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TIME PERIOD:1.5 - 3 days post-ovulation
The zygote now begins to cleave, with each division occurring into two cells called blastomeres. The zygote's first cell division begins a series of divisions, with each division occurring approximately every twenty hours. Each blastomere within the zona pellucida becomes smaller and smaller with each subsequent division.
When cell division generates about sixteen cells, the zygote becomes a morula (mulberry shaped). It leaves the fallopian tube and enters the uterine cavity three to four days after fertilization.
Research in August 2014, revealed that a handful of genes are clearly signaling to each other at the two-cell and four-cell stage. This happens within days after an egg has been fertilized by sperm and before the embryo has implanted into the uterus. Among the identified genes are several belonging to the WNT signaling pathway, well-known for cell-to-cell communication.
The research used single-cell RNA sequencing to look at every gene in the mouse genome. Published in 2014 in the journal Genome Research and reviewed in The Visible Embryo.
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