The blastocoel (), 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 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 (f ...

during very early embryonic development. At this stage in mammals the blastula develops into the blastocyst containing an inner cell mass, and outer trophectoderm. It develops following cleavage of the

zygote A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicell ...

after

fertilization Fertilisation or fertilization (see spelling differences), also known as generative fertilisation, syngamy and impregnation, is the fusion of gametes to give rise to a new individual organism or offspring and initiate its development. Proce ...

. It is the first cell cavity formed as the embryo enlarges, and is the essential precursor for the differentiated

gastrula Gastrulation is the stage in the early embryonic development of most animals, during which the blastula (a single-layered hollow sphere of cells), or in mammals the blastocyst is reorganized into a multilayered structure known as the gastrula. ...

. In the ''

Xenopus ''Xenopus'' () (Gk., ξενος, ''xenos''=strange, πους, ''pous''=foot, commonly known as the clawed frog) is a genus of highly aquatic frogs native to sub-Saharan Africa. Twenty species are currently described within it. The two best-know ...

'' a very small cavity has been described in the two-cell stage of development.

In mammals

After fertilization, the

undergoes

mitotic In cell biology, mitosis () is a part of the cell cycle in which replicated chromosomes are separated into two new nuclei. Cell division by mitosis gives rise to genetically identical cells in which the total number of chromosomes is maintai ...

divisions during rotational cleavage resulting in daughter cells known as

blastomere In biology, a blastomere is a type of cell produced by cell division (cleavage) of the zygote after fertilization; blastomeres are an essential part of blastula formation, and blastocyst formation in mammals. Human blastomere characteristics In ...

s until they are at the 16-cell stage called the

morula A morula (Latin, ''morus'': mulberry) is an early-stage embryo consisting of a solid ball of cells called blastomeres, contained in mammals, and other animals within the zona pellucida shell. The blastomeres are the daughter cells of the zygot ...

. The morula is a solid ball of cells that has a small group of internal cells surrounded by a larger group of external cells. Following further cleavages and compaction the blastomeres undergo cellular differentiation that gives rise to an internal group of cells called the inner cell mass, (also ''embryoblast'') and an external layer of surrounding cells known as

trophoblast The trophoblast (from Greek : to feed; and : germinator) is the outer layer of cells of the blastocyst. Trophoblasts are present four days after fertilization in humans. They provide nutrients to the embryo and develop into a large part of the p ...

s. The inner cell mass will go on to become the actual embryo. The external, surrounding cells develop into the

cells. At this stage there is no cavity within the morula; the embryo is still a ball of dividing cells. In a process called cavitation, the trophoblast cells secrete fluid into the morula to create a blastocoel, the fluid-filled cavity. The membranes of the trophoblast cells contain sodium (Na⁺) pumps, Na⁺/K⁺- ATPase and Na⁺/H⁺ exchangers, that pump sodium into the centrally forming cavity. The accumulation of sodium pulls in water osmotically, creating and enlarging the blastocoel. 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 blastocoel expands and the inner cell mass is positioned on one side of the trophoblast cells forming a mammalian blastula, called a blastocyst.

In amphibians

An amphibian embryo in the 128- cell stage is considered a blastula as the blastocoel in the embryo becomes apparent during this stage. The fluid-filled cavity forms in the animal hemisphere of the frog. However, the early formation of the blastocoel has been traced back to the very first

cleavage furrow In cell biology, the cleavage furrow is the indentation of the cell's surface that begins the progression of cleavage, by which animal and some algal cells undergo cytokinesis, the final splitting of the membrane, in the process of cell division. ...

. 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. As cleavage continues, the cavity expands to become the developed blastocoel. The blastocoel is a crucial component of amphibian embryo development. It permits cell migration during gastrulation and prevents the cells beneath the blastocoel from interacting prematurely with the cells above the blastocoel. For instance, the blastocoel prevents the vegetal cells destined to become endoderm from coming in contact with those cells in the ectoderm fated to give rise to the skin and nerves.

