One of the most important ideas in developmental biology over the last 75 years was put forth by Hans Spemann based on transplantation in amphibian embryos. This is the idea that certain regions of the embryo, called organizers, are responsible for instructing other tissues to follow particular developmental pathways (1). The particular region that Spemann identified, the dorsal lip of the blastopore, is usually referred to as the Spemann organizer in his honor.
Early in embryogenesis, when the amphibian embryo consists of a ball of cells called the blastula, the formation of an invagination called the blastopore leads to the formation of the three germ layers. These three germ layers—the endoderm, mesoderm, and ectoderm—are responsible for forming the various tissues and organs of the embryo. The ectoderm will form both the epidermis and most of the nervous system, but requires a signal from the cells of the dorsal lip of the blastopore in order to form the neural tube. In a series of classic experiments, Mangold and Spemann transplanted cells from the dorsal lip of the blastopore of a donor salamander embryo to a region on the opposite side of a host embryo of a different species of salamander (the two different species were used to allow the donor and host tissues to be distinguished after the transplantation). Not only did a neural tube develop in the normal position, but a second neural tube formed near the transplanted dorsal lip cells. This second neural tube was partially formed from cells in the host embryo that had been instructed, or induced, by the transplanted cells. Similar induction events have been shown to be important in the formation of other ectodermal structures, such as the lens of the vertebrate eye (2).
Since the Spemann organizer region was first described, many experiments have been done to identify the molecules involved. A large number of molecules have been shown to induce when misexpressed, but the nature of the real signal is still in debate. Several different signaling pathways have been implicated in the Spemann organizer, including the wingless Wnt signaling pathway (3-5), the transforming growth factor b pathway (6, 7), and the fibroblast growth factor pathway (8, 9). The same signaling pathways have also been shown to be involved in developmental induction in Drosophila (10, 11). Although the induction events in the embryo, both in amphibians and in Drosophila, appear to involve signaling between a number of different tissues and several interrelated cascades of gene interactions, the combination of developmental and molecular experiments has come a long way in furthering our understanding of the nature and function of organizer regions in development.
