Zygote Formation

• Shortly after fertilization, which occurs in the oviduct, the male and female pronuclei fuse to form the zygote which is a single cell that contains a complete set of 46 chromosomes. (10)

Cleavage 

• The zygote goes through multiple rounds of mitotic divisions to form smaller cells called blastomeres. The divisions result in 2, 4, 8, 16, and 32- cell stages. The total amount of cytoplasmic volume stays the same during this time but the blastomeres get increasingly smaller. (10)

Morula Formation

• Around the 16 cell-stage, the blastomeres begin to express adhesion factors and therefore adhere to one another. At this point, the zygote is referred to as a morula due to its lobulated appearance that resembles a mulberry. (10)

Compaction 

• At the same time, the cells begin to undergo compaction where they flatten out against each other and begin to form adheren junctions. At this point the cells also begin to differentiate into two distinct groups. (10)

Differentiation

• The outer cells of the morula differentiate into trophoblasts which make up the trophectoderm. The trophoblasts are connected by tight junctions that control the passage of molecules into the cell. The trophectoderm will eventually give rise to the placenta. The inner cells differentiate to form the inner cell mass which will give rise to the embryo. These cells are connected primarily by gap junctions which allow for easy communication between cells. (10)

Blastocoel Formation 

• Active ion pumps on the outer cells begin to move ions (Na and K) against their concentration gradient towards the center of the morula which causes water to also enter the cell due to osmosis. The water accumulates within the cell to form a fluid filled cavity known as the blastocoel. This fluid filled cavity pushes the trophectoderm to the perimeter of the cell and the inner cell mass to one end of the cell. (10)

Breakdown of the Zona Pellucida 

• As more fluid enters the cell due to the pumping of ions towards the center of the cell, the internal pressure of the cell increases, putting increased tension on the zona pellucida. At the same time, the trophoblasts produce proteolytic enzymes which begin to breakdown and weaken the zona pellucida. The blastocyst will begin contracting and relaxing to create pressure pulses and all of these factors contribute to the zona pellucida eventually cracking and allowing the blastocyst to exit. (10)

Free Floating Blastocyst  

• When the blastocyst escapes from the zona pellucida, it becomes free floating and begins to travel towards the uterine lumen. The free-floating blastocyst consists of the trophoblasts making up the trophectoderm which acts as the cell wall, the inner cell mass which will give rise to the embryo, and the fluid filled blastocoel. Once the blastocyst makes it to the uterine lumen it will begin developing the fetal membranes and will eventually implant itself into the endometrium of the ewe. (10)