![]() ![]() The thesis subject was the development of the liver and pancreas of the frog embryo, and Holtfreter commented that this subject was not of great interest to Spemann or himself.ĭuring this time, he shared a laboratory bench with Hilde Mangold (nee Pröscholdt), who was in the process of discovering the amphibian gastrula organizer, a discovery later acknowledged in the award of the 1935 Nobel Prize to Spemann. Nevertheless, Holtfreter began studying embryology and in 1924 received a doctoral degree in natural sciences based on thesis research completed in Spemann's laboratory. His replacement was Hans Spemann, whose work as the preeminent embryologist of the time was unknown to Holtfreter. However, the professor died shortly before Holtfreter arrived. He pursued natural science at the Universities of Rostock and Leipzig from 1917 to 1919 and then transferred to the University of Freiburg, attracted by the hiking and skiing in the area and by the possibility of working with a renowned naturalist on the faculty (Professor Doflein). ![]() Yet he felt confirmed in his inclination as an incipient field biologist. Analysis of the three kinds of region-specific morphogenetic activities of cells in the amphibian gastrula and the integration of this information into a unified view of gastrulation (1942–43).Discovery of cell sorting and analysis of tissue affinities and tissue segregation in embryos (1939, 1955).Analysis of the role of the notochord and somites in shaping the floor plate and walls of the neural tube (1933).Analysis of minimal conditions of pH extremes, Ca ++ depletion, and hypotonicity (sublethal cytolysis) to obtain neural development in ectodermal fragments (1944–51).The use of interspecies (xenoplastic) grafting experiments (urodele-anuran) to demonstrate the species-specific competence of tissues to respond to the organizer's signals, yet the cross-species commonality of the organizers inductive signals (1935–36).These embryos provided evidence that the organizer may exclusively transmit neuralizing signals to the ectoderm by a vertical path in urodeles (1933). Discovery of conditions to produce urodele exogastrulae in which neural tissue does not form.The use of these conditions to test the autonomous differentiation capacity of small clusters of cells from various parts of the urodele or anuran gastrula embryo and the contribution of data to specification maps, competence maps, and distribution maps of head inducers and trunktail inducers in the early gastrula (1938).His improvement of the sandwich assay for inducers, by which the experimentalist can define the responding tissue and control its contact with inducing tissue or extracted test material (1933).His discovery that dead and disintegrated organizer tissue could still induce locally organized parts of secondary axes (1932–38) and his findings that most tissues of embryos and adults of representative members of many animal phyla contain substances that induce neural development, findings that set off an international search for the true inducer.The invention of Holtfreter's medium (a balanced salt solution in which operated embryos and clumps of embryonic cells survive and differentiate) and the introduction of sterile technique (1931).Holtfreter's particular contributions include: Our present-day concepts of secreted inductive signals, cell competence, and cellular morphogenetic activities sprang from Holtfreter's findings and insights. ![]() The signals from the organizer mostly evoke or release this development, rather than provide detailed instructions for it. For embryologists, his research shifted their view from the developing embryo as a supracellular organismal entity to the embryo as a complex population of interacting cells in which the numerous cells surrounding the organizer have a high competence for development, held in a latent state. He initiated and contributed substantially to many lines of experimentation that are still ongoing in the analysis of the embryonic organizer and of embryonic induction. His research was done entirely with amphibian embryos, the favored material of the time. JOHANNES HOLTFRETER WAS the world's foremost experimental embryologist in the decades between 19. Johannes Holtfreter January 9, 1901–November 13, 1992 ![]()
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