COELOM: CHARACTERISTICS, FUNCTIONS, TYPES, CLASSIFICATION - BIOLOGY - 2025

The coelom is an anatomical cavity surrounded by mesoderm, one of the three sheets or layers of embryonic tissue in triblastic animals. It is a fluid-filled cavity between the body wall and the digestive tract.

The structures of most animals develop from three embryonic sheets or layers of tissue known as germ layers: the ectoderm, the mesoderm, and the endoderm.

Diagram of a segmental section of a typical oligochaete worm showing body systems and organs (Source: KDS444 via Wikimedia Commons)

The ectoderm forms the outer covering of the body and the nervous system. The endoderm, the inner layer, lines the digestive tract and its attached organs. Cnidarians and Ctenophores only have these two embryonic layers, which is why they are classified as diblastic or diploblastic animals.

The mesoderm or middle layer is the one from which most of the body structures arise such as the skeleton, muscles, and circulatory system (when they have it) of triblastic or tripoblastic animals.

The coelom is recognized as the cavity between the tissues derived from the ectoderm (the body wall) and the endoderm (the digestive tract); and coelominated animals are characterized by a "tube within a tube" body organization.

From a functional point of view, some authors consider that the development of the coelom as an internal hydrostatic skeleton was necessary for animals that adopted a benthic, crawling and burrowing way of life.

It also had many advantages for locomotion and a circulatory environment and space for the development of more complex organs and organ systems.

Despite the above, even today the evolutionary theories about the formation of the coelom are quite controversial, especially in relation to the different forms of embryonic development that exist and that give rise to the same type of cavity.

Formation and characteristics

Polychaeta anatomy, where you can see the coelom.

Source: © Hans Hillewaert / wikimedia commons

The coelom can be formed by two main pathways: the schizocelic pathway (schizocelia) and the enterocelic pathway (enterocelia). These terms refer to the nature of its formation: "schizo", by division, and "whole" by digestive tract.

Schizocelic pathway

The coelom of schizocelic origin is formed by the division of mesodermal bands from the blastopore region, which is the opening of the archenteron (primitive digestive tube) in the gastrula. These bands grow between the ectodermal and endodermal tissues.

The number of paired coeloms that arise from the growth and cupping of the bilateral paired masses of mesodermal origin during the formation of the coelom, varies depending on the type of animal considered and is usually associated with the number of body segments of the same.

Enterocelic pathway

The coelom of enterocelic origin arises from evaginations of the archenteron during embryonic development. In the most direct and simple form of formation, the emergence of the mesoderm and the coelom occurs as a single and indivisible process, better known in the literature as "archenteric evagination."

This process begins with the formation of one or more "bags" or "sacs" in the walls of the digestive tract, which detach as coelomatic compartments whose walls correspond to the mesoderm.

In other cases, the mesoderm originates from the walls of the archenteron, which are initially sheets or leaves that are subsequently hollowed out.

Protostomes and deuterostomes

The coelom of protostomate organisms is formed by the schizocelic pathway, whereas deuterostomates generally have coeloms of enterocelic origin.

A protostomized organism is one in which, during embryonic development, the mouth is formed from the first embryonic opening, that is, the blastopore. Protostomized animals are characterized by spiral segmentation during embryonic development in the morula phase.

In deuterostomes, the first embryological opening gives rise to the anus and these organisms are characterized by radial segmentation during initial development.

Features

The earthworm is a coelomed animal Source: pixabay.com

The fluid-filled internal cavity that the coelom represents has general functions as a “hydrostatic skeleton” and a buffer between the digestive tract and its attached organs and the body wall.

The function of the hydrostatic skeleton is to provide a rigid and flexible cavity at the same time, being the muscles present in the body wall responsible for movement and changes in shape in animals.

The development of the coelom allowed the appearance of new types of locomotion and body movements in animals, movements impossible for animals lacking internal cavities.

The coelom also provides a greater surface area for the diffusion of gases, nutrients and wastes to and from the organs. It also has storage functions, as a vehicle for the elimination of waste and reproductive products and, evolutionarily speaking, it contributed to the increase in body size.

