Blastulation is the formation of a blastula from a morula. The morula is an embryo filled evenly with cells (blastomeres), but the blastula contains a fluid cavity called blastocoel. In mammals, the blastula is called a blastocyst which consists of inner cell mass, trophoblast and blastocoel (Figure 4.2).
During blastulation, cells continue to divide and begin to differentiate. The outer cells of the morula are polarized. That is, one side of the cell differs from the other side. The outer, polar cells give rise to trophoblast and the inner, apolar cells become the inner cell mass. The watery fluid of the blastocoel is secreted by trophoblast cells and transported in from the exterior. For the human, the blastocyst is formed by days 5 to 6 after fertilization. At this time, the blastocyst has reached the uterus, but has not yet implanted into the uterine wall. On day 5, preimplantation human embryo contains 200 to 250 cells, only 30 to 34 of which are inner cell mass cells.
In subsequent development, the cells of the inner cell mass will give rise to all tissues of the embryo's body. The embryonic stem cells are derived from the inner cell mass.