Developments in Embryology
Developments in Embryology
At the beginning of the twentieth century, embryologists, following the principles established by Wilhelm Roux (1850-1924) and Hans Adolf Eduard Driesch (1867-1941), were actively investigating the question of how factors intrinsic or extrinsic to an egg could govern the development of the embryo. Hans Spemann (1869-1941) refined the techniques of experimental embryology and carried out systematic studies of embryonic development. The "organizer experiment," performed by Spemann's doctoral student, Hilde Mangold, demonstrated that, when the embryonic region known as the dorsal lip was grafted onto a host embryo, it induced the formation of a new embryo. Moreover, the secondary embryo was composed of a mosaic of cells from the host and the donor. The dorsal lip region was, therefore, called the "organizer region."
Wilhelm Roux, who saw himself as the founder of a new discipline that he called "developmental mechanics," argued that embryologists must adopt experimental methods as the tools that would make possible the analysis of the immediate causes of development. The primary question Roux posed was whether development proceeded by means of self-differentiation or correlative dependent differentiation. Self-differentiation was defined as the capacity of the egg or of any part of the embryo to undergo further differentiation independently of extraneous factors or of neighboring parts in the embryo. In turn, correlative dependent differentiation was defined as being dependent on extraneous stimuli or on other parts of the embryo. These were operational definitions that could be tested by experiment, that is, by transplantation and isolation. Based on his most famous—but seriously flawed—experiment, Roux believed that self-differentiation served as the mechanism of development. In other words, Roux tended to visualize the fertilized egg as similar to a complex machine and development as a process that involved the distribution of parts of the machine to the appropriate daughter cells. In his experiment, Roux destroyed one of the cells of a frog embryo at the two-cell stage. The undamaged cell then developed into a half-embryo. This result supported Roux's belief that each cell normally develops independently of its neighbors and that total development is the sum of the separate differentiations of each part.
When other scientists repeated Roux's experiment with embryos of different species, they obtained quite different results. Nevertheless, as late as the 1930s embryologists still cited Roux as the scientist who had formulated the core questions of embryology. Hans Driesch, however, argued that his results provided definitive evidence against Roux's model of development. Driesch used sea urchin eggs instead of frog embryos and was able to separate early embryonic cells by shaking. The separated cells developed into embryos that were normal in configuration but half the size of their normal counterparts. Driesch's experiments suggested that at some fundamental level all the parts of the embryo were uniform; therefore, the fate of any given cell was a function of its relative position to the whole.
Roux and Driesch's followers were able to plan and perform increasingly subtle experiments on living embryos by means of isolation, transplantation, and tissue culture. Hans Spemann in Germany and Ross G. Harrison (1870-1959) in the United States were especially important for their role in refining the techniques of experimental embryology. Spemann carried out systematic studies of early organ determination, whereas Harrison established the experimental foundations of tissue culture and neurogenesis.
Hans Spemann was the son of Johann Wilhelm Spemann, a prosperous publisher and bookseller in Stuttgart. Spemann left school when he was nineteen and joined his father's business. After a year of military service, Spemann became fascinated by the writings of Goethe and Ernst Haeckel (1834-1919) and decided to resume his education. In 1891 he was admitted to the University of Heidelberg as a medical student. His mentors eventually convinced him to give up medicine and study general biology, comparative anatomy, and embryology at the Zoological Institute of the University of Würzburg with the cytologist Theodor Boveri (1962-1915), who introduced him to the use of the amphibian embryo as a model system. In 1895 Spemann received his degrees in botany, physics, and zoology. While enduring a rest cure for tuberculosis, Spemann read August Weismann's book The Germ Plasm: A Theory of Heredity (1892). He began to formulate an experimental approach, based on Roux's work, to the question of the relationship between the reproductive cells and embryological development and differentiation. Spemann, however, selected the salamander as his model system and developed his own methods of separating and rearranging the cells of the early embryo. His studies led to a series of papers called "Developmental Physiological Studies on the Triton Egg" (1901-1903) that introduced the technique of manipulating and constricting the egg with a loop of fine baby hair. Spemann had found that, if he constricted fertilized salamander eggs without completely separating the cells, he could produce animals with one trunk and tail but two heads. Passages in his autobiography reveal his fascination with watching the behavior of such "twin embryos." The mystery of this result and the pleasure derived from such experiments led to his total commitment to embryological research.
