Blastozysten

Blastozysten, or blastocysts, are a stage in mammalian embryonic development that follows the morula. In humans and other mammals, a blastocyst forms about five days after fertilization and contains about 200-300 cells. It features a fluid-filled cavity called the blastocoel, an outer cell layer of trophoblast cells that will contribute to the placenta, and an inner cell mass that gives rise to the developing embryo. The external shell, the zona pellucida, surrounds the early embryo and typically ruptures (hatches) to allow implantation into the uterine lining.

During the transition from morula to blastocyst, cells differentiate into trophoblast and inner cell mass; this differentiation is a key step in lineage commitment. The blastocyst hatches from the zona pellucida and then implants into the endometrium, where the trophoblast interacts with maternal tissue to establish nutrient exchange and immune modulation. The placenta develops from trophoblast derivatives and supports fetal growth.

In clinical and research contexts, blastocyst formation is a critical milestone in assisted reproductive technology (ART). Day-5 to day-6 blastocyst transfer can improve implantation and pregnancy rates compared with earlier-stage embryos, but selection criteria vary. Blastocysts are commonly graded for expansion, quality of the inner cell mass and trophectoderm. Biopsy of blastocysts for preimplantation genetic testing may involve cells from the trophectoderm. Cryopreservation of blastocysts by vitrification is routine, enabling single cycles to maximize success while reducing multiple pregnancies.