Cleavage and Blastulation

Cleavage is rapid mitotic cell division that occurs without growth as the fertilized zygote travels through the fallopian tube. The zygote divides within the zona pellucida to form progressively smaller daughter cells called blastomeres. Each successive division produces blastomeres that are smaller than their parent cells, establishing the foundation for subsequent developmental stages.

At the eight-blastomere stage, compaction occurs when blastomeres tightly push against each other, becoming nearly indistinguishable. Tight junctions form among exterior blastomeres while interior cells form gap junctions allowing ion and molecule movement. E-cadherin, a calcium-dependent adhesion molecule, further stabilizes the tightly packed structure, preparing the embryo for morula formation.

A morula forms at approximately 32 blastomeres and marks the end of cleavage with outer and inner cell layers plus a fluid-filled cavity. The blastocyst develops further with specialized structures: the trophoblast, which becomes the placenta, and the inner cell mass containing embryonic stem cells. The zona pellucida then dissolves, allowing blastocyst implantation into the uterus wall.

Zygotic genome activation (ZGA) occurs by the eight-cell stage of cleavage, when embryonic genes become activated. This transition shifts genetic control from maternal mRNAs to the embryo's own genes. Although maternal mRNAs are degraded, previously translated proteins may remain through later developmental stages, supporting continued growth and differentiation.

Mammals have holoblastic rotational cleavage because they possess sparse, evenly distributed yolk throughout the egg. This uniform yolk distribution allows the cleavage furrow to extend completely through the entire embryo, unlike meroblastic cleavage where the furrow cannot penetrate yolk-dense regions. This complete division pattern is essential for proper mammalian embryonic development.

During compaction, tight junctions form among exterior blastomeres to stabilize the tightly packed structure and maintain cell-cell adhesion. Interior blastomeres form gap junctions that permit movement of ions and small molecules between cells, enabling intercellular communication. Together with E-cadherin adhesion molecules, these junctions create a cohesive, organized embryonic structure.

During blastulation, the trophoblast develops and will eventually become the placenta, while the inner cell mass contains embryonic stem cells that form the embryo itself. These specialized cell layers create distinct regions with different developmental fates. The zona pellucida dissolves at this stage, enabling the blastocyst to implant into the uterus wall and progress toward gastrulation.