Initial Stage Characteristics
Gastrulation is preceded by cleavage. In amphibians, the original egg is mesolecithal, meaning it contains a moderate amount of yolk concentrated at one end—the vegetal pole.
Due to this uneven distribution of yolk, zygote cleavage is specific: it is complete (holoblastic), but unequal and asynchronous. At the nutrient-heavy vegetal pole, cell division occurs much slower than at the opposite pole. As a result, an amphiblastula forms—a multilayered embryo with pronounced asymmetry.
The cells of the animal pole (blastula roof) are small, while the cells of the vegetal pole (blastula floor) are very large. The primary cavity (blastocoel) is also displaced toward the animal pole roof.
Mechanisms of Cell Movement
Amphibian gastrulation is driven by two interrelated mechanisms resulting from differential rates of cell division.
- Epiboly (spreading). This is the primary mode of gastrulation in amphibians. Small, rapidly dividing cells of the animal pole actively crawl over and gradually overgrow the slowly dividing, large, yolk-rich cells of the vegetal pole.
- Invagination (folding inward). This supplementary mechanism starts at the marginal zone between the animal and vegetal regions of the amphiblastula. A characteristic structure—the gray/dorsal lip of the blastopore (or blastoporal groove)—appears on the surface of the embryo here.
During this process, animal pole cells not only overgrow the embryo externally but also turn inward over the lips of the blastopore, actively migrating into the interior.
Dynamics of Internal Cavities
As cells move and internalize, the internal architecture of the embryo undergoes dramatic changes involving space redistribution:
- Gastrocoel (archenteron cavity). The primary invagination deepens, forming a brand-new cavity. As cells migrate inward, the gastrocoel expands.
- Blastopore. The opening that leads into the gastrocoel, functioning as the embryonic blastopore.
- Blastocoel (primary cavity). The original blastula cavity is displaced by the expanding gastrocoel, shifting laterally and continuously decreasing in volume.
The outcome of this stage is the internalization of the vegetal region, which fills about half of the gastrocoel volume. All three germ layers, including the mesoderm, are established within the newly formed gastrula wall.
Histological Appearance of the Gastrula
When examining a microscope slide (such as a frog gastrula), the results of these cell movements and embryonic poles are clearly visible:
- Animal pole is represented by a layer of small, actively dividing cells.
- Vegetal pole is formed by massive, large cells packed with yolk inclusions.
The cross-section clearly demonstrates the spatial relationship of the cell masses, showing small cells enveloping the large ones. A deep, slit-like space—the gastrocoel—is clearly visible between these two cell populations.