, the embryo compacts to form the morula, with increased cell contact that morphologically obscures cell boundaries. Symmetric cell divisions result in similar daughter cells. Asymmetric divisions provide topologically distinct outer cells, which are trophectoderm precursors (forming the placenta), and inner cells, which establish the inner cell mass (ICM) of the fetus. Blastocoel formation defines the blastocyst, and the eccentric localization of the ICM localizes the polar and mural trophectoderm, the latter of which initiates uterine implantation.