Blastocyst

A blastocyst is a stage of the mammalian embryo that follows the morula and precedes implantation into the uterus. In humans, it forms about five to six days after fertilization. The blastocyst is a hollow sphere with a fluid-filled cavity called the blastocoel.

The blastocyst has three main components. The outer cell layer, the trophoblast, will contribute to the placenta and other extraembryonic membranes. The inner cell mass, sometimes called the embryoblast, will develop into the embryo proper. The surrounding blastocoel distinguishes the structure from earlier cell clusters.

Before implantation, the blastocyst is enclosed by the zona pellucida, a protective glycoprotein coat. To implant, the blastocyst undergoes hatching, breaking free of the zona pellucida and exposing the trophoblast to the uterine lining. Implantation begins as the trophoblast adheres to the endometrium, with invasion and interaction that support placental development and sustained pregnancy.

In development, the blastocyst marks the transition from pre-implantation to post-implantation stages. The trophoblast differentiates into cytotrophoblast and syncytiotrophoblast, contributing to placental formation, while the embryoblast differentiates into tissues of the future embryo. The blastocyst stage is a key reference point in reproductive biology and is used in assisted reproductive technologies to assess embryo viability and timing of transfer.