Blastocoel

The blastocoel is the fluid-filled cavity inside a blastocyst, which forms around day 5 or 6 of embryo development. It fills with fluid to help the blastocyst expand, a process that prepares the embryo to hatch from its outer shell (zona)

outside of the blastocyst showing the trophectoderm cells and inside showing the blastocoel and inner cell mass (ICM)
Modified from CNX OpenStax, CC BY 4.0

The blastocyst is like a tiny water balloon:

  • The trophectoderm cells form the “skin” of the balloon.
  • The blastocoel is the water-filled space inside.
  • The inner cell mass (ICM), which will develop into the fetus, is attached to the wall of this cavity on one side.