Blastulation

Blastulation is the embryonic process in which cleavage-stage cells organize into a blastula, often forming a fluid-filled cavity called the blastocoel.

Cleavage (embryology): Rapid embryonic cell divisions that partition the zygote without increasing the embryo's overall size. These divisions supply the cells that reorganize during blastulation.
Zygote: A diploid cell formed by the fusion of an egg and a sperm. Blastulation follows the zygote's initial cleavage divisions.
Blastocyst: The mammalian developmental stage with an outer trophoblast, an inner cell mass, and a fluid-filled cavity. It is the specialized mammalian form of the blastula produced by blastulation.
Sea urchin development: The embryonic development of sea urchins, including radial cleavage and formation of a hollow blastula. Sea urchins provide a clear example of blastocoel formation during blastulation.
Blastocoel: The fluid-filled cavity inside many blastulae. Its formation is a defining structural outcome of blastulation in many animals.
Morula: A compact ball of cells formed during early embryonic cleavage. In many animals, the morula stage precedes formation of a blastula.
Gastrulation: The developmental process that reorganizes an embryo into germ layers and establishes body axes. It follows blastulation and transforms the blastula into a more complex body plan.
Xenopus development: The embryonic development of the frog Xenopus, whose blastula forms after rapid cleavage of a yolky egg. Its blastula has a displaced cavity and illustrates how yolk affects blastulation.
Blastomere: A cell produced by cleavage divisions of an early embryo. Blastomeres change position, adhesion, and organization as the blastula forms.
Cell polarity: The unequal distribution of molecules and structures that gives a cell distinct spatial axes. Polarity helps blastomeres establish the organized surfaces needed in some blastulae.

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