Image caption Is the idea of growing hands a myth or a far-off reality?

It might seem unbelievable, but researchers can grow organs in the laboratory. There are patients walking around with body parts which have been designed and built by doctors out of a patient's own cells.

Over the past few weeks on the BBC News website we have looked at the potential for bionic body parts and artificial organs to repair the human body. Now we take a look at "growing-your-own".

There is a pressing need. A shortage of available organs means many die on waiting lists and those that get an organ must spend a lifetime on immunosuppressant drugs to avoid rejection.

The idea is that using a patient's own stem cells to grow new body parts avoids the whole issue of rejection as well as waiting for a donor.

Dr Anthony Atala, director of the Institute for Regenerative Medicine at the Wake Forest Baptist Medical Center in North Carolina, US, has made breakthroughs in building bladders and urethras.

He breaks tissue-building into four levels of complexity.

Flat structures, such as the skin, are the simplest to engineer as they are generally made up of just the one type of cell.

Tubes, such as blood vessels and urethras, which have two types of cells and act as a conduit.

Hollow non-tubular organs like the bladder and the stomach, which have more complex structures and functions.

Solid organs, such as the kidney, heart and liver, are the most complex to engineer. They are exponentially more complex, have many different cell types, and more challenges in the blood supply.

"We've been able to implant the first three in humans. We don't have any examples yet of solid organs in humans because its much more complex," Dr Atala told the BBC.

Bladder builders

His technique for growing bladders starts with taking a tissue sample, about half the size of a postage stamp, from the bladder that is being repaired.

Over about a month, the cells are grown in the laboratory in large quantities. Meanwhile a scaffold in the shape of the organ, or part of the organ, being replaced is built.

Image caption A bladder scaffold is used to hold together the new structure

"We coat the scaffold, basically like creating a layer cake. We place the cells on the structure one layer at a time with the cells in the correct positions," Dr Atala said.

The cake is then "baked" for a two weeks in an oven, which has the same conditions as the inside of the human body. The new bladder is then ready to be implanted back into the body.

Eventually the scaffold is absorbed by the body, leaving the cells in place.

Building a scaffold for the bladder is one thing, building one for the heart is far more complicated. One of the problems when you move to larger organs is the getting the blood supply to work, connecting arteries, capillaries and veins to keep the organ alive.

It is why some researchers are investigating "decellularisation" - taking an existing donated organ, stripping out the original cells and replacing them with new cells from the patient who will receive the organ.

Prof Martin Birchall, a surgeon at University College London, has been involved in a number of windpipe transplants performed in this way.

The technique starts with a donor windpipe which is then effectively put through a washing machine. Repeated cycles of enzymes and detergents break down and wash away the host cells.

What is left behind is a web of proteins, mostly collagens and elastins, which give the windpipe its structure. It would look and feel like a windpipe, just without cells - a natural scaffold.

The next steps are very similar to those for making the bladder. Stem cells are taken, this time from bone marrow, and grown in a lab before being layered onto the scaffold.

Thefirst patient was fitted with one of these windpipes in Spain in 2008.

Prof Birchall said: "We've made some inroads by starting with the windpipe. We're looking at some other tissues now like the oesophagus and diaphragm and overseas the big breakthroughs have been in building the bladder and urethra.

"Those are the areas in which immediate breakthroughs have occurred, but I see a raft of further first-in-man studies in other organs happening in the next five years."

Heartbeat

There are already strong hints of what the next steps could be.

Five routes to a solid organ Build it on a scaffold

Strip an old organ of cells and put new ones in their place

Use a "bioprinter" to built an organ layer by layer

Inject cells into a living organ to repair

Use chemicals to trigger an organ to repair itself

Dr Doris Taylor, who is about to move to the Texas Heart Institute, has used the decellurisation technique on rats' hearts andproduced beating organs.

The cells were stripped away leaving a "ghost heart" and were then injected with heart cells. Eight days later the heart was beating, albeit at just 2% of normal heart function.

She said the technique could "absolutely" be used on any organ that had a blood supply.

She told the BBC: "It's not science fiction any more, but moving that to more complex organs is the challenge ahead of us."

Other groups have also produced miniature organs or "organoids". They are not the full-blown thing, but they perform the same functions at a smaller scale.

Wake Forest researchers haveproduced liver organoidswhich can break down drugs.

Dr Atala said: "The challenge for us is - how do we scale up?"

Bioprinting, just like an office printer except it "prints" cells layer by layer, has been used to "print" a kidney.

While these findings are a very long way from making it into hospitals, if indeed they ever do, the scientists involved are convinced these techniques will come good.

"The vision has to be tempered by the past and the number of false dawns that have occurred," Prof Birchall said.

"But I genuinely do believe stem-cell technologies and tissue engineering is going to completely transform healthcare delivery in the future.

"I see it incrementally reaching out to replace transplantation. The writing is on the wall for it to do wonderful things."

Dr Atala said: "The strategies are out there to someday be able to target every organ in the body we are not there yet. We are nowhere near there yet.

"But the goal of the field is to keep on advancing the number of tissues that we can target."

Of course growing a hand is even more challenging than anything being tried in laboratories so far. Will it ever be possible?

"You never say never, but certainly it's something I will most likely not see in my lifetime," Dr Atala concluded.