Fig. 1.

Stages of human pre-implantation embryo development. Phase-contrast images of human embryo development from day (d) 0 to day 7. Following fertilization, embryos undergo a series of mitotic cell divisions. Arrowheads in d0 and d1 indicate pronuclei. On or around day 4, the embryo compacts, resulting in the formation of a morula that consists of cells (or blastomeres) in a compact cluster contained within the zona pellucida (the glycoprotein layer that surrounds the embryo). The blastocyst, which forms on day 5, is a fluid-filled structure composed of an inner cell mass (white arrowhead) and trophectoderm (gray arrowhead). On day 6, the blastocyst ‘hatches’ from the zona pellucida and it is ready to implant into the uterine wall on day 7.