Term Paper: History of Embryology

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[. . .] These mutant genes, however, were considered to be alleles at the same locus. The answer to this genetic quandary was to be found in the embryology of the mouse (Burian et al.).

The effect of the two alleles T. And to on notochord and mesoderm might suggest that the two alleles act on two different structures. However, if considered from the embryological point-of-view, the notochord and mesoderm of the mouse have the same origin, namely in the tissue of the wall of the primitive gut. This brought the problem back to what had been thought of as the mammalian equivalent of the dorsal blastopore lip. Gluecksohn-Schoenheimer was trying to do with mutants what Spemann and the Mangolds had done by transplantations. As she would summarize in 1949, "The study of this material makes it very likely that in mammals the notochord plays a role in processes of early organization similar to that of the notochord in amphibians as analyzed with the techniques of experimental embryology." More than that, Gluecksohn-Schoenheimer thought that she could do with the T-locus what Spemann's group and the Cambridge laboratory of Needham and Waddington could not do: Find the inducer molecule, itself. Gluecksohn-Waelsch would later write, "It was therefore hoped that the identification of the mode of action of T-locus genes -- and the nature of their gene products-might provide leads towards the molecular analysis of normal inductive mechanisms."

The T-locus alleles weren't the only mutations that appeared to control induction. The phenotypes caused by another, closely linked, mutation, Kinked were interpreted in terms derived directly from Spemann's work on amphibian embryonic regulation. Homozygous mutants of Kinked were found to have duplications of their dorsal axis, sometimes forming twin embryos.

Their striking resemblance to the double-monsters obtained by constriction experiments of amphibian embryos at the two-cell stage led to the suggestion that an "organizer" region analogous to that identified experimentally in amphibians existed in mammalian embryos and that its normal functioning was severely affected in FuKi / FuKi embryos. There is no doubt that all these interpretations of mutational effect on the developmental mechanism was strongly influenced by the orientation of the particular investigators and their view of development as depending on a series of inductive interactions.

Gluecksohn-Schoenheimer interpreted all three genes (T, to, and FuKi) as disturbing "specific organizer relationships." She interpreted the action of the Kinked gene as causing constrictions analogous to those done experimentally by Spemann and Holtfreter on salamander eggs (Van Speybroeck, De Waele and Van de Vijver). These famous studies had shown that the constriction of the egg down the medial plane caused the formation of two organizers, each of which formed embryonic axes, thereby creating twin larvae. Constriction in the frontal plane, however, caused the formation of one normal larva and one BauchstYch, an amorphous tissue mass consisting chiefly of endoderm and blood cells. Partial constrictions, moreover, caused conjoined larvae, an observation that Spemann had related to mammalian teratology.

According to Gluecksohn-Schoenheimer, the Kinked mutants had an inducing mesoderm that was divided in two, just like Spemann's and Holtfreter's constricted embryos. The duplicated axes formed when this constriction was in the medial plane, and the BauchstYch-like mass seen in several of the Kinked embryos also "might well be the result of a frontal constriction."

Gluecksohn-Schoenheimer was aware of her integrating embryology and genetics. She announced that her research on the Kinked gene "was undertaken both from the point-of-view of the embryologist interested in the causal analysis of development and that of the geneticist interested in the analysis of gene effects." Linking organizers to genes meant linking embryology to genetics.

During this investigation of axial development, other tailless or short-tailed mutant mice were found. One of these tailless mutants was due to the Sd/Sd genotype that also caused the lack of kidneys. Gluecksohn-Schoenheimer wrote that when confronted with such cases, the developmental geneticist must reverse the order of the experimental embryologist and work backward from effect to cause. She demonstrated that the ureteric bud normally grew into the area of the metanephrogenic mesenchyme. When that occurred, the ureter continued to grow and branch, and the mesenchyme condensed into tubules. In the Sd/Sd mutant, however, the ureteric bud failed to reach the mesenchyme and no kidney was formed. In Sd/+ heterozygotes, some tips of the short ureteric bud did find there way into the metanephrogenic mesenchyme, and a small kidney resulted. These findings indicate strongly the existence of an inductive relationship between the ureter and kidney -- such as has been shown experimentally to exist in other vertebrates.

