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The fourth cleavage event in the early developing embryo yields the late morula, and results in a nonpolarized inner cell mass (ICM, pluriblast, embryoblast), enveloped by the outer, polarized trophoblast layer of cells, which cavitates to form an inner cavity (blastocoele), whose formation indicates the bastocyst stage. While the trophoblast will ultimately form the outer chorionic sac and the fetal component of the placenta, the pluriblast forms the inner cell mass which will give rise to all embryonic tissues and to some of the extraembryonic membranes.
The blastocyst stage includes dramatic events such as, hatching of the blastocyst from the zona pellucida, and implantation of the embryo in the uterine wall. Implantation is a complex stage that occurs via the formation of the enveloping syncytiotrophoblast layer and cytotrophoblast layer, both of which originate in the trophoblast.
During human development, at Carnegie stage 3, the trophoblastic cells are distinguishable from the embryonic cells proper due to their peripheral position. The trophoblastic cells that cover the inner cell mass are referred to as polar and represent the future site of embryo implantation. The remaining trophoblast is termed mural. The trophoblast forms the fetal layers of the placenta (the villous and extravillous cytotrophoblast and multinucleated syncytiotrophoblast layers).