WHAT IS GASTRULATION?

The gastrulation is an event of embryonic development where a massive reorganization occurs from a mass of cells - the blastula - to a highly organized structure, formed by several germ layers.

To temporarily place ourselves in the stages of embryonic development, fertilization occurs first, then segmentation and the formation of the blastula and the organization of this cell mass is gastrulation. It is the most important event in the third week of gestation in humans.

Source: Pidalka44

Diploblastic animals are made up of two embryonic layers: the endoderm and the ectoderm, while triblastic animals are made up of three embryonic layers: the endoderm, mesoderm and ectoderm. The correct organization of these structures and the appropriate cellular localization of them occurs during gastrulation.

Each of the embryonic layers formed during gastrulation will give rise to specific organs and systems of the adult organism.

The process varies depending on the animal lineage studied. However, there are certain common events such as changes in the motility of cells, in their shape and in the way they are connected.

Principles of embryonic development

Crucial steps during development

To understand the concept of gastrulation, it is necessary to know certain key aspects of the development of an embryo. We will list and briefly describe each of the pre-gastrulation steps to get into context.

Although animal development is a highly variable event between lineages, there are four common stages: fertilization, blastula formation, gastrulation, and organogenesis.

Step 1. Fertilization

The first step is fertilization: the union between the female and male gametes. After this event, a series of changes and transformations occur in the zygote. The phenomenon involving the passage from a single cell to a fully formed embryo is the goal of embryonic developmental biology.

Step 2. Segmentation and formation of the blastula

After fertilization, the repetitive and massive division of cells occurs, which at this stage is called the blastomere. In this period called segmentation, there is no increase in size, only divisions of the large initial cell mass. When this process is finished, a mass of cells called a blastula has formed.

In most animals, the cells arrange themselves around a fluid-filled central cavity called a blastocele. Here a germ layer is formed, and it is a step that all multicellular animals undergo during their development.

It is important to mention that during the segmentation phenomenon the embryo takes on a polarity. That is, they differ in animal and plant poles. The animal is characterized by being rich in cytoplasm and little yolk sac.

Step 3. Gastrulation and formation of two germ layers

After the formation of the first germ layer, the formation of a second layer occurs. This process will be described in detail in this article.

Step 4. Organogenesis

Once the three germ layers have been established, cells begin to interact with their partners and the formation of tissues and organs occurs in an event called organogenesis.

Several organs contain cells from more than one germ layer. It is not unusual for the exterior of the organ to be derived from one germ layer and the interior from another. The clearest example of this heterogeneous origin is the skin, which is derived from the ectoderm and also from the mesoderm.

As in gastrulation, during organogenesis, cells migrate long distances to reach their final position.

Embryonic layers

The crucial event of gastrulation is cellular organization in various embryonic layers. The number of embryonic layers allows the classification of animals into two categories: diblastic and triploblastic.

Diblastics are animals with a very simple structure and only have two germ layers: the endoderm and the ectoderm.

In contrast, triblastic organisms have three embryonic layers: those possessed by diblastics, and an additional one, the mesoderm.

Gastrulation: origin of the three germ layers

During gastrulation, the spherical blastula described in the previous section begins to increase in complexity, leading to the formation of germ layers from the totipotent cells of the epiblastic layer.

Initially, the term gastrulation was used to describe the event of bowel formation. However, it is now used in a much broader sense, describing the formation of a trilaminar embryo.

In this process, the body plan of the developing organism is established. The cells that are part of the endoderm and mesoderm are located inside the gastrula, while the cells that make up the skin and the nervous system are scattered on the surface of it.

Cell movement - or migrations - during gastrulation involves the entire embryo and is an event that must be finely coordinated.

Gastrulation is a highly variable process

Depending on the animal group studied, we find marked variations in the gastrulation process

In particular, this process varies in different classes of vertebrates. This occurs, in part, due to the geometric restrictions that each gastrula exhibits.

Despite these marked variations, the post-gastrula stages are very similar between the groups. In fact, the resemblance between vertebrate embryos is a fact of popular knowledge.

It is very difficult - even for the leading authorities in embryology - to differentiate an embryo from a bird and a reptile in its incipient stages.

Classification of bilateral animals according to the fate of the blastopore.

During gastrulation, an opening called the blastopore forms. The final destination of this allows the classification of bilaterated animals into two large groups: protostomes and deuterostomes.

In the protostomates - a term from the Greek roots "first mouth" - the mentioned opening gives rise to the mouth. This lineage contains the mollusks, arthropods and various phyla of vermiform animals.

In deuterostomes, the blastopore gives rise to the anus, and the mouth is formed from another secondary opening. In this group we find echinoderms and chordates - we humans find ourselves within chordates.

Other embryonic and molecular characteristics have helped validate the existence of these two evolutionary lineages.

Types of movements during gastrulation

The fate of the blastula depends on several factors, including the type of egg and segmentation. The most common types of gastrulation are the following:

Gastrulation by invagination

In most animal groups, the segmentation is of the holoblastic type, where the blastula resembles a ball with nothing inside and the cavity is called a blastocele.

During invagination gastrulation, a portion of the blastula invaginates inward and gives rise to the second germ layer: the endoderm. In addition, a new cavity appears, called the archenteron.

It is analogous to taking a ball or soft plastic ball, and with the finger we press until we form a hole: this is the invagination to which we refer. Sea urchins exhibit this type of gastrulation.

Grastrulation by epibolia

This type of gastrulation occurs in eggs that have appreciable amounts of yolk in the vegetative pole. For these reasons, it becomes difficult to create an intussusception (the process we described in the previous type of gastrulation).

For the formation of the germ layers, there is a multiplication of the micromeres located in the animal pole, sinking and surrounding the macromers. At this point the blastopore has already formed at the vegetative pole, The ectoderm will be formed by the micromeres, while the macromers will give rise to the endoderm.

This type of gastrulation is found in the formation of the ectoderm of very heterogeneous animal groups, such as amphibians, sea urchins and tunicates (also known as sea squirts or sea syringes).

Involution gastrulation

This process occurs when the egg exhibits enormous amounts of yolk - more than in the case of epibolian gastrulation. The process consists of the collapse of the cells located on the periphery of the disc.

These cells then move backward and form a second layer that folds inside, forming the endoderm and ectoderm. The mesoderm of amphibians is formed by following this pattern of development.

Gastrulation by delamination

The endoderm originates from divisions of ectodermal cells. In addition, these cells migrate and sink. Then, the separation into two layers of cells occurs, which will be the ectoderm and the endoderm.

This type of gastrulation is not frequent and there is no blastopore. Hypoblast formation in birds and mammals occurs by delamination.

Ingression gastrulation

It resembles gastrulation by delamination in several respects, with the exception that the endoderm is formed from the movement of cells from the ectoderm.

As in gastrulation by delamination, the formation of a blastopore does not occur and is characteristic of less complex animals, such as sea sponges and marrow. The mesoderm of sea urchins is formed in this way, and so are neuroblasts in the genus Drosophila.

References

Carlson, BM (2018). Human Embryology and Developmental Biology E-Book. Elsevier.
Hall, BK (2012). Evolutionary developmental biology. Springer Science & Business Media.
Hickman, CP (2007). Integrated principles of zoology. McGraw-Hill.
Holtfreter, J. (1944). A study of the mechanics of gastrulation. Journal of experimental zoology, 95 (2), 171-212.
Kalthoff, K. (1996). Analysis of biological development. New York: McGraw-Hill.
Nance, J., Lee, JY, & Goldstein, B. (2005). Gastrulation in C. elegans. In WormBook: The Online Review of C. elegans Biology. WormBook.