The coelom is a body cavity found in metazoa (animals that develop from an embryo with three layers of tissue: ectoderm, mesoderm, and endoderm). The cells in each tissue layer become differentiated during development and become different tissues, organs, and digestive tract.
The coelom derived from the mesoderm is located between the intestinal canal and the body wall and is lined with mesodermal epithelium. The mesodermal tissue also makes up blood, bones, digestive tract, gonads, kidneys, and other organs. Organisms that have a real coelom are called (true) coelomates.
True coelomates are often divided into two categories: protostomes and deuterostomes. This distinction is based on patterns of cell division, coelom formation, and the fate of the blastopore. In protostomes, the blastopore becomes the mouth. In deuterostomes, the blastopore becomes the anus.
Organisms that have a body cavity that is not completely lined with mesodermal epithelium are called pseudocoelomates, while organisms that lack a body cavity are called acoelomates.
Functions of a Coelom and its Importance
The coelomic cavity is filled with a fluid known as coelomic fluid. It serves to separate the organs from the outer body and ultimately protects the organs from mechanical shock or trauma.
The separation between the organs and the outer body also allows for greater ranges of motion and flexibility, as the organs are not disturbed by slight displacement when the body bends or stretches.
In soft-bodied animals such as earthworms and many other invertebrates, the fluid-filled coelom can act as a hydrostatic skeleton. A hydrostatic skeleton is a type of structure that functions as a skeleton but is supported by fluid pressure rather than bone. This allows movement in soft-bodied animals.
Immune System Support
Coelomocytes play a key role in the immune system of most invertebrates. They are macrophage-like cells that are involved in important functions such as phagocytosis, inflammation, and the secretion of humoral factors that confer humoral immunity.
The coelomic fluid also facilitates the transport of gases, nutrients, and waste products between different parts of the body. Nutrients absorbed in the coelomic fluid are distributed to all parts of the body in a similar way to a circulatory system, and any unneeded substances remaining after metabolic processing are excreted through the coelomic fluid.
Indeed, the appearance of the coelom in organisms has enabled the development of larger body sizes as this facilitates the transport of materials.
Evolution and Development of the Coelom
The evolutionary history of the coelom is uncertain. There are two competing theories about the origin of the coelom: the acoelomate theory and the enterocoel theory. The acoelomate assumes that the coelom developed from an acoelomate ancestor, while the enterocoel theory assumes that the coelom developed from the gastric pouches of a cnidarian ancestor.
While neither has been proven wrong, there is more research to support the enterocole theory. The development of the coelom begins at the gastrula stage and can be formed by one of two processes: schizocoely or enterocoely.
Schizocoely, a blind pouch called the archenteron forms in which the digestive tract of the embryo develops. The mesoderm divides into two layers, one attached to the ectoderm (which becomes the parietal layer) and the other to the endoderm (which becomes the visceral layer). The space between these two layers becomes the coelom of the organism.
In enterocoely, the mesoderm buds are hollowed out from the walls of the archenteron to form the coelom cavity.
Examples of Coelomates
Mollusks, Annelids, and Some Arthropods
Clams, snails, slugs, octopuses, earthworms, and protostome coelomates, that is, they are formed from head to foot (or from mouth to foot). The mouth first develops from the blastopore, the first development opening. Protostomes spiral and determine cleavage in the early embryonic stages, and the coelomis formed through the process of schizokoely.
Echinoderms and Chordates
Starfish, sea urchins, fish, and humans are deuterostome coelomates, which means that they are formed from the anus to the head. The blastopore becomes the anus, and the mouth is formed later. Deuterostomes are split radically and indefinitely in the early embryonic stage; The coelom is formed by the process of enterocoely.