The essential functions of the coelom in organisms as complex as the human being become evident in some related pathologies, which result from congenital malformations related to the intra-embryonic coelom.

Among these is the diaphragmatic hernia, very common in neonates, which can be fatal in specific cases such as Bochdalek's congenital diaphragmatic hernia, in which the abdominal viscera (stomach, spleen and part of the liver) occupy the cavity. thoracic, moving the heart forward and compressing both lungs.

Types

In coelominated animals such as humans, a distinction can be made between an extra-embryonic coelom during the formation of the yolk sac and an intra-embryonic coelom, which in the adult will form three compartments, namely:

- The pericardial cavity (which includes the heart).

- The pleural cavities (which contain the lungs).

- The abdominal cavity (which houses the viscera under the diaphragm).

The pericardial and pleural cavities are found in the thoracic cavity. The thoracic and abdominal cavities are separated by the diaphragm and the pericardial cavity and the pleural cavities by a membrane called the pleuropericardial membrane.

Extra-embryonic coelom

The extra-embryonic coelom surrounds the primitive yolk sac and the amniotic cavity. This is formed by the fusion of cavities in the extra-embryonic mesoderm, a loose and delicate tissue that has its origin in the trophoblast, which is the outer layer of cells that surrounds the blastocyst and that subsequently gives rise to the placenta. and therefore disappears after delivery.

Intra-embryonic coelom

This type of coelom is the space limited by the splanchnic sheet of the mesoderm, which is continuous with the mesoderm of the yolk sac, and by the somatic sheet of the mesoderm, which is continuous with the extra-embryonic mesoderm that covers the wall of the amniotic cavity..

Initially, the extra- and intra-embryonic coelomas are communicated by their right and left portions. However, as the body of the embryo bends and folds, this connection is lost and the intra-embryonic coelom forms a large space extending from the thoracic region to the pelvic region.

Classification of animals according to coelom

The presence or absence of a coelom is an important determinant in the evolutionary advance of animals with bilateral symmetry.

Triblastic animals can be classified according to the presence and characteristics of the body cavity represented by the coelom. Thus, the Acelomados animals (without internal cavity), the Pseudocoelomados or Blastocoelomados (that have a false cavity) and the Eucoelomados or Coelomados are commonly recognized.

Cellophane

The acellomates are triblastic animals (with three embryonic layers) that are solid or compact, since they do not have a cavity similar to the coelom.

Some cells between the endoderm and the ectoderm are loosely organized in a tissue known as the parenchyma. These cells are not specialized in any particular function.

In this group are flatworms or flatworms, endoprocts or parasites of the anus, gnathostomulids or mandibular worms and gastrotricos.

Pseudocoelomates or blastocoelomates

Many triblastic animals such as rotifers and nematodes have cavities of varying sizes that do not originate from the mesoderm or are delimited by tissue derived from it, which is why they are known as pseudocoelomates (with a false coelom).

In these animals the organs are free inside these cavities, bathed by their own fluids. No connective or muscular tissue associates with the digestive tract, no mesodermal layer covers the surface of the body wall, and no membrane helps to suspend organs.

They are also called blastocoelomates since these cavities correspond to the remains of the embryonic blastocele (fluid-filled cavity where the cells of the blastula are arranged during embryonic development).

Eucelomados or celomados

In coelomed animals, the coelom is a true cavity surrounded by a thin tissue derived from the mesoderm known as the peritoneum. In this cavity the organs are not free, but are separated from the coelomatic space by the peritoneum.

The peritoneum forms particular structures that help in the suspension of the organs and that are known as mesenteries. The coelomatic cavity in these animals has tissues derived from the mesoderm such as muscles and other connective tissues, associated with internal organs.

In vertebrates, the coelom originates from the lateral plate of the mesoderm, the specification of which is controlled by various molecular factors.

Eucelomates include mollusks, annelids or segmented worms and arthropods (insects, spiders and crustaceans), echinoderms (sea urchins and starfish) and chordates (including vertebrates such as mammals, birds, fish, amphibians and reptiles).

References

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