TRIBLASTICS: CHARACTERISTICS AND LAYERING - BIOLOGY - 2024

The term triblastic or triploblastic refers to the presence of three germ layers or leaflets during embryonic development. These layers are the ectoderm, mesoderm, and endoderm, from which the tissues and organs of the body of the vast majority of animals are derived. This type of body organization is closely related to bilateral symmetry.

The vast majority of existing zoological groups are triblastic (metazoan animals). However, there is the exception with those groups considered diblastic, such as coelenterates, in which there are only two germ layers (ectoderm and endoderm) and an intermediate layer called mesoglea.

Source: Abigail Pyne

characteristics

The origin of the triblastic organization has been widely discussed by identifying triblastic characteristics in groups such as the Cnidarians (diblastics). In triblastic organisms there is a third germ layer called mesoderm, between the ectoderm and endoderm.

These last layers are also found in diblastic organisms, being the only cell layers on the body surfaces in this group of animals.

The endoderm and ectoderm maintain contact with external media, regulating interactions and exchange with them. The endoderm is in contact with the gastric system and the ectoderm with the epidermis.

The mesodermal tissue, in contrast, is only in contact with these two layers, thus constituting a physiologically self-controlled compartment. The mesoderm can form a compact mesenchyme and surround cavities such as muscles and blood vessels.

Formation of germ layers

The existence of three germ layers is a synapomorphy in triploblastic animals, while in paraphyletic diblastic animals it is an ancestral condition. Most tripoblastic animals have an organ-level organization system.

During the gastrulation process, the blastomeres of the blastula begin to differentiate due to their migration.

During gastrulation, the three germ layers are generated from which specific organs and tissues of each layer are produced through the process of organogenesis. The mesoderm constitutes the additional layer not present in diblastic organisms.

Ectoderm

The ectoderm is the germ layer that remains outside the embryo and gives rise to the epidermis and its derivatives (feathers, hair, various glands, enamel, among others), the membrane of the mouth and cloaca and the nervous system.

Mesoderm

It is the germ layer from which muscles, connective tissue and the blood system usually develop. During the gastrulation process the mesoderm is situated between the ectoderm and endoderm.

Endoderm

It is the innermost germ sheet of the embryo and gives rise to the digestive tract and associated organs such as the liver and pancreas. In birds and mammals it forms the allantois, and in birds the yolk sac.

Formation of the mesoderm in protostomes

In protostome organisms, the mesoderm is formed by endoderm cells near the blastocele that travel to the blastocele. From this migration of cells, three types of organisms can be formed, depending on their body plans: acellomates, pseudocoelomates and true coelomates.

Formation of the mesoderm in deuterostomes

In deuterostomized animals, the mesoderm is formed by the growth of cells from the inner lining of the intestine. These cells grow outward, expanding to the blastocele in the form of sacs.

The growing cells form a mesodermal ring and as they expand they enclose a space that will be called a coelom. The sacs formed by the expanding cells break away forming a coelomic cavity surrounded by mesoderm.

This coelom completely fills the blastocele, thus forming the second body cavity of the animals. True coelomes are also surrounded by a thin cell membrane (peritoneum).

Triblastic animal groups and organization patterns

Triblastic organisms can be cataloged with a progressive level of organization. These are classified into two large groups, the protostomates and the deuterostomes. These two groups are differentiated by four important characteristics during their embryonic development.

The first characteristic is the position of the cells when dividing (it can be radial or spiral). The form of excision of the cytoplasm, which can be regulatory or mosaic, is the second characteristic.

The third characteristic is the formation of the coelom that can occur in an enterocelic or schizocelic form and the fourth and best known distinguishing characteristic is the fate of the blastomere, which can form the mouth or anus of the organism.

In triblastic animals we observe a growing organization pattern that can be subdivided based on the presence or absence of a body cavity called a coelom.

Having this coelom provides certain advantages such as the generation of space for the formation of organs, a storage area, as well as the diffusion of gases, nutrients and waste outside the organs. It also provides a hydrostatic skeleton and the facilitation of size increase in these organisms.

Cellophane

In acellomates, the mesoderm cells completely cover the blastocele, forming a relatively solid mass of cells called the parenchyma, between the ectoderm and the endoderm.

This parenchyma develops from embryonic connective tissue and performs functions of transport and assimilation of food, as well as the elimination of metabolic waste. In animals with this body plan, the only internal cavity is the intestinal cavity.

Pseudocoelomates

When the mesodermal cells surround the blastocele on its external face, the body plan of the pseudocoelomates is formed. In these organisms, two body cavities are formed, the intestinal cavity and the blastocele that is constantly formed and is called pseudocoelom or false coelom.

This false coelom lacks muscle and connective tissue associated with the intestine. In this body plan, the internal organs are not surrounded by a membrane and there is no mesodermal layer covering the inner body surface.

Celomados

In coelomates, the blastocele is completely invaded by the cells of the mesoderm, forming a solid layer that surrounds the intestine. Subsequently, a series of programmed apoptosis within this layer allows a hollow cavity to form called a coelom.

The coelom is completely enveloped by the mesoderm and a thin mesodermal membrane covers the internal surface of the body. The peritoneum extends to cover the organs, in which case it is called serosa.

The peritoneum and serosa continue to form the mesentery, which is a lamina that supports the visceral structures of the coelomic cavity.

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