Figure 1: The blastocyst is a hollow sphere made of approximately 150 cells and
contains three distinct areas: the trophoblast, which is the surrounding outer layer that
contains the trophoblast stem cells and later becomes the
placenta, the blastocoel, which is a fluid - filled cavity within the blastocyst, and the inner cell mass, also known as the embryoblast, which can become the embryo proper, or fetus, and is where
human embryonic stem cells are isolated from.