Blastozysts

Blastozyst (plural blastozysts) is a stage of mammalian embryonic development that follows the morula. It is a hollow ball of cells containing a fluid-filled cavity called the blastocoel. The blastocyst comprises two main cell populations: the outer trophoblast, which will contribute to the placenta, and the inner cell mass, which gives rise to the embryo proper. The blastocoel forms as fluid accumulates inside, and the surrounding zona pellucida is typically shed as the embryo prepares for implantation.

In humans, blastocyst formation occurs about five to six days after fertilization. The blastocyst then hatches from the zona pellucida and attaches to the maternal endometrium, initiating implantation. The trophoblast differentiates into the cytotrophoblast and the syncytiotrophoblast, which participate in placental formation, while the inner cell mass reorganizes into a bilaminar embryonic disc consisting of the epiblast and hypoblast.

In clinical and research contexts, the blastocyst stage is important for assisted reproduction and embryology. In vitro fertilization often involves transferring blastocysts to improve implantation rates, and blastocyst biopsy can be used for preimplantation genetic testing. Blastocysts are also a source of embryonic stem cells for research, though their use is governed by ethical and legal frameworks and varies by jurisdiction. Surplus blastocysts may be cryopreserved for later use.

Notes on terminology: while the exact terminology varies by language and literature, blastozyst is commonly used as an alternative spelling for blastocyst and refers to the same developmental stage.