A blastocyst is an embryo that has reached the 5th day of development. At this time, its structure is made up of a multitude of cells that have already begun to form a specialization and, therefore, it is possible to distinguish several parts of a blastocyst:
- The trofoectoderm
- are the cells that come in contact with the endometrium and, subsequently, will originate the placenta.
- The internal cell mass (ICM)
- is inside the trofoectoderm and is the structure that will give rise to the fetus.
- The blastocele
- is the central cavity of the embryo that is filled with fluid.
- The zona pellucida
- is a protective layer that surrounds the blastocyst until hatching occurs and the blastocyst detaches from it in order to implant.