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Spemann's Lens Induction and Constriction Experiments
Embryology was rather a new scientific field back in 1897 when Hans Spemann began his scientific experiments and while he did base his early work on ideas explored before, his discoveries refuted many preconceived notions and led to a better understanding of how life evolved.
In 1910-1912, he carried out lens induction experiments and attempted to analyze eye lens development in newts. He reached the conclusion that the optic cup influenced the newt eye lens development in the ectoderm cells. But then the Czech embryologist Emmanuel Mencl and the American embryologist Helen King reported free lens development in their respective experiments with the Rana palustris species. The debate between induction and self-differentiation in eye lens development ended when Spemann, with further experiments, discovered that both were possible in amphibians.
Spemann’s constriction experiments of 1897-1905 were technically simple. Using a fine baby hair from his daughter Margrete to tie a knot between two blastomeres of a two-celled salamander embryo, Spemann created a constriction that separated the two parts. He observed that -
- Each part developed separately and produced two embryos smaller than the original.
- If the constriction wasn't tight enough, the two developed embryos fused together. Sometimes the Duplicitas anterior malformation developed, producing an embryo with two heads and a common tail.
- The plane of division played an important role in how the embryos would develop.
- If the constriction separated the dorsal and ventral sides, the dorsal side developed into an embryo while the ventral side produced a mass of cells. This indicated an existing difference between the two regions.
- Regulative capacity of embryos decreased in later developmental stages.
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Spemann's Transplantation and Nuclear Somatic Transfer Experiments
To discover at which stage irreversible determination took place in embryos, Spemann carried out transplantation experiments in 1915-1918 and found that –
- Transplants from early gastrula stage embryos took on the characteristic of the host – which meant, their fate hadn’t been determined.
- Transplants from end of gastrula stage embryos preserved their histological specificity – which meant irreversible determination had taken place.
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Spemann’s experiments with embryos with developed blastopores (exterior mouth opening of a hollow sphere of undifferentiated cells) led him to believe that, in these, it was the transplanted blastopore lip that influenced the developmental fate of host cells. But, as graft and host cells came from similarly pigmented species, it was hard to determine conclusively whether it was the host or the graft that perpetuated the secondary axis. Taking a page from the American scientist Ross Harrison, who in 1903 had successfully carried out embryonic grafts between different frog species, Rana palustris and Rana sylvatica, Spemann decided to use different pigmented newt species, the white Triton cristasus and the dark Triton taeniatus and Triton alpestris. He assigned the actual experimentation to his PhD student Hilde Proescholdt (later Mangold) as her doctoral dissertation. Of the hundreds of embryos she experimented on only five survived, and in these, histological sections showed clear distinctions between the graft and host cells; the graft cells formed mesodermal elements of a secondary embryo under the gastrula surface and above the host ectoderm cells gave rise to the neural tube of the secondary embryo. Spemann published a paper detailing this experiment in 1924.
In 1928, Spemann carried out the first ever somatic nuclear transfer, a step in the direction of cloning. He transferred the nucleus of a differentiated salamander embryo into an enucleated single-cell salamander embryo. The cell adapted to the nucleus and developed into a normal salamander. Spemann next separated a salamander embryo nucleus from the cytoplasm with a hair knot constriction and allowed the nucleus side to develop into a sixteen-cell embryo. Then, moving the nucleus into the cytoplasm side, he tightened the constriction and let the cells on this side divide to form an embryo. The twin embryos developed into normal salamanders. Spemann detailed these experiments in his 1938 book 'Embryonic Development and Induction'.
This success led Spemann to speculate whether the nuclear transfer method experiment might also work using nuclei from older embryos or even adult organisms. He didn't manage to technically bring this about, but Thomas King and Robert Briggs proved him right in 1952.