Spemann became co-director of the Division of Developmental Mechanics at the Kaiser Wilhelm Institute for Biology in Dahlem (a suburb of Berlin) in 1914. Although the institute provided good research facilities and stimulating colleagues, World War I delayed construction of Spemann's own laboratory and malnutrition related to the war had a severe impact on his health. In 1919 Spemann became the director of the Zoological Institute of the University of Freiburg.
Spemann called his work the study of the "physiology of development." The goal of his research program was to discover the precise moment when a particular embryonic structure became irrevocably determined in its path towards differentiation. The excitement generated by his results made the idea of determination a dominant theme in development research for many years. His colleagues and students considered him a master of the art of microdissection. Spemann guided his associates through a series of experiments in which selected parts of one embryo were transferred to a specific region of another. By using embryos of different species or varieties, Spemann could rely on color differences to follow the fate of the transplanted area during development.
Hilde Proescholdt, who had joined Spemann's laboratory in 1920, was assigned the task of transplanting a region known as the dorsal lip. In 1921 Hilde Proescholdt married Otto Mangold, who was also one of Spemann's students. Hilde Mangold appears as the second author of the landmark paper "Induction of Embryonic Primordia by Implantation of Organizers from a Different Species" (1924). Although Spemann's other students had been allowed to publish their thesis work as sole authors, Spemann insisted on adding his name to Hilde Mangold's thesis publication; he also insisted on being first author. After the discovery of the organizer region, Hilde Mangold abruptly disappears from accounts of the history of embryology. She died in 1924, at the age of 26, from severe burns caused by the explosion of a gasoline heater.
The "organizer experiment," performed by Hilde Mangold as her Ph.D. thesis, has been called the culmination of Spemann's achievements. When the region known as the dorsal lip was grafted onto a host embryo, it seemed to lead to the formation of a new embryo. Mangold's experiment involved grafting the dorsal lip from an unpigmented species onto the flank of a pigmented host embryo. Three days after the graft, an almost complete secondary embryo appeared on the host embryo.
Mangold proved that the secondary embryo was composed of a mosaic of host and donor cells. Therefore, the transplanted dorsal lip and the host embryo both participated in the formation of the secondary embryo. Because the tiny amount of tissue from the donor embryo was powerful enough to cause the formation of a new embryo, the dorsal lip was called the "organizer region." Keeping such embryos alive was very difficult, but Mangold described five experimental cases in detail and briefly noted several others. Some years later, using antibiotics and special growth media, Johannes Holtfreter and his coworkers were able to keep similar embryos alive into even more advanced stages. Remarkably, the secondary embryos in Holtfreter's experiments were as complete as the primary embryos of the hosts.
Through various refinements of the organizer experiment, Spemann discovered other organizer regions. He expected that this approach would eventually lead to a complete understanding of embryonic development. Spemann's discovery of the organizer and his concept of induction can be seen as the culmination of the approach advanced by Roux and Driesch. In 1935, the year in which he retired from the University of Freiburg-im-Breisgau, Spemann was awarded the Nobel Prize in physiology or medicine for his discovery of the "organizer effect" in the induction of embryonic development. One year later, Spemann published his final account of his ideas and experiments in Embryonic Development and Induction.
In the 1930s other embryologists demonstrated that the organizer region could induce a secondary embryo even after it had been killed by heating, freezing, or alcohol treatment. Spemann had previously discovered that the organizer was active after its cells had been killed by crushing but had not pursued an explanation for such observations. The idea that dead embryonic tissues could induce differentiation had a profound impact on experimental embryology, suggesting new biochemical approaches to development and differentiation. Investigators found that various bits of animal tissues—alive or dead—could induce complex structures, including heads, internal organs, and tails. In Embryonic Development and Induction, Spemann complained that a "dead organizer" was a contradiction in terms. Other scientists, however, hoped to find some "magic molecules" released by the inducing tissues and to discover the basis for their organizing and inducing powers. The failure to find such molecules eventually led to a loss of interest in organizer regions, but organizer theory had helped to stimulate a chemistry-based approach to embryology. The explosion of discoveries that followed the identification of DNA as the genetic material would make possible an entirely new approach to solving the fundamental questions of embryology.
LOIS N. MAGNER
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