Blastulation is the stage in early animal embryonic development that produces the blastula. 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.[1][2] 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.[3]

Blastulation: from 1. morula to 2. blastula
Gives rise toGastrula
Anatomical terminology
A. Morula and B. cross section of a blastula displaying the blastocoel and blastoderm of early animal embryonic development

A common feature of a vertebrate blastula is that it consists of a layer of blastomeres, known as the blastoderm, which surrounds the blastocoel.[4][5] In mammals, the blastula is referred to as a blastocyst. 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.[3][6]

During blastulation, a significant amount of activity occurs within the early embryo to establish cell polarity, cell specification, axis formation, and to regulate gene expression.[7] In many animals, such as Drosophila and Xenopus, the mid blastula transition (MBT) is a crucial step in development during which the maternal mRNA is degraded and control over development is passed to the embryo.[8] Many of the interactions between blastomeres are dependent on cadherin expression, particularly E-cadherin in mammals and EP-cadherin in amphibians.[7]

The study of the blastula, and of cell specification has many implications in stem cell research, and assisted reproductive technology.[6] In Xenopus, blastomeres behave as pluripotent stem cells which can migrate down several pathways, depending on cell signaling.[9] By manipulating the cell signals during the blastula stage of development, various tissues can be formed. This potential can be instrumental in regenerative medicine for disease and injury cases. In vitro fertilisation involves implantation of a blastocyst into a mother's uterus.[10] Blastula cell implantation could serve to eliminate infertility.[citation needed]

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