Technical and ethical challenges abound after first hurdle of taking cells from millennia-old bodies is cleared
Even a well-preserved carcass like this baby woolly mammoth is unlikely to provide viable cells for cloning, as used to create Dolly. Photograph: Aaron Tam/Getty
By Ian Wilmut
It is unlikely that a mammoth could be cloned in the way we created Dolly the sheep, as has been proposed following the discovery of mammoth bones in northern Siberia. However, the idea prompts us to consider the feasibility of other avenues. Even if the Dolly method is not possible, there are other ways in which it would be biologically interesting to work with viable mammoth cells if they can be found.
In order for a Dolly-like clone to be born it is necessary to have females of a closely related species to provide unfertilised eggs, and, if cloned embryos are produced, to carry the pregnancies. Cloning depends on having two cells. One is an egg recovered from an animal around the time when usually she would be mated.
In reality there would be a need for not just one, but several hundred or even several thousand eggs to allow an opportunity to optimise the cloning techniques. The cloning procedure is very inefficient. After all, after several years of research with sheep eggs, Dolly was the only one to develop from 277 cloned embryos. In species in which research has continued, the typical success rate is still only around 5% at best.
Photo: Rubén Portas
In this case the suggestion is to use eggs from elephants. Because there is a danger of elephants becoming extinct it is clearly not appropriate to try to obtain 500 eggs from elephants. But there is an alternative.
There is a considerable similarity in the mechanisms that regulate function of the ovaries in different mammals. It has been shown that maturation of elephant eggs is stimulated if ovarian tissue from elephants is transplanted into mice.
In this way it might be possible to obtain a considerable number of elephant eggs over a period of time if ovarian tissue is obtained from elephants that die.
Cells from mammoths are required to provide the genetic information to control development. The suggestion is to recover cells from the marrow of bones emerging from the frozen north of Siberia. However, these cells will degenerate rapidly at the temperature of melting snow and ice. This means that cells in the bones may well become useless for this capacity as they thaw.
The chances of cells being viable would be increased if bones could be recovered from the lowest possible temperature rather than waiting until they emerge from snow. The cells can then be warmed rapidly. Alternatively, the nuclei could be transferred directly into eggs.
The very first stages of embryo development are controlled by proteins that are in the egg when it is shed by the ovary. One for example has a critical role in cell division. Together these proteins have an extraordinary ability to repair damaged nuclei so it may not be strictly necessary for the cells to be viable. It would be best if the mammoth nucleus could be introduced into an egg immediately, by injection of the contents of the damaged cell into the egg.
Research in 2008 found that when nuclei from freeze-dried sheep cells were transferred into eggs, some of the cloned embryos developed for a few days, but not to term. This was a very clear indication of the ability of the egg to repair damaged nuclei. However, freeze-dried cells are likely to be more stable than those that have been frozen with liquid still present. In the case of the mammoth, the cells would likely be killed by large ice crystals formed from the liquid.
Finally, if embryos that developed normally for a few days could be produced, they would have to be transferred to surrogate mothers to develop through pregnancy. Embryo transfer is only carried out routinely in fewer than a dozen species, and the elephant is not one of them. Success in embryo transfer depends upon introducing the embryo to a womb that is in a receptive state. The mechanisms that bring about this state in elephants are currently being defined by research in a number of zoos.
Taken together, it can be seen that there is biological uncertainty about the availability of viable cells, and that several complex techniques would have to be developed for cloning of mammoths to be successful. There is no guarantee that these techniques are even biologically possible. There may be unknown differences between species that would prevent the procedures that we developed in sheep being successful in mammoths.
Copyright: Royal BC Museum
Mammoth stem cells
An alternative ambition would be to try to use mammoth cells to produce stem cells. In several different species it is possible simply by the introduction of four selected proteins to give adult cells the characteristics of embryo stem cells. The four factors give embryo stem cells their unique characteristics and were found to be able to impose these characteristics on skin cells. This type of stem cell can be grown for very long periods in the laboratory while retaining the ability to form all of the tissues of the body.
They would provide extraordinary opportunities to compare mammoth cells with those of elephants. This knowledge would be of fundamental biological interest. It would enable us to begin to answer groundbreaking questions. What are the differences between the cells and tissues of these species? What are the similarities? The mammoth lived in a different climate, so was the metabolism of their cells different? Does this information cast any light on the cause of extinction of mammoths?
Stem cells of this type can also be induced to form gametes. If the cells were from a female, this might provide an alternative source of eggs for use in research, and perhaps in breeding, including the cloning of mammoths.
From a male, they would be sperm, and they might be able to fertilise eggs to produce a new mammoth embryo. It would be interesting to know if mammoth sperm could fertilise eggs of the elephant. If so, would the embryos develop to term to produce a hybrid animal?
Only a small proportion of mixed matings between species produces viable offspring, but the mule is one example and has been used by humans for centuries.
In all of these discussions it is necessary to consider the welfare of the animals. Mammoths lived in cold climates, whereas their current relatives including potential surrogate mothers live in warmer regions.
It would be essential to provide mother and clone with the appropriate environment of temperature, moisture and diet. It would almost certainly be necessary to keep the animals in captivity, so it would be essential to provide as interesting an environment as possible. Ideally this should include other elephants, mammoths or hybrids to provide social interaction for the animal.
So while unlikely at present, the development of some form of mammoth creature or hybrid might be possible in the longer term, the research of which could lead to major biological discoveries and advances.
This is another area of biology where studies of stem cells would be very rewarding. In stem cell research rather than cloning there would also be fewer concerns over animal welfare, or the effect of the animal on the environment. All in all, research to produce mammoth stem cells would be the appropriate choice, and extraordinarily scientifically rewarding, should it be possible to find viable mammoth cells.