Trophectoderm in the context of Blastocysts

Blastulation is the stage in early animal embryonic development that produces the blastula. In mammalian development, the blastula develops into the blastocyst with a differentiated inner cell mass and an outer trophectoderm. The blastula (from Greek βλαστός (blastos meaning sprout)) is a hollow sphere of cells known as blastomeres surrounding an inner fluid-filled cavity called the blastocoel. Embryonic development begins with a sperm fertilizing an egg cell to become a zygote, which undergoes many cleavages to develop into a ball of cells called a morula. Only when the blastocoel is formed does the early embryo become a blastula. The blastula precedes the formation of the gastrula in which the germ layers of the embryo form.

A common feature of a vertebrate blastula is that it consists of a layer of blastomeres, known as the blastoderm, which surrounds the blastocoel. In mammals, the blastocyst contains an embryoblast (or inner cell mass) that will eventually give rise to the definitive structures of the fetus, and a trophoblast which goes on to form the extra-embryonic tissues.

The blastocoel (/ˈblæstəˌsiːl/), also spelled blastocoele and blastocele, and also called cleavage cavity, or segmentation cavity is a fluid-filled or yolk-filled cavity that forms in the blastula during very early embryonic development. At this stage in mammals the blastula is called the blastocyst, which consists of an outer epithelium, the trophectoderm, enveloping the inner cell mass and the blastocoel.

It develops following cleavage of the zygote after fertilization. It is the first fluid-filled cavity or lumen formed as the embryo enlarges, and is the essential precursor for the differentiated gastrula. In the Xenopus a very small cavity has been described in the two-cell stage of development.

The blastocyst is a structure formed in the early embryonic development of mammals. It possesses an inner cell mass (ICM) also known as the embryoblast which subsequently forms the embryo, and an outer layer of trophoblast cells called the trophectoderm. This layer surrounds the inner cell mass and a fluid-filled cavity or lumen known as the blastocoel. In the late blastocyst, the trophectoderm is known as the trophoblast. The trophoblast gives rise to the chorion and amnion, the two fetal membranes that surround the embryo. The placenta derives from the embryonic chorion (the portion of the chorion that develops villi). The corresponding structure in non-mammalian animals is an undifferentiated ball of cells called the blastula.

In humans, blastocyst formation begins about five days after fertilization when a fluid-filled cavity opens up in the morula, the early embryonic stage of a ball of 16 cells.The blastocyst has a diameter of about 0.1–0.2 mm and comprises 100-200 cells following 7-8 rounds of cleavage (cell division without cell growth). About seven days after fertilization, the blastocyst undergoes implantation, embedding into the endometrium of the uterine wall where it will undergo further developmental processes, including gastrulation. Embedding of the blastocyst into the endometrium requires that it hatches from the zona pellucida, the egg coat that prevents adherence to the fallopian tube as the pre-embryo makes its way to the uterus.

The inner cell mass (ICM) or embryoblast (known as the pluriblast in marsupials) is a structure in the early development of an embryo. It is the mass of cells inside the blastocyst that will eventually give rise to the definitive structures of the fetus. The inner cell mass forms in the earliest stages of embryonic development, before implantation into the endometrium of the uterus. The ICM is entirely surrounded by the single layer of trophoblast cells of the trophectoderm.