Animation 42.4: The Primary Embryonic Organizer

INTRODUCTION

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.

Video titled: Animation 42.4: The Primary Embryonic Organizer

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CONCLUSION

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.

Textbook Reference: Key Concept 42.3 Gastrulation Generates Multiple Tissue Layers

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