Embryonic stem cells are considered to be master human tissue building material because they are exceptionally versatile, primitive cells able to develop into any tissue of the body and scientists hope that in the future they could be used to repair tissue damage, replace organs and reverse degenerative diseases.
The scientists at the Advanced Cell Technology laboratory in Massachusetts say each time the nuclei of rabbit, mice and cow embryos were replaced with a human nucleus, something went wrong and instead of turning on the right genes the animal eggs would turn them off.
The cloning technique called somatic cell nuclear transfer involves removing the nucleus from an egg cell and replacing it with the nucleus from another type of cell from the donor animal or person who is to be cloned and the scientific team used a new method called global gene expression analysis to see which genes were turned on and off as the eggs grew.
If successful the cloning technique process starts the egg growing and dividing as if it had been fertilized by a sperm, but the resulting embryo carries mostly the DNA of the donor.
Their research did nevertheless provide some encouraging results as a significant advance in the cloning of human embryos, which could be a path to producing a host of patient-specific treatments, was made. Dr Robert Lanza, the chief scientific officer at Advanced Cell Technology, and co-author of the study says for the very first time it was seen that cloning really does work and that DNA is reprogrammed but mixing human and animal cells does not appear to programme the egg properly.
Dr Lanza and his team were able to replace the nucleus of a number of embryos and bring the clones to the morula stage, where they had divided into eight to 16 cells - in the human embryos, they were able to prove that the DNA was reprogrammed because the same genes were activated as in a normal embryo.
Lanza's team used a new method called global gene expression analysis to see which genes were turned on and off as the eggs grew.
Researchers were optimistic that cloned animal eggs could be used to create human embryonic stem cells by coaxing them into becoming lab-dish replacements for heart, liver, skin, eye, brain, nerve and other cells destroyed by disease, accidents, war or normal wear-and-tear.
Scientists have discovered two potential ways to avoid the dangers of organ or tissue rejection - by reprogramming skin cells so they behave like embryonic stem cells or by cloning embryos so that they have the same DNA or tissue type as the patient.
Reprogrammed 'induced pluripotent stem cells' (iPS) are currently created using harmful viruses and are not safe for clinical use whereas cloned embryos could be safe for clinical use.
As yet researchers have not derived an embryonic stem cell line from a cloned embryo or found an efficient way to clone human embryos and there was a hope that animal eggs could be used as a substitute for human embryos, which are difficult to harvest, controversial to use and impractical because the high failure rate means it takes hundreds of eggs to create a single stem cell line.
Experts say the research indicates that animal cells are extremely unlikely to be suitable as recipients for use in human nuclear transfer and the production of patient-specific stem cells by this means would be impracticable.
The research is published in the journal Cloning and Stem Cells.