Blastotsüst

Blastotsüst, commonly known in English as the blastocyst, is a stage of early mammalian embryonic development. It typically forms about five to six days after fertilization in humans and other primates, and serves as a key transitional structure before implantation. The blastocyst is a hollow sphere composed of three parts: an outer layer of trophoblast cells, a fluid-filled cavity called the blastocoel, and an inner cell mass that will give rise to the embryo proper. The trophoblast ultimately contributes to the placenta and supporting tissues, while the inner cell mass differentiates into the embryo’s tissues.

Formation begins as the earlier morula develops a fluid-filled cavity, resulting in the blastocoel. The outer trophoblast cells establish contact with the uterine lining, and the inner cell mass becomes positioned at one pole of the structure. Around day six to seven, the blastocyst begins the process of implantation into the endometrium, a critical step in establishing pregnancy. Successful implantation depends on proper trophoblast invasion, signaling between the embryo and maternal tissues, and a receptive uterine environment.

Clinical relevance is notable in assisted reproduction. Culturing embryos to the blastocyst stage allows for more informed selection and can improve implantation rates in IVF. Blastocysts are also used in preimplantation genetic testing and can be cryopreserved for later transfer. Variations exist among species in timing and exact morphology, but the blastocyst generally represents a conserved developmental milestone linking early cleavage to embryo–maternal interaction and subsequent development.

Etymology derives from Greek roots meaning “bud” and “sac,” reflecting its vesicular nature. In some languages, the term is rendered as blastotsüst.