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 development.
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.
In amphibians, specific sperm-binding sites are found only in the animal hemisphere, so sperm always enter the egg in that hemisphere. Following fertilization, the dark cortical cytoplasm of the zygote rotates relative to the inner cytoplasm.
The rotation of the darkly pigmented cortical cytoplasm leaves a region of diffusely pigmented equatorial cytoplasm on the side opposite sperm entry. Due to its appearance in the microscope, it is known as the gray crescent, and it will be the site of important developmental events.
What effect do these cytoplasmic rearrangements have on the development of the embryo, and what is the significance of the gray crescent? Such questions caught the attention of biologist Hans Spemann at the turn of the twentieth century.
Working with a recently fertilized salamander egg, Spemann used a fine baby hair to split the zygote in half in the plane of the first cleavage, bisecting the gray crescent. When he did this, he found that both halves of the zygote developed into normal embryos.
In another experiment, however, one half received the entire gray crescent and the other half received none. In this case, the half containing the entire gray crescent developed normally, while the other half became a clump of unorganized tissue lacking dorsal structures. Spemann dubbed this the Bauchstück, or "belly piece."
Why should these two experiments give different results? A hint came from fate maps that showed the gray crescent region gives rise to the cells that initiate gastrulation.
Spemann and his student, Hilde Mangold, next tried transplanting the dorsal lip from one gastrula to another.
Gastrulation in frogs begins when cells in the gray crescent—called bottle cells because of their shape—move inward to form a fold called the dorsal lip.
They transplanted the dorsal lip onto an area destined to become the organism's ventral epidermis.
The transplant stimulated gastrulation at the new site. A second invagination formed, and gastrulation proceeded at both sites.
In fact, development proceeded normally at both gastrulation sites, resulting in two complete embryos attached at the belly.
Spemann and Mangold named the dorsal lip cells the primary embryonic organizer. These cells organized host and donor tissues into a complete secondary embryo. Since its discovery, researchers have studied this region intensively to uncover the molecular basis of its action.
When gastrulation begins, the zygote consists of a mass of undifferentiated cells. Spemann performed a number of transplantation experiments. The results demonstrated that if cells from one part of the embryo are moved to a different location early in gastrulation, the cells will develop into tissues appropriate for their new location. The cells did not "know" their fate; their fate was determined by their surroundings. If the transplants took place late in gastrulation, however, the transplanted cells developed into tissues appropriate for their original location. This demonstrated that during the process of gastrulation, cell fates become determined.
In Spemann and Mangold's famous transplant experiment, they took the dorsal lip of the blastopore from one gastrula and transplanted it onto another gastrula on the ventral side (opposite the endogenous dorsal lip). Both the endogenous lip and the transplanted lip became gastrulation sites, and each developed normally into a complete embryo. The result was two whole embryos joined belly-to-belly. The experiment proved that the dorsal lip of the blastopore was capable of inducing the formation of an entire embryo. Spemann and Mangold therefore dubbed it the primary embryonic organizer. The organizer has been studied extensively in recent years to determine the molecular mechanism of its unique capability.