Briggs & King clone tadpoles.
|In 1952, Robert
Briggs and Thomas
cloned northern leopard frogs using a method of nuclear
kicked off nearly two decades of heavy scientific
interest in cloning research. Though at the time they
were unaware of the earlier work of Hans Spemann, Briggs' and King's experiment
was similar to that envisioned by Hans Spemann in his
1938 proposal for a "fantastical
Robert Briggs envisioned the cloning experiment as a way to study the activation and deactivation of genes during cell development. He intended to transfer the nucleus of a blastula cell, which is an embryo cell during the period in which a young embryo consists of only around eight to sixteen thousand cells, into a fertilized egg whose nucleus had been removed. Thomas King was found by Briggs to actually perform the experiment.
Using a glass pipette wider than the cell's nucleus, but smaller than the width of the cell, King extracted the nucleus of a blastula cell. Because the outer part of the cell was wider than the pipette, it was crushed and broke away from the nucleus as the cell was sucked into the pipette, leaving only the cell's nucleus intact inside the pipette. Next, the nucleus of the fertilized egg was extracted using a glass needle. An incision was then cut into the egg's coating and the blastula cell nucleus was inserted into the egg.
Briggs' and King's early attempts failed, but by the project's completion they had successfully cloned thirty-five complete embryos and twenty-seven tadpoles from one-hundred and four successful nuclear transfers. Continuing their research, Briggs and King attempted to clone tadpoles using the nuclei of older embryo cells and even differentiated cells. They found as the embryo cells developed it became much more difficult to produce clones from them. The few tadpoles cloned from differentiated cells that survived to become tadpoles grew abnormally. Their results led Briggs and King to believe that genetic potential diminished as a cell differentiated and that it was impossible to clone an organism from adult differentiated cells.