Disabling strokes could one day be treated by replacing damaged brain tissue with stem cells, scientists have shown.
Researchers used a new technique to insert therapeutic stem cells into the brains of rats with pinpoint accuracy.
Once implanted the cells began to form new brain tissue and nerve connections.
The work is at an early stage and does not yet prove that stroke symptoms such as paralysis can be reversed.
But it demonstrates that lost brain tissue can be replaced with stem cells targeted at sites of damage.
Stem cells are immature cells with the ability to take on any of a number of specialist roles.
In previous animal experiments, stem cells implanted into the brain have tended to migrate to surrounding healthy tissue rather than fill up the hole left by a stroke.
Scientists from King's College London and the University of Nottingham overcame the problem by loading the cells onto biodegradable particles.
These were then injected through a fine needle to the precise site of damage, located using a magnetic resonance imaging (MRI) scanner.
Once implanted, the particles disappeared leaving gaps for the growth of new tissue and nourishing blood vessels.
The cells, derived from stem cells taken from mouse embryos, had already progressed some of the way to becoming neurons.
They were attached to particles made from a biodegradable plastic-like polymer called PLGA.
Dr Mike Modo, leading the King's College team from the university's Institute of Psychiatry, said: "The stem cell-loaded PLGA particles can be injected through a very fine needle and then adopt the precise shape of the cavity. In this process the cells fill the cavity and can make connections with other cells, which helps to establish the tissue.
"Over a few days we can see cells migrating along the scaffold particles and forming a primitive brain tissue that interacts with the host brain. Gradually the particles biodegrade leaving more gaps and conduits for tissue, fibres and blood vessels to move into."
Colleague Kevin Shakesheff, Professor of Advanced Drug Delivery and Tissue Engineering at the University of Nottingham, said: "This was a great collaborative project with the Kings College team and hopefully this technology will be taken to the clinical setting soon. Repairing damaged brain tissue is one of the ultimate challenges in medicine and science. It is great that we are now one step closer to achieving that goal."
The next stage will be to apply a "growth factor" chemical called VEGF with the particles to encourage the creation of new blood vessels.
The research, funded by the Biotechnology and Biological Sciences Research Council (BBSRC), is reported in the journal Biomaterials.
Strokes occur when part of the brain dies off because of a blocked or burst blood vessel.
An estimated 150,000 strokes occur in the UK each year, 67,000 of which are fatal.
Strokes cause more disability than any other chronic condition. Around 300,000 people in the UK are moderately or severely disabled because of a stroke.
Joe Korner, from the Stroke Association charity, said: "This research is another step towards using stem cell therapy in treating and reversing the brain damage caused by stroke. It is exciting because researchers have shown they are able to overcome some of the many challenges in translating the potential of using stem cells into reality.
"The potential to reverse the disabling effects of stroke seems to have been proved. However the development of stem cell therapy for stroke survivors is still in the early stages and much more research will be needed before it can be tested in humans or used in practice.
"Every five minutes someone in the UK has a stroke and it is vital that we do all we can to help those affected by stroke."
Professor Douglas Kell, chief executive of the BBSRC, said: "Stroke is a leading cause of disability in industrialised countries. It is reassuring to know that the technology for treating stroke by repairing brain damage is getting ever closer to translation into the clinic. This crucial groundwork by Dr Modo and his colleagues will surely be a solid foundation of basic research for much better treatments in the future."
Anthony Hollander, Professor of Rheumatology and Tissue Engineering at the University of Bristol, said: "We are only just beginning to understand how to use tissue engineering to cure diseases. This study shows the exciting possibility of using a biomaterial to deliver stem cells to a very specific location in the brain. It is too early to say if it will be clinically effective in patients but the more we explore these possibilities the more likely it is that we will develop successful therapies."