This term is used to describe a fluid-filled cavity.
The coelom is the internal cavity that appears in the embryonic stage and that, in adults, houses internal organs.
The word comes from the Greek korma, which means hollow or cavity.
This cavity 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 most bilateral animals. When divided in half, both the left and right sides are the same as vertebrates or animals with a backbone.
A coelom acts as a cushion and a protective barrier.
It prevents the organs from hitting against the outer wall of the body, protecting the internal organs from 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. Animals with a coelom are called coelomates.
Vertebrate placental development has extraembryonic (outside the embryo) and intra-embryonic (within the source) coelomes.
The intraembryonic coelom
The intraembryonic coelom is initially a single space located in the mesoderm of the lateral plate, which will later form the three main cavities of the body: pleural, pericardial, and peritoneal.
All intraembryonic cavities are filled with fluid, and developing organs push against one cavity wall, creating a double layer (serosa adventitia) surrounding an organ (e.g., 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, and mammals called “coelomates.”
This cavity is surrounded by a membrane of mesodermal origin at the origin of various organs such as the dermis, and the blood circulatory system, among others.
The coelom, filled with a liquid, has several 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 is 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 extraembryonic coelom
Extraembryonic coelomates include the yolk sac, amniotic cavity, and chorionic cavity; they can also be found in the developing placenta.
The extraembryonic coelom is filled with fluid (amniotic fluid) initially formed by epiblast and then lined by ectoderm and the surrounding extraembryonic 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. With the embryonic disc folded, this sac is removed ventrally to wrap (cover) the entire embryo and the fetus.
The presence of this membrane led to the description of reptiles, birds, and mammals as “amniotes.”
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 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.
It is an extraembryonic membrane of endoderm origin and covered with extraembryonic 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 extraembryonic mesoderm differentiates to form blood and blood vessels in the yolk system.
In reptiles and birds, the yolk sac is associated with nutrition.
The yolk sac is a source of primordial germ cells and blood cells in mammals.
In early development (week 2), a structure called the «primitive yolk sac» is formed, a cavity formed by a thin membrane called Heuser’s membrane, derived from the endoderm, which lines a large part of the extraembryonic mesoderm.
Some recent findings
The coelomic cavity is part of the extraembryonic mesoderm, which surrounds the amniotic cavity, the embryo, and the yolk sac in early gestation.
It is now believed to represent a vital transfer interface and nutrient reservoir for the embryo.
Puncture performed by ultrasound-guided transvaginal cellocentesis offers more accessible 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, and 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 gastrulation, and eventually, these layers of cells develop into different parts of the body.
This coelom and pseudocolor 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 and triploblastic animals, categorized according to the embryo’s development.
As their name implies, Diploblastic animals have two layers of cells: the outer layer, 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 triploblastic 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 by identifying the type of divisions during early development, the coelom forms, and how the mouth develops about an opening called the blastopore.
Among the coelomates, 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 any great detail, humans fall into deuterostomes, and animals like snails and squid are classified as protostomes.
In summary, the main difference between the two types lies in how the divisions occur during the organism’s development.
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.
We see this blind pouch or archenteron shape in protostomes, and deuterostomes are the mesoderm folds inward.
The two types start to differ, which will be explained below.
Vertebrates are coelomates organisms with a coelomic cavity divided into compartments by partitions throughout evolution.
The compartments range from two in fish (pericardium and peritoneal cavity) to four in mammals (two pleural cavities for the lungs and 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. Therefore the appearance of the coelomic holes automatically implies the formation of a coelomic epithelium.
This is not true because the mesodermal layers within which the definitive coelomic cavities appear (somatopleure and splanchnopleura) are not strictly speaking “epithelia”; they are polarized cells with lateral adhesions and resting on a basal lamina.
The primitive somatopleure 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 this review is that this embryonic coelomic epithelium becomes locally activated, giving rise to new populations of mesodermal cells.
The lateral mesoderm is divided into the bodily and the splanchnic layer below the physical 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.