Blastocysten

A blastocyst is an early stage in mammalian embryonic development that forms about five days after fertilization in humans. It is a hollow structure containing a fluid-filled cavity called the blastocoel, an outer cell layer called the trophoblast, and a cluster of cells inside called the inner cell mass or embryoblast. The trophoblast contributes to the formation of the placenta and supporting membranes, while the embryoblast gives rise to the developing embryo. The blastocyst is surrounded by the zona pellucida until it hatches to enable implantation.

Development and formation: Following fertilization in the oviduct, the zygote undergoes successive cell divisions and becomes a morula by about day 3. By days 4 to 5, the morula develops a blastocoel, forming a blastocyst. The blastocyst travels to the uterus and typically implants into the endometrium around day 6 to 7. The inner cell mass differentiates to form the embryo proper, and the trophoblast differentiates to establish the placenta and other extraembryonic tissues. Later embryonic development proceeds through gastrulation, forming the three germ layers.

Clinical and research relevance: In assisted reproductive technology, transferring blastocysts can improve implantation rates and may influence success in embryo selection. Blastocysts may be cryopreserved for later transfer. Abnormal blastocyst development or implantation failure can contribute to infertility. The biology of blastocysts is studied across mammals, with species-specific differences in development and timing.

Size and structure: A human blastocyst is roughly 100 to 150 micrometers in diameter. It comprises three main components: the trophoblast, the embryoblast, and the blastocoel. The trophoblast forms the placenta and membranes, the embryoblast becomes the fetus, and the blastocoel is the fluid-filled cavity that supports growth.