One of the most exciting stories in animal development is the discovery of how cells in the embryo become committed to their specific fate. After a sperm joins with an egg, the zygote goes through an initial series of rapid cell divisions that subdivides the cytoplasm into a mass of smaller undifferentiated cells. Cell fate-regulating molecules in the form of mRNAs or proteins become asymmetrically distributed starting at the one-cell stage. This uneven distribution of molecules is maintained through successive divisions and provides positional information that results in the determination of cells—their commitment to a particular role in the body plan. An orderly series of cell movements, called gastrulation, then creates multiple cell layers and sets up new cell-to-cell contacts that trigger further steps of determination.
In the accompanying animation, we examine the experiments conducted by German biologist Hans Spemann and his student Hilde Mangold in which they determined how the embryo becomes organized.
Much of our understanding of induction comes from work that began on amphibian embryos in the early twentieth century and has continued since. When Hans Spemann and his student Hilde Mangold were studying the dorsal lip of the blastopore in frog eggs, they transplanted this region to another embryo at the same stage of development. The results were momentous. This small piece of tissue stimulated a second site of gastrulation, and a second complete embryo formed. Because the dorsal blastopore lip of amphibians was apparently capable of inducing host tissue to form an entire embryo, Spemann and Mangold dubbed it the organizer, and its action became known as primary embryonic induction. We now know that the dorsal blastopore lip is not responsible for the first inductive event in development, but because of its extreme importance, it is still referred to as the organizer. For more than 80 years, the organizer has been an active area of research.
Textbook Reference: Concept 38.4 Gastrulation Sets the Stage for Organogenesis and Neurulation in Chordates