Blastocyst

Definition

A blastocyst is an early stage embryo, approximately five to six days after fertilization, consisting of a sphere made up of an outer layer of cells (trophectoderm), a fluid-filled cavity (blastocoel), and a cluster of cells (inner cell mass) from which the embryo develops. It is the structure that implants into the uterine wall during the process of embryonic development.

Related terms

Trophectoderm: The outer layer of cells in the blastocyst that contributes to forming part of the placenta.

Inner Cell Mass: A group of cells within the blastocyst that will eventually develop into the fetus.

Implantation: The process by which a blastocyst attaches itself to the lining of the uterus, beginning its growth into an embryo