A blastocele, also spelled blastocoele or 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. The blastula is a stage of development that occurs after fertilization, when the zygote undergoes multiple cell divisions to form a ball of cells. The blastocele is the first cell cavity formed as the embryo enlarges, and is the essential precursor for the differentiated gastrula, which will give rise to the three germ layers of the embryo.

The formation of the blastocele depends on the type of animal and the amount of yolk in the egg. In mammals, such as humans, the egg has very little yolk and is called an isolecithal egg. After fertilization, the zygote undergoes rotational cleavage, resulting in daughter cells called blastomeres. When the blastomeres reach the 16-cell stage, they form a solid ball of cells called a morula. The morula has a small group of internal cells surrounded by a larger group of external cells. The internal cells will become the inner cell mass, which will give rise to the embryo proper, while the external cells will become the trophoblasts, which will form the placenta and other extraembryonic membranes.

In a process called cavitation, the trophoblast cells secrete fluid into the morula to create a blastocele. The accumulation of fluid pulls in water osmotically, creating and enlarging the blastocele. The trophoblast cells have sodium pumps and exchangers that pump sodium into the centrally forming cavity. The oviduct cells stimulate these trophoblast sodium pumps as the fertilized egg travels down the fallopian tube towards the uterus. As the embryo further divides, the blastocele expands and the inner cell mass is positioned on one side of the trophoblast cells forming a mammalian blastula, called a blastocyst. The blastocyst sheds its protective layer called zona pellucida before implanting into the uterine wall.

In amphibians, such as frogs, the egg has a lot of yolk and is called a telolecithal egg. The yolk is concentrated at one pole of the egg, called the vegetal pole, while the other pole, called the animal pole, has less yolk and more cytoplasm. After fertilization, the zygote undergoes holoblastic cleavage, resulting in daughter cells that are smaller at the animal pole and larger at the vegetal pole. When the cells reach the 128-cell stage, they form a hollow ball of cells called a blastula with a small cavity called a blastocele. The fluid-filled cavity forms in the animal hemisphere of
the frog. However, the early formation of the blastocele has been traced back to
the very first cleavage furrow. It was demonstrated in the frog embryo that
the first cleavage furrow widens in the animal hemisphere creating a small
intercellular cavity that is sealed off via tight junctions.

The blastocele has several functions for embryonic development. First, it creates
a space for cell migration and differentiation during gastrulation, which is
the process of forming three germ layers: ectoderm, mesoderm and endoderm.
These germ layers will give rise to all the tissues and organs of
the adult animal.

Second, it helps regulate cell fate and gene expression by creating different
microenvironments for different cell types. For example, in mammals,
the inner cell mass cells are exposed to different signals and nutrients than
the trophoblast cells because they are separated by
the blastocele. This influences their developmental potential and gene expression patterns.

Third, it helps prevent cell death by providing mechanical support and preventing
cell compression. As