## Amphibians make it possible to observe embryogenesis outside the mother's body

The fact that many amphibians undergo fertilisation of eggs and subsequent development of the embryo outside the mother's body makes them an excellent model for developmental biology. It should also be added that many species of amphibian (e.g., the clawed frog Xenopus laevis) produce a large number eggs that are quite big in size, which makes it possible to obtain a substantial amount of experimental material.

After fertilisation, the cells divide for several generations synchronously, so it is possible to study what happens during the individual phases of the cell cycle. After the multiplication of the mass of cells, a spherical structure of the blastula is formed, which is comprised of undifferentiated cells covered by a single layer of cells. Subsequently, the creation of the so-called gastrula, which results in three germ layers (ectoderm, endoderm and mesoderm), from which individual tissues later develop (Figure 20.8).

Gastrulation begins in a specific location of the blastula, which is referred to as the "organiser". Cells from the organiser region migrate into the blastula and eventually form the three-layered gastrula. But how do the cells in the organiser area "know" to start migrating? This interested a German embryologist, Hans Spemann (1869 – 1941), and his student Hilde Mangold (1898 – 1924). They hypothesised that a substance is formed in the organiser area that provides the cells with the instruction to start migrating. To test their hypothesis, Mangold transplanted an organiser from an embryo that had just begun gastrulation into the blastula of another embryo. She observed that in the second embryo, gastrulation began in two places: in the area of the original and the area of the implanted organiser. The result was two mirror-orientated connected gills (Figure 20.8). The identification of the substance that is the inducer for starting gastrulation took a few more years, but it turned out that it is basically the same in both amphibians and mammals. The Nobel Prize for Spemann in 1935 underlined the importance of this discovery for human medicine.

John B. Gurdon also used the advantages of working with amphibians in his experiments, where he proved that the nucleus of differentiated body cells can be "reprogrammed" to the state in which it was in the fertilised egg stage. In his experiments, Gurdon removed the nucleus from the differentiated body cell of the frog and introduced it into an unfertilised egg, the nucleus of which he had previously destroyed. Subsequently, he stimulated the egg cell with the "new" nucleus to divide, and the result was that embryogenesis started, leading to a frog that was a genetic clone of the nucleus donor (Figure 20.9). This experiment indicated that it should be possible to clone even more complex animals. Considering that the whole procedure is not technically trivial, it took several years before Gurdon's experiment was also carried out in mammals. The result was Dolly the sheep, whose nucleus came from the udder cell of an adult sheep. The fact that the nucleus of a differentiated body cell can be reprogrammed opened the way for an experiment whose goal was to find out what the molecular mechanism of this "dedifferentiation" is. Japanese biologist Shinya Yamanaka identified these reprogramming factors in human cells. He discovered that the activation of these factors in body cells leads to a change in their properties so that, similar to embryonic cells, they become so-called pluripotent. This means that such reprogrammed cells (induced pluripotent cells), like embryonic cells, are capable of giving rise to many types differentiated cell. This opened the door to the so-called therapeutic cloning, in which, for example, it is possible to take a population of body cells from the patient's body, reprogram them to be pluripotent, correct genetic disorders in them and, after re-differentiation, bring them back to the patient and thus help him heal. Gurdon's experiments on amphibians thus led to a model that Yamanaka used to make a discovery with previously unimaginable therapeutic potential. Both therefore deservedly shared the Nobel Prize for Physiology or Medicine in 2012.