A cellular mass formed during the development of mammalian embryos. The mass is divided into an inner and outer cell mass, and consists of about 100 cells. The overall appearance of the blastocyst is that of a thin-walled, hollow structure containing a cluster of cells.
This stage of development occurs prior to implantation. The inner cell mass, or embryoblast, eventually develops into the fetus, while the outer cell mass, or trophoblast, eventually becomes the placenta.
Embryonic stem cell research is performed using embryos that are typically only 4-5 days old (i.e. in embryoblast form). The embryos are products of in vitro fertilization that were not needed and have been donated, with consent of the parents, to research.
This use of embryos for stem cell research has become the topic of much ethical debate.