In Gluecksohn-Schoenheimer's work during this period, there is a reciprocity between genetics and embryology. Genetics could be used to analyze development in areas where experimental techniques had not yet been perfected. Embryology could identify the effects of these genes whose functions were necessary for the construction of the embryo. These early embryonic abnormalities "represent the end result of a chain of events at the beginning of which stands the gene. The analysis of the action of the gene is our ultimate goal."

This programmatic statement of 1945 differs from that of 1938. In 1938, the developmental geneticist was content to draw conclusions on the nature of the "experiment" performed by the genes. Now, the further charge was to understand the nature of gene activity. But Gluecksohn-Schoenheimer's goal would have to await the techniques of molecular biology. She did not start analyzing gene activity until the mid-1970s when she turned the direction of her laboratory from morphological mutations in mice to the analysis of the biochemical defects caused by the deletion of a specific portion of mouse chromosome 7. In Salome Gluecksohn-Schoenheimer's research on developmentally lethal genes, we see the enormous role that experimental embryology, especially Spemann's constriction and transplant experiments, had in the propounding of developmental genetics.

Conrad Hal Waddington identified himself as a student of "diachronic biology," a science of "embryology-genetics-evolution which again form a group whose interconnections are obvious and unavoidable." That Waddington did not see these three disciplines as distinct entities is reflected in his peripatetic training as a biologist. After graduating with a first class degree in geology from Cambridge University in 1926, he began pursuing graduate work in paleontology. His thesis work involved analyzing ammonites, a group of extinct cephalopods that, he would later claim, "forces on one's attention the Whiteheadian point that the organisms undergoing the process of evolution are themselves processes. The whole developmental process is preserved so that one cannot avoid examining it. Waddington admitted to being very much influenced by Whitehead during his last two years as an undergraduate, and his research in paleontology was partially funded by a philosophy studentship. This work in paleontology did not lead to a PhD. He had met Gregory Bateson, and the friendship between these men caused Waddington's interest to move from paleontology towards genetics. However, Waddington felt that it was not possible to earn a living as a geneticist in Britain during the 1920s, so he looked elsewhere.

In 1929, Dame Honor Fell, director of the Strangeways Laboratory, was told about a "bright young paleontologist (who also had a scholarship in philosophy) who had been reading Spemann's papers and wondered if the organ culture method developed here could be used for the experimental study of avian and mammalian embryos." Indeed, she was impressed with Waddington's ideas, and soon Waddington began working at the Strangeways Laboratory. By 1930, at the International Congress of Experimental Cytology in Amsterdam, Waddington was able to present his first results on culturing chick embryos. Another participant at this congress, Richard Goldschmidt, invited Waddington to come to Germany. However, when Waddington received funds to work in Germany, he decided to work with Otto Mangold "for the purpose of learning the technique of amphibian operations," rather than to pursue genetics in Goldschmidt's laboratory. Mangold, a former graduate student of Spemann who was now his collaborator, was working on the problems of neural induction in amphibians. Waddington would adopt this set of problems for himself. From 1932 to 1938, Waddington attempted to clarify the nature of amphibian neural induction and to transfer the techniques of amphibian experimental embryology to the study of chick development.

The events up to this point describe in detail the origins of this discipline that is embryology. Further significant developments took place after this place. However, it was the events of the late 1800's and early 1900's that clearly delineated this field from others.

Works Cited

Burian, R.M., et al. "Selected Bibliography on History of Embryology and Development." Hist Philos Life Sci 22.3 (2000): 325-33.

Churchill, F.B. "The Rise of Classical Descriptive Embryology." Dev Biol (N Y 1985) 7 (1991): 1-29.

Horder, T.J. "The Organizer Concept and Modern Embryology: Anglo-American Perspectives." Int J. Dev Biol 45.1 (2001): 97-132.

Kuratani, S., S. Kuraku, and Y. Murakami. "Lamprey as an Evo-Devo Model: Lessons from Comparative Embryology… [END OF PREVIEW]

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