Gastrulation in amphibians

Characteristics of the initial stage

The gastrulation process is preceded by the cleavage stage. In amphibians, the original egg cell is polylecithal and moderately telolecithal, meaning it contains a large amount of yolk displaced to one side — the vegetal pole.

Due to this uneven yolk loading, zygote cleavage proceeds specifically: it is total, but unequal and asynchronous. At the vegetal pole, overloaded with nutrients, cell division is significantly slower than at the opposite pole. As a result, an amphiblastula is formed — a multi-layered embryo with pronounced asymmetry.

Cells of the animal pole (blastula roof) are small, while cells of the vegetal pole (blastula floor) are very large. The primary cavity itself is also displaced toward the roof, closer to the animal pole.

Mechanisms of cell movements

Gastrulation in amphibians is realized through two interrelated mechanisms driven by different cell division rates.

1. Epiboly (overgrowth). This is the main method of gastrulation in amphibians. The essence of the process is that small cells of the animal pole, which divide very rapidly, actively begin to "crawl over" the vegetal part. They gradually overgrow the slowly dividing, large, yolk-rich cells of the vegetal pole.
2. Invagination (infolding). This additional mechanism starts at the border between the animal and vegetal parts of the amphiblastula. It is here that a characteristic structure appears on the embryo surface — the gray crescent.

During the process, the cells of the animal pole not only overgrow the embryo externally but also fold over the edges of the gray crescent, actively penetrating inward.

Dynamics of internal cavities

As cells move and infold inward, the internal architecture of the embryo changes cardinally. A redistribution of spaces takes place:

• Gastrocoel (archenteron cavity). The primary invagination deepens, and a completely new cavity is formed. As cells penetrate inward, the gastrocoel becomes larger.
• Blastopore. This is the entrance opening leading into the gastrocoel. It functions as the primary mouth of the embryo.
• Blastocoel (primary cavity). The former cavity of the amphiblastula is displaced by the growing gastrocoel. The blastocoel shifts sideways and continuously decreases in volume.

The outcome of this stage is the immersion of the vegetal part into the interior of the embryo, where it fills about half of the gastrocoel volume. All three germ layers, including the middle one, are laid down in the wall of the formed gastrula.

Histological picture of the gastrula

When studying a micropreparation (e.g., a frog gastrula), one can clearly visualize the results of the described cell movements and differentiate the poles of the embryo.

• Animal pole is represented by a layer of small, actively dividing cells.
• Vegetal pole is formed by massive, large cells whose cytoplasm is rich in yolk inclusions.

In cross-section, the spatial relationship of cell masses is clearly traceable: one can see how small cells overgrow large ones. Between these two groups of cells, a deep slit-like space is defined, which represents the formed gastrocoel.

Class and exam questions

What else to study on this topic

• Formation of the three germ layers in the gastrula wall
• Structure and functions of the blastopore (primary mouth)
• Detailed analysis of the frog gastrula micropreparation
• Effect of yolk on the division rate of vegetal pole cells

Other topics in "Gastrulation, formation of axial organ rudiments, and extraembryonic membranes"

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