Early development and implantation of the embryo.
A. The zygote stage begins upon fertilization of the secondary oocyte by the sperm. The zygote contains both pro-nuclei and is contained within the zona pellucida, until the blastocyst stage.
B. The morula stage. Following compaction and symmetrical cleavage divisions of the blastomeres (the cells of the early embryo), the embryo contains 8 (early morula) to 32 cells (morula). The inner cells will give rise to the inner cell mass, whereas the outer cells will give rise to the trophoblast, which forms a cavity called the blastocoele cavity.
C. The blastocyst stage. The developing embryo is defined as a blastocyst from the appearance of the blastocoele cavity, and now contains two cell populations- the surrounding outer trophoblast cells, and the inner cell mass cells, located at one side of the inner cavity. The portion of the trophoblast nearest to the inner cell mass is called the polar trophoblast (embryonic pole) and the portion of the trophoblast farthest and opposing the inner cell mass is called the mural trophoblast (abembryonic pole). In the blastocyst stage, the embryo hatches from the zona pellucida layer and is now able to begin implantation into the maternal endometrium.
D. The hatched human embryo makes contact with and implants into the maternal endometrium at the polar trophectoderm. Meanwhile, the inner cell mass differentiates to two layers collectively called the bilaminar embryonic disc. The layer closest to the trophoblast is called the epiblast, and will give rise to the embryo proper, and also contribute to several extraembryonic tissues. The layer bordering the blastocoele cavity is the hypoblast (primitive endoderm), which is a transient structure that contributes to the development of the extraembryonic mesoderm and yolk sac and plays a key role in signaling to establish axial patterning in the embryo itself.
E. The embryo implants into the maternal endometrium as the trophoblast differentiates to the cytotrophoblast and syncytiotrophoblast layers. At the stage shown here, the embryo is fully implanted and lacunae filled with maternal blood form in the syncytiotrophoblast, enabling exchange of nutrients. The three embryonic cavities (the amniotic cavity, the yolk sac cavity and the blatocoele cavity) have already formed, and the bilaminar embryo is situated between the amnion and the yolk sac.