The use of human admixed embryos as stem-cell factories is only one area of research among many. Last year, researchers at Toronto University found stem cells in the pancreases of mice that could produce insulin, offering hope that the discovery may lead to treatments for diabetes.

Germ cells – the cells that eventually become sperm and egg cells (as opposed to the cells of colds and other diseases) – are another hot area of research, for example. They are considered interesting because they exhibit totipotency, the ability to create an entire organism from the information contained in those cells.

One the leading scientists in the field is Professor Azim Surani, of the University of Cambridge’s Gurdon Institute, who is studying the cells in mice and sees germ cells as “an enduring link between all generations”.

As with embryonic stem cells, germ cells undergo a form of reprogramming. “All of the information gets erased, very similar to wiping a computer disk and this prepares the genome for the next generation,” Surani says. He says that understanding the germ cell’s reprogramming mechanism might lead to an understanding of how other body cells, such as skin cells, can be reprogrammed for other uses.

The research promises to help in stem-cell research, Surani says. “The current procedures are not very efficient and one of the problems is that we are quite ignorant of the mechanisms involved. We can get these cells to go back, but we don’t understand the factors and molecules involved.”

At some point in the medium term, Surani believes that his work may lead to advances in reproductive medicine.

Professor Austin Smith, of the Wellcome Trust Centre for Stem Cell Research in Cambridge, says his team has been studying stem cells for more than 20 years. As well as pluripotent stem cells from embryos that can turn into any type of body cell, Smith’s team is also looking at neural stem cells, which can develop only into cell types of the central nervous system, such as the brain. Research into these cells may have applications in understanding neuro-degenerative diseases and basic neurobiology.

“The reason we are interested in these cells is to understand the relationship between them and pluripotent stem cells,” Smith says. “They are somewhere halfway between pluripotent cells and differentiated cells – they have some options open but not all. We are interested in understanding that distinction and understanding what pluripotency really is.”

Although stem-cell research appears to be at an early stage, some doctors are claiming remarkable success with the techniques. Dr Andrew Gennery of the University of Newcastle works as a clinician at one of only two specialist units in the UK for babies born without immune systems, at Newcastle General Hospital. He says new bone marrow transplant techniques using stem cells have increased the survival rate of so-called “bubble babies” from 50 per cent to 88 per cent.

HOW CROSS-SPECIES EMBRYOS CAN BENEFIT HUMANS

The Human Fertilisation and Embryology Bill identifies four types of human admixed embryos that may be created under licence:

Cytoplasmic hybrid embryos are created by inserting the nucleus of an adult human cell such as a skin cell into an animal egg which has had essentially all its genetic material removed.

Transgenic human embryos are human embryos into which sections of animal DNA have been inserted. The use of these will allow further investigation into how stem cells differentiate into specific cell types.

Chimeric human embryos are human embryos into which one or more animal cells have been integrated. These can be used to examine the relationship between embryonic stem cells and normal embryo cells and to investigate how stem cells differentiate into specialised cell types.

True hybrid embryos are created by the fusion of human and animal gametes. This is the most controversial of the four. Scientists have admitted that there is little need for “true” hybrids at the moment, but advances in the field could make them useful in future.