25.4: Cleavage and Blastulation

After a large-single-celled zygote is produced via fertilization, the process of cleavage occurs while zygotes travel through the uterine tube. Cleavage is a mitotic cell division that does not result in growth. With each round of successive cell division, daughter cells get increasingly smaller.

Zygotic Genome Activation

At the beginning of embryogenesis, maternal mRNAs control development. However, by the eight-cell stage of cleavage, embryonic genes become activated in a process called zygotic genome activation (ZGA). As a result, maternal mRNAs get degraded, and ZGA causes a transition from maternal to zygotic genetic control of developing an embryo. Although maternal mRNAs get degraded, previously translated proteins may remain in the embryo through later stages of development.

Cleavage Pattern

Cleavage patterns vary between organisms depending on the presence and distribution of egg yolk amongst other factors. For example, mammals have a holoblastic rotational cleavage pattern. They are holoblastic because they have sparse, but evenly distributed yolk and therefore end up with a cleavage furrow that extends through the entire embryo as opposed to being meroblastic where the cleavage furrow does not extend through the yolk-dense portion of the cytoplasm.

At the onset of cleavage, rotational cleavage begins when the zygote first divides to form two smaller daughter cells called blastomeres. During this first cleavage event, division occurs in an austral fashion. The two daughter blastomeres then go through mitosis to each form two new blastomeres. During this second cleavage event, one daughter blastomere cleaves in an austral manner while the second cleaves equatorially. This pattern continues so that the resultant blastomeres end up being smaller than their respective parent cells.

Compaction

At the eight-blastomere stage, compaction starts to occur—blastomeres tightly push against each other and appear to be one cell where individual cells are indistinguishable from one another. To stabilize the tightly packed blastomeres, tight junctions are formed among the exterior blastomeres while the interior blastomeres form gap junctions that allow the movement of ions and small molecules in between cells. E-cadherin, a calcium-dependent adhesion molecule, helps to further adhere blastomeres to each other.

Morula Formation

Once there are approximately thirty-two blastomeres, the zygote becomes a morula. Morula formation marks the end of cleavage. The morula then becomes a blastula that goes through further differentiation during the subsequent stages of development.

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.