Celoma: What is it? Types, Formation and Anatomy of This Body Cavity

This term is used to describe a fluid-filled cavity.

The coelom is the internal cavity that appears in the embryonic state , and that, in adults, houses internal organs.

The word comes from the Greek koilma, which means hollow or cavity.

This is a cavity that in man and some animals develops between the viscera and the wall of the body.

The coelom is filled with fluid and completely lined by tissue created from the mesoderm, the middle layer of the primary cells found in an embryo .

It is typically found in multicellular organisms and in most bilateral animals, those that when divided in half, both the left side and a right side are exactly the same, as well as vertebrates or animals with a backbone.

A coelom acts as a cushion and a protective barrier.

Basically, it prevents the organs from hitting against the outer wall of the body, protecting the internal organs from any kind of massive real trauma on a day-to-day basis.

In complex organisms like us humans, this allows our organs to develop and grow.

The coelom is a structure that has evolved over time. Animals with a coelom are called coelomats.

Vertebrate placental development has both extra-embryonic (outside the embryo) and intra-embryonic (within the embryo) coelomes.

Coelom types

The intraembryonic coelom

The intraembryonic coelom is initially a single space located in the mesoderm of the lateral plate, which will later form the 3 main cavities of the body: pleural, pericardial and peritoneal.

All intraembryonic cavities are filled with fluid, and developing organs push against one wall of the cavity, creating a double layer (serosa adventitalia) that surrounds an organ (eg, the lungs.

The serous membrane is the epithelium (squamous) and its associated underlying loose connective tissue.

The coelom is a cavity that forms during the embryonic development of certain groups of animals such as annelids, echinoderms, mammals, called for this reason “coelomatous”.

This cavity is surrounded by a membrane of mesodermal origin at the origin of various organs such as the dermis, the blood circulatory system, among others.

The coelom, filled with a liquid, has a number of advantages:

  • It leaves room for the growth and movement of organs such as the digestive tract or the heart.
  • It is a shock absorber for mechanical shock and thermal variations.
  • It constitutes a hydrostatic skeleton.
  • It facilitates the excretion of metabolic wastes through the fluid that floods the organs.

In humans, the coelomic cavities and their walls form the pleural cavities of the lungs, the pericardial cavity that houses the heart, and the peritoneal cavity that surrounds the viscera.

Some recent findings

The coelomic cavities of vertebrates are lined by a mesothelium that develops from the mesoderm of the lateral plate.

During development, the coelomic epithelium is a highly active cell layer, which can locally supply mesenchymal cells that contribute to the mesodermal elements of many organs and provide signals that are necessary for their development.

The wall of the body, the heart, the liver, the lungs, the gonads and the gastrointestinal tract are occupied by cells derived from the coelomic epithelium that contribute to its connective and vascular tissues.

And sometimes to specialized cell types such as the stellate cells of the liver, the interstitial cells of Cajal of the intestine or the Sertoli cells of the testis.

The extra-embryonic coelom

Extra-embryonic coelomas include the yolk sac, amniotic cavity, and chorionic cavity, it can also be found in the developing placenta.

The extra-embryonic coelom is filled with fluid (amniotic fluid) initially formed by epiblast and then lined by ectoderm and the surrounding extra-embryonic mesoderm.

In humans, it forms the innermost fetal membrane, causing the amniotic fluid to expand to eventually fuse with the chorionic membrane during the eighth week of development.

This fluid-filled sac is initially located on the trilaminar embryonic disc and with the embryonic disc folded, this sac is removed ventrally to wrap (cover) the entire embryo, and then the fetus.

The presence of this membrane led to the description of reptiles, birds, and mammals as “amniotes.”

Chorionic cavity

The extraembryonic coelom is filled with fluid initially formed from the trophoblast and the extraembryonic mesoderm that forms the placenta. The chorion is incorporated into the development of the placenta.

The avian and reptile chorion is located next to the eggshell and allows gas exchange.

In humans, this cavity is lost during the eighth week when the amniotic cavity expands and fuses with the chorion.

Viteline sack

It is an extra-embryonic membrane of endoderm origin and covered with extra-embryonic mesoderm.

The yolk sac is outside the embryo initially connected by a yolk stalk to the midgut with which it is continued.

The endodermal lining is continuous with the endoderm of the gastrointestinal tract.

The extra-embryonic mesoderm differentiates to form blood and blood vessels of the yolk system.

In reptiles and birds, the yolk sac has a function associated with nutrition.

In mammals, the yolk sac acts as a source of primordial germ cells and blood cells.

In early development (week 2) a structure called the «primitive yolk sac» is formed, which is a cavity formed by a thin membrane called Heuser’s membrane, derived from the endoderm, which lines a large part of the extra-embryonic mesoderm.

Some recent findings

The coelomic cavity is part of the extra-embryonic mesoderm, which surrounds the amniotic cavity, the embryo, and the yolk sac in early gestation.

It is now believed to represent an important transfer interface and nutrient reservoir for the embryo.

