The stem cells produced are known as pluripotent, that is, they can be induced to develop into a variety of different organs.
The next stage is to make pluripotent cells totipotent – ie to create sperm and eggs. This is what the Nature paper reports. Both the original breakthrough and the new study come from Japan. Mitinori Saitou’s team at Kyoto University has created mouse sperm and egg cells from pluripotent stem cells that have gone on to produce viable fertile mice.
What is most striking about this is that a few factors – four in the case of reprogramming adult specialised cells and three in the case of creating sex cells, can throw the master switches for the enormous cascade of development that results in an adult. At the other end of the scale, a recent evolutionary study of the genes that control the development of mammalian limbs has shown that compared to mice and macaques 3000 human enhancers and 2000 promoters are active in limb development. These are not completely different genes to those of mice and macaques but genes of common origin that show changes in the human line selected during evolution.
Which means that the early big decisions, ie male or female, can be controlled by three or four genes but the fine tuning that makes the human form subtly different to that of other mammals requires endless tinkering at many stages of development. Once you know this it makes sense. Naively we might think that the factors that set in train the fertilisation of an egg and subsequent growth must be the base of the pyramid. But they are the tip. This why the embryos of many creatures, not just mammals, look virtually identical. All the complex tweaking occurs downstream.
Nature, 8 August, 2013, pp. 158-9 and Cell, 2013, 154, pp. 185-196.
Nature, 12 September, 2013, pp. 222-226.