Gastrulation

Formation of Germ Layers

During the migration of cells and entire sections of the blastula wall, the fundamental layers of the embryo, known as germ layers, are formed. All types of tissues and organs will subsequently develop from this initially relatively homogeneous cellular mass.

There are three main germ layers:

• Ectoderm (from Greek ectos — outside) is the outermost germ layer of the embryo.
• Endoderm (from Greek entos — inside) is the inner germ layer.
• Mesoderm (from Greek mesos — middle) is the third, or middle, germ layer, developing between the first two.

Main Methods of Gastrulation

Depending on egg structure and animal species, biology distinguishes four basic types of gastrulation, as well as their combinations.

1. Invagination (folding inward). In this mechanism, a specific area of the blastoderm at the vegetal pole begins to bend inward until it reaches the opposite animal pole. This forms a bilayered gastrula. The internal cellular layer (endoderm) thus lines the cavity of the archenteron, or gastrocoel. The opening connecting this cavity to the external environment is called the blastopore (primitive mouth). In protostomes (worms, mollusks, arthropods), the blastopore turns into the mouth opening. In deuterostomes (chordates, e.g., amphioxus), the anus forms at its site, while the true mouth breaks through at the opposite end of the body.

1. Immigration (cell migration). Some blastoderm cells leave the embryo surface and move inward into the blastocoel cavity. Gradually, the blastocoel fills with these cells, forming an inner layer (endoderm) directly beneath the outer ectoderm. This type of development is characteristic of cnidarians.

1. Epiboly (overgrowth). The process is driven by the rapid proliferation of small cells (micromeres) that overgrow the vegetal pole externally. As a result, larger cells (macromeres) end up inside the embryo. An important feature of epiboly is that no blastopore is formed and there is no gastrocoel. It occurs in amphibians and animals developing from telolecithal eggs.

1. Delamination (splitting or delamination). Blastoderm cells divide in such a way that a single sheet splits into outer (ectoderm) and inner (endoderm) layers. This pathway is also characteristic of cnidarians.

A mixed type—a combination of different basic methods—is often found in nature. For example, birds and mammals exhibit a combination of immigration and delamination, while amphibians show epiboly combined with invagination.

Mesoderm Formation

The third germ layer (mesoderm) forms via two fundamentally different pathways, serving as an important criterion in animal classification.

Transition to Histo- and Organogenesis

Immediately after the completion of gastrulation and mesoderm formation, the stage of histo- and organogenesis begins. Histo- and organogenesis (from Latin histos — tissue) is the formation of specialized tissues and organs through cell differentiation.

Differentiation refers to the process of emergence and progressive increase of differences between cells of the developing embryo. These differences involve three aspects:

• Morphological (changes in shape and structure);
• Biochemical (synthesis of specific proteins);
• Functional (acquisition of narrow specialization).

The pattern of organogenesis is such that axial organs are always laid down first, followed by all others. Notably, in amphioxus, the mesoderm forms not before, but simultaneously with the axial organ complex during organogenesis.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Morphological, biochemical, and functional differentiation of cells
• Histogenesis and organogenesis: sequence of system development
• Formation of the axial organ complex in deuterostomes
• Specifics of embryonic development and notochord formation in amphioxus

Other topics in the section «Periodization of Ontogenesis»

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