The development of the human embryo can be characterized by the stages of preimplantation, gastrulation, and organogenesis that lead to the formation of a fetus (day 56). Preimplantation development culminates in the formation of a mature blastocyst consisting of a group of cells that form an internal tissue called an epiblast (EPI, in red), whose potential is largely limited to the formation of the fetus, and extraembryonic cells forming the trophectoderm (TE, in blue) and the primitive endoderm (PrE, in green) that, together, envelop and support the development of the epiblast and mediate implantation in the uterus. After implantation, supported by the extraembryonic and uterine environments, the epiblast continues to develop and forms the outline and the major axis of the body plan (gastrulation process). Once this outline is laid, the embryo forms the organs until the fetus is formed (56 d.p.f.), thereby closing the period of greatest transformation. The image of the zygote and the 2-cell stages are from the European Society for Human Reproduction and Endocrinology. The image of the blastocyst is from the laboratory of Laurent David. The postimplantation stage images are embryos and a fetus from the Kyoto Human Embryo Visualization Project.
