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The zygote is the first stage of the early embryo, following the fusion of the haploid male and female pronucleus, wherein the chromosome number in the fertilized ovum becomes diploid (2n). Fertilization, which normally takes place in the ampulla of the uterine tube, requires contact of spermatozoa with the zona pellucida of a secondary oocyte, penetration of one or more spermatozoa through the zona pellucida and the ooplasm, swelling of the spermatozoal head, extrusion of the second polar body, formation of the male and female pronuclei, completion of the second meiotic division and commencement of the first mitotic division, or cleavage, of the zygote.
While the zygote is transported through the Fallopian tubes to the uterus, its cells undergo subsequent mitotic divisions called cleavages, forming the 2, 4 and 8-cell embryo. From the 8-cell stage (also known as the early morula), the embryonic cells undergo compaction, forming the morula. As they continue to divide, the cells segregate to the inner and outer cells, forming a cavity (the blastocoele cavity), thereby defining the blastocyst-stage embryo. The inner group of cells will become the embryo, while the outer group of cells will become the extraembryonic membranes.
Human embryonic development begins with relative transcriptional silence, with an oocyte-to-embryo transition that lasts for approximately 3 days and encompasses fusion of the egg and sperm, migration and fusion of the germ cell pronuclei, genetic and epigenetic reprogramming and a series of cleavage divisions that culminate with a major wave of embryonic genome activation between the 4- and 8-cell stages.