- A stage in the early embryonic development of mammals in which there is a hollow sphere with an outer layer of cells and inside the hollow sphere, there is a cluster of cells called the inner cell mass. If development continues, the outer layer of cells gives rise to the placenta and other supporting tissues needed for fetal development within the uterus while the inner cell mass cells gives rise to the tissues of the body.
* * *The modified blastula stage of mammalian embryos, consisting of the inner cell mass and a thin trophoblast layer enclosing the blastocele. SYN: blastodermic vesicle. [blasto- + G. kystis, bladder]
* * *blas·to·cyst 'blas-tə-.sist n the modified blastula of a placental mammal
* * *n.an early stage of embryonic development that consists of a hollow ball of cells with a localized thickening (the inner cell mass) that will develop into the actual embryo; the remainder of the blastocyst is composed of trophoblast. At first the blastocyst is unattached, but it soon implants in the wall of the uterus. See also implantation.
* * *blas·to·cyst (blasґto-sist) [blasto- + Gr. kystis bladder] the mammalian conceptus in the postmorula stage; it is like a blastula in having a fluid-filled cavity, but unlike it in having the surface layer not exclusively embryoblast but mainly or entirely trophoblast, in having an eccentric embryoblast, and in not being limited to one germ layer. The human blastocyst consists of an embryoblast (inner cell mass) and a thin trophoblast layer enclosing the blastocyst cavity.
Medical dictionary. 2011.