Principles of Development
The Primary Organizer
In the 1920s and 1930s, research in embryology was dominated by one issue: embryonic induction, the capacity of one tissue to influence the developmental fate of another. The new paradigm of induction began with the work of German embryologist Hans Spemann (1869 to 1941) who set out to discover how different parts of an embryo influence one another. In experiments carried out in 1916, Spemann had noted the capacity of tissue transplanted from the dorsal lip of the salamander gastrula to transform the tissue it touched. These delicate experiments were repeated in 1921 and 1922 by his student Hilde Pröscholdt, who, despite great difficulties with the amphibian material used, produced six successful embryos in which the transplanted tissue had induced the host embryo to form a secondary embryo. Spemann designated the dorsal lip tissue the primary organizer because it was the only tissue that had the capacity to organize, by induction, the principal axis of a secondary embryo. The classic experiments were published in 1924 but Hilde, who in the meantime had married the embryologist Otto Mangold, had already died as the result of a household accident. Spemann (above, photographed in his laboratory) was awarded the Nobel Prize in 1935, the only biologist ever cited purely for research in embryology. By demonstrating the central importance of induction, Spemann had ushered in the golden age of embryology, which continued until after World War II when induction research began to yield to studies of genetic control of body form.
How is it possible that a tiny, spherical fertilized human egg, scarcely visible to the naked eye, can unfold into a fully formed, unique person, consisting of thousands of billions of cells, each cell performing a predestined functional or structural role? How is this marvelous unfolding controlled? Clearly all information needed must originate from the nucleus and in the surrounding cytoplasm. But knowing where the control system lies is very different from understanding how it guides the conversion of a fertilized egg into a fully differentiated animal. Despite intense scrutiny by thousands of scientists over many decades, it seemed until very recently that developmental biology, almost alone among the biological sciences, lacked a satisfactory conceptual coherence. This now has changed. During the last two decades the combination of genetics with modern techniques of cellular and molecular biology produced an avalanche of information that solved many questions. Causal relationships between development and evolution have also become the focus of research. We do at last appear to have a conceptual framework to account for development.
In the 1920s and 1930s, research in embryology was dominated by one issue: embryonic induction, the capacity of one tissue to influence the developmental fate of another. The new paradigm of induction began with the work of German embryologist Hans Spemann (1869 to 1941) who set out to discover how different parts of an embryo influence one another. In experiments carried out in 1916, Spemann had noted the capacity of tissue transplanted from the dorsal lip of the salamander gastrula to transform the tissue it touched. These delicate experiments were repeated in 1921 and 1922 by his student Hilde Pröscholdt, who, despite great difficulties with the amphibian material used, produced six successful embryos in which the transplanted tissue had induced the host embryo to form a secondary embryo. Spemann designated the dorsal lip tissue the primary organizer because it was the only tissue that had the capacity to organize, by induction, the principal axis of a secondary embryo. The classic experiments were published in 1924 but Hilde, who in the meantime had married the embryologist Otto Mangold, had already died as the result of a household accident. Spemann (above, photographed in his laboratory) was awarded the Nobel Prize in 1935, the only biologist ever cited purely for research in embryology. By demonstrating the central importance of induction, Spemann had ushered in the golden age of embryology, which continued until after World War II when induction research began to yield to studies of genetic control of body form.
How is it possible that a tiny, spherical fertilized human egg, scarcely visible to the naked eye, can unfold into a fully formed, unique person, consisting of thousands of billions of cells, each cell performing a predestined functional or structural role? How is this marvelous unfolding controlled? Clearly all information needed must originate from the nucleus and in the surrounding cytoplasm. But knowing where the control system lies is very different from understanding how it guides the conversion of a fertilized egg into a fully differentiated animal. Despite intense scrutiny by thousands of scientists over many decades, it seemed until very recently that developmental biology, almost alone among the biological sciences, lacked a satisfactory conceptual coherence. This now has changed. During the last two decades the combination of genetics with modern techniques of cellular and molecular biology produced an avalanche of information that solved many questions. Causal relationships between development and evolution have also become the focus of research. We do at last appear to have a conceptual framework to account for development.
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