Blastocyst development

Abstract

(Greek, blastos = sprout + cystos = cavity) or blastula, the term used to describe the hollow cellular mass that forms in early development. The blastocyst consists of cells forming an outer trophoblast layer, an inner cell mass and a fluid-filled cavity. The blastocyst inner cell mass is the source of true embryonic stem cells capable of forming all cell types within the embryo. In humans, this stage occurs in the first and second weeks after the zygote forms a solid cellular mass morula stage) and before implantation.