Cleavage and Blastulation

In human embryonic development as the fertilized zygote travels down the fallopian tube into the uterus the process of cleavage, rapid mitotic cell division that does not result in growth begins and is followed by blastulation the first stage of cellular differentiation.

At the onset of cleavage the zygote first divides within a protective membrane called the zona pellucida to form two smaller daughter cells called blastomeres. Which then go through another round of mitosis resulting in four new blastomeres that are smaller than their parent cells.

Beginning with the eight blastomere stage, compaction starts to occur. Blastomeres tightly push against each other making tight making tight junctions and becoming almost indistinguishable from one another. At the 16 blastomere stage the embryo becomes a morula and the cells continue to divide and form an outer and an inner cell layer along with an inner fluid filled cavity.

This stage of development the blastocyst stage in humans is marked by the development of the trophoblast which will will become the placenta and the inner cell mass which are the embryonic stem cells that will continue developing into the embryo. Finally, the zona pellucida will dissolve to allow the blastocyst to implant into the uterus wall and begin the gastrulation stage.