Human Blastocyst Formation and Development

The preimplantation period of human embryo development is remarkable and characterized by successive changes in terms of genetic control, physiology, and morphology of the embryo. Human preimplantation embryo development is characterized by the initial phase of embryo development, the phase before the embryo implantation process. In normal conditions, after fertilization, the embryo grows until the blastocyst stage. The blastocyst grows as the cells divide and the cavity expands, where it “hatches” from the zona pellucida to implant into the endometrium. Reprogramming and programming are continuous processes in the embryo that encompasses fusion of the egg and sperm pronuclei; epigenetic reprogramming and modification, an extensive wave of degradation of maternal transcripts, and activation of the nascent human embryonic genome and aneuploidy can occur in this stage. The embryo produces cytokines, growth factors, and receptors for endometrial signals in the apposition stage.