Blastopore

The blastopore is a line of invagination on amphibian embryos for gastrulation (1). After fertilization, the amphibian zygote divides to form a mass of cells called blastomeres. The upper surface is called the animal pole, and the lower surface is called the vegetal pole. During gastrulation, the cells at the vegetal pole move inside the embryo, and the cells of the animal pole are left to form the outer surface of the embryo. The animal cells left at the surface will form the ectoderm, including the nervous system. The cells that move inside during gastrulation form the mesoderm and endoderm.

The process of gastrulation begins with the formation of a small indentation between the animal and vegetal poles on one side of the embryo. This small indentation is the beginning of formation of the blastopore. As gastrulation proceeds, the blastopore expands laterally to form a crescent, and eventually it encircles the entire embryo as gastrulation proceeds. The vegetal cells and the marginal cells between the vegetal and animal cells invaginate through the blastopore and form a second layer inside the embryo. As gastrulation continues, the blastopore shrinks to cover the remaining vegetal cells, called the yolk plug. When the blastopore almost completely covers the yolk plug, organogenesis begins. Interactions between various groups of cells during gastrulation cause cells to become determined to form various organs and structures. The determination of cells caused by interactions with their neighboring cells is called developmental induction. One of the most well-known examples is the organizer region of the amphibian embryo that specifies the determination of the nervous system. This is the dorsal lip of the blastopore and is often called the Spemann organizer, because it was first characterized by Hans Spemann (2).

References

1. B. I. Balinsky (1975) An Introduction to Embryology, 4th ed., W. B. Saunders, Philadelphia, pp. 161–164. 

2. H. Spemann (1938) Embryonic Development and Induction, Yale University Press, New Haven.