Damage to blastocoel

The blastocoel can be damaged and abolished if the adhesion between blastomeres, provided by cell adhesion molecules like EP-cadherin, is destroyed as mRNA by

oligonucleotide Oligonucleotides are short DNA or RNA molecules, oligomers, that have a wide range of applications in genetic testing, research, and forensics. Commonly made in the laboratory by solid-phase chemical synthesis, these small bits of nucleic acids ...

s. If the mRNA is destroyed, then there’s no EP-cadherin, little to no blastomere adhesion and the blastocoel is non-existent. During the next stage of embryonic development, amphibian gastrulation, the blastocoel is displaced by the formation of the

archenteron The primary gut that forms during gastrulation in a developing embryo is known as the archenteron, the gastrocoel or the primitive digestive tube. It develops into the endoderm and mesoderm of an animal. Formation in sea urchins As primary mesen ...

, during mid-gastrulation. At the end of gastrulation, the blastocoel has been obliterated.

In sea urchins

At the 120- cell stage, the sea urchin embryo is considered a blastula because of its developed blastocoel, which every embryonic cell surrounds and touches. Every cell is in contact with the proteinaceous fluid of the blastocoel on the inside and touches the hyaline layer on the outside. The loosely connected blastomeres are now tightly connected because of

tight junction Tight junctions, also known as occluding junctions or ''zonulae occludentes'' (singular, ''zonula occludens''), are multiprotein junctional complexes whose canonical function is to prevent leakage of solutes and water and seals between the epith ...

s that create a seamless epithelium that completely encircles the blastocoel. Even as the blastomeres continue to divide, the blastula remains one-cell thick and thins out as the embryo expands outward. This is accomplished in part due to the influx of water that expands the blastocoel and pushes the cells surrounding it outwards. At this point, the cells have become specified and are ciliated on the opposite side of the blastocoel. The vegetal plate and animal hemisphere develop and secrete a hatching enzyme that digests the fertilization envelope and allows the embryo to now become a free-swimming hatched blastula.

Development of primary mesenchyme

Important to the sea urchin blastula is the ingression of the

primary mesenchyme Mesenchymal stem cells (MSCs) also known as mesenchymal stromal cells or medicinal signaling cells are multipotent stromal cells that can differentiate into a variety of cell types, including osteoblasts (bone cells), chondrocytes (cartilage cel ...

. After the blastula hatches from the fertilization envelope, the vegetal side of the blastula begins to flatten and thicken as a small cluster of these cells develop long, thin processes called

filopodia Filopodia (singular filopodium) are slender cytoplasmic projections that extend beyond the leading edge of lamellipodia in migrating cells. Within the lamellipodium, actin ribs are known as ''microspikes'', and when they extend beyond the lame ...

. These cells then dissociate and ingress into the blastocoel and are called the primary mesenchyme. The cells move randomly along the inside of the blastocoel, until they become localized in the ventrolateral region of the blastocoel.

In birds

Similar to mammals, fertilization of the avian ovum occurs in the oviduct. From there the blastodisc, a small cluster of cells in the animal pole of the egg, then undergoes discoidal

meroblastic In embryology, cleavage is the division of cells in the early development of the embryo, following fertilization. The zygotes of many species undergo rapid cell cycles with no significant overall growth, producing a cluster of cells the same siz ...

cleavage. The blastoderm develops into the epiblast and hypoblast and it is between these layers that the blastocoel will form. The shape and formation of the avian blastodisc differs from amphibian, fish, and echinoderm blastulas, but the overall spatial relationship of the blastocoel remains the same.

Formation of primitive streak

The avian blastocoel is important during the development of the primitive streak. The ingression of the endodermal precursor cells form the epiblast into the blastocoel and the migration of lateral cells of the posterior epiblast towards the center form the early primitive streak. As these cells converge inward, a depression forms called the primitive groove and functions as an opening through which cells travel into the blastocoel. As cells migrate into the blastocoel, they undergo an epithelial-to-mesenchymal transformation.

Zebrafish: a special case

Unlike amphibian, echinoderm, mammalian, and avian embryos,

zebrafish The zebrafish (''Danio rerio'') is a freshwater fish belonging to the minnow family (Cyprinidae) of the order Cypriniformes. Native to South Asia, it is a popular aquarium fish, frequently sold under the trade name zebra danio (and thus often ca ...

do not have a defined blastocoel. Rather, they have small, irregular extracellular spaces that are formed between the cells of the blastodisc sitting atop the yolk.