Puncture performed by ultrasound-guided transvaginal cellocentesis offers easier access to the human embryo, starting 28 days after fertilization.

However, despite some studies on its biochemical composition, our knowledge about the presence of cellular elements and their quality in this compartment is still limited.

Human coelomic fluids sampled from 48 days to 10 weeks of gestation were studied, demonstrating the presence of functional embryonic erythroid precursors, that is, megaloblasts in the coelomic cavity.

This cavity is surrounded by three layers of cells called ectoderm (outer layer), endoderm (inner layer), and mesoderm (middle layer).

These layers of cells form in the embryo through a process called gastrulation, and eventually these layers of cells develop into different parts of the body.

This coelom and pseudocoelom act as a hydrostatic skeleton and spread pressure throughout the body to minimize damage to internal organs.

The coelom acts as a shock absorber and a hydrostatic skeleton.

Longitudinal and circular waves can be efficiently transmitted through the hydrostatic skeleton.

There are two types of animals, diploblastic animals and triptoblastic animals, which have been categorized according to the development of the embryo.

Diploblastic animals, as their name implies, have two layers of cells, that is, the outer layer, which is called the ectoderm, and the inner layer called the endoderm.

Tryptoblastic animals have an extra layer of cells between the ectoderm and endoderm called the mesoderm.

Only triptoblastic animals have body cavities.


In organisms, a coelom can form in two different ways, depending on the category of coelomate.

Scientists divide organisms into two different categories of coelomates: protostomes and deuterostomes.

The way to tell which is which is by identifying the type of divisions that occur during early development, the way the coelom forms, and how the mouth develops in relation to an opening called the blastopore.

Among the coelomatous, the members of the animals are classified according to how the coelom is formed during embryonic development, in protostomes the division is spiral-shaped and in deuterostomes, the division is made radially.

Without going into any great detail, humans fall into the category of deuterostomes, and animals like snails and squid are classified as protostomes.

In summary, the main difference between the two types lies in the way in which the divisions occur during the development of the organism.

In both protostomes and deuterostomes, the coelom forms in one area along with the mesoderm.

The formation differs in that the protostomes form a coelom when the mesoderm divides, whereas the deuterostomes form a coelom when the mesoderm cells combine, resulting in the body cavity.

This formation occurs during the gastrulation or three-layer structure stage of development.

In both protostomes and deuterostomes we see this blind pouch or archenteron shape as the mesoderm folds inward.

This is where the two types start to differ, which will be explained below.

Vertebrates are coelomatous organisms with a coelomic cavity that is divided into compartments by partitions throughout evolution.

The number of compartments ranges from two in fish (pericardium and peritoneal cavity) to four in mammals (two pleural cavities for the lungs in addition to the previous one).

All these cavities are lined by a coelomic epithelium, which is derived from the mesoderm of the lateral plate.

This shunt has been well established, but there is a lack of information in the recent literature on how the coelomic epithelium differs.

This is probably due to the assumption that the coelomic epithelium is just the cell lining of the coelomic cavity, and therefore the appearance of the coelomic cavities automatically implies the formation of a coelomic epithelium.

This is not true, because the mesodermal layers within which the definitive coelomic cavities appear (somatopleura and splanchnopleura) are not strictly speaking “epithelia”, that is, they are polarized cells with lateral adhesions and resting on a basal lamina.

The primitive somatopleura and splanchnopleura are highly active cell layers that give rise to an abundant population of mesodermal cells that form the body wall and the mesodermal compartment of many viscera.

The splanchopleura also gives rise to the primitive heart tube.

Therefore, the coelomic epithelium, sensu stricto, appears when the adluminal layer of somatopleural and splanchnopleural cells acquires epithelial characteristics, basoapical polarization and basal lamina.

Somatopleura gives rise to the coelomic epithelium of the body wall, while splanchnopleura is the origin of the coelomic epithelium that covers the organs contained in the coelomic cavity.

What is relevant in the context of this review is that this embryonic coelomic epithelium becomes locally activated giving rise to new populations of mesodermal cells.

Coelom anatomy

The lateral mesoderm is divided into two layers: the somatic layer and below the somatic layer, the splanchnic layer. The space in between is the coelom.

As the embryo’s body folds, its coelom becomes a single closed cavity.

In it you can recognize, at a regional level, a pericardial cavity (heart cavity), two pleural canals (for the lungs) and a peritoneal cavity (for abdominal content).

A thick plate of mesoderm, the transverse septum, forms a partial partition just anterior to the developing liver. Two pairs of membranes grow from the septum.

One set separates the pericardial cavity from the two pleural cavities; These membranes later expand into the pericardium and enclose the heart.

The other pair of membranes separates the pleural cavities from the peritoneal cavity of the abdomen.

The definitive diaphragm is a compound partition, much of which is provided by the transverse septum; Minor contributions are from the lateral walls of the body and the paired membranes that separated the pleural and peritoneal cavities.