Blastulation

Blastula
Blastulation: 1 - morula, 2 - blastula.
Details
Days	4
Precursor	Morula
Gives rise to	Gastrula
Dorlands /Elsevier	b_14/12188083
Anatomical terminology []

The blastula (from Greek (blastos), meaning "") is a hollow sphere of cells, referred to as blastomeres, surrounding an inner fluid-filled cavity called the formed during an early stage of embryonic development in animals.Embryo development begins with a sperm fertilizing an egg to become a zygote which undergoes many cleavages to develop into a ball of cells called a morula. Only when the blastocoele 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 blastocoele. In mammals the blastula is referred to as a . The blastocyst contains an embryoblast (or inner cell mass) that will eventually give rise to the definitive structures of the fetus, and the trophoblast, which goes on to form the extra-embryonic tissues.

During the blastula stage of development, a significant amount of activity occurs within the early embryo to establish cell polarity, cell specification, axis formation, and regulate gene expression. 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. Many of the interactions between blastomeres are dependent on cadherin expression, particularly E-cadherin in mammals and EP-cadherin in amphibians.