blastocyste

A blastocyst is a developmental stage in mammals that forms about five to six days after fertilization in humans. It is a hollow sphere containing a fluid-filled cavity called the blastocoel, an outer cell layer known as the trophectoderm, and an inner cell mass called the embryoblast. The blastocyst arises from the morula as cells begin to differentiate and the zona pellucida surrounding the embryo loosens and is shed during hatching.

Differentiation within the blastocyst sets up two primary lineages: the trophectoderm, which contributes to the placenta and fetal membranes, and the inner cell mass, which will form the embryo proper. Inside the inner cell mass the epiblast and hypoblast lineages give rise to extraembryonic tissues and, later, the three germ layers during gastrulation.

Timing and implantation: In humans, the blastocyst typically implants into the uterine lining around day 6–7 after fertilization. Successful implantation involves attachment to the endometrium, followed by invasion and formation of the placenta.

Clinical relevance: In assisted reproduction, blastocyst-stage transfer is common, and blastocysts may be biopsied for preimplantation genetic testing. They can also be cryopreserved by vitrification for later use. The blastocyst represents a key milestone in early development, balancing embryonic growth with the establishment of supporting extraembryonic tissues.