blastocysta

Blastocysta, known in English as blastocyst, is an early stage in mammalian embryonic development that follows the morula. It is a roughly spherical structure around 100-120 micrometers in diameter in humans. It contains a fluid-filled cavity called the blastocoel, an outer layer of cells called the trophoblast, and a cluster of cells inside known as the inner cell mass (embryoblast). The trophoblast will contribute to the placenta, while the inner cell mass will form the future embryo.

During its formation the blastocyst uses active transport and osmosis to create the blastocoel. The zona pellucida, a surrounding glycoprotein shell, is present during early blastocyst stages and is typically shed (hatches) to allow implantation.

Formation timeline: after fertilization (day 0), the embryo usually becomes a morula by day 3 or 4, then a blastocyst by day 5-6. In humans, implantation into the uterine lining typically occurs around day 6-9, mediated by contact between trophoblast and endometrium. Trophoblast differentiates into an inner cytotrophoblast and a multinucleated syncytiotrophoblast that invade the endometrium and contribute to placenta development.

Clinical relevance: In assisted reproduction, transfer of blastocysts (day 5-6) is common and associated with higher implantation and pregnancy rates than transfer of earlier-stage embryos, though not all embryos reach the blastocyst stage in culture. Blastocyst quality is used to assess viability and guide selection, balancing implantation potential with the risk of cycle cancellation.

Across mammals, blastocyst formation is a conserved feature of early development, though timings and details vary among species.