Tissue engineers make penises for rabbits; humans next
North Carolina researchers have grown erectile tissue in the lab for rabbit penises, a feat that could have broad applications for adult males who have had damage to their organs and for children with congenital abnormalities. Eventually, the procedure even could be used in men with erectile dysfunction who do not respond to drugs such as Viagra and Cialis -- about a quarter of all men with the problem. The team worked with rabbit penises because their physiology is very similar to that of humans, so reproducing the results in humans should be relatively easy.
Current devices to produce erections range from simple plastic inserts that can be inflated to produce an erection to sophisticated systems with pumps, valves and reservoirs that ultimately achieve the same end result. Most men find all of them less than satisfactory, however, because lovemaking must be interrupted to inflate them. And many children are born with deformed organs that cannot achieve an erection by any means. It was these children that drove Dr. Anthony Atala of Wake Forest University Baptist Medical Center's Institute for Regenerative Medicine to explore ways to produce what is, in effect, an artificial penis. He has been working on the project for 18 years.
Atala has been working on a variety of replacement organs. His biggest success has been with replacement bladders, which have now functioned successfully in humans for several years. The difficulty of producing replacement organs varies with the complexity of the organs, he noted. Skin, which is essentially flat, is by far the easiest organ to reproduce, even though it took researchers years to achieve it. Hollow organs such as a bladder are more difficult. Solid organs like the penis are the most difficult of all because they require several types of cells, and a blood supply must be achieved throughout the organ.
Atala's team had previously produced short segments of erectile tissue that were partially successful at producing erections. The new results were achieved, he said, by starting out with a much larger number of cells.
Atala and his colleagues first harvested smooth muscle cells and endothelial cells (which line blood vessels) from rabbits, then multiplied their numbers in laboratory dishes. For scaffolding on which to grow the organ, they used cartilage from rabbit penises that had been stripped of all cells. The cartilage was then seeded with about 3.25 million endothelial cells and as many as 118 million smooth muscle cells. Within a month after the scaffold was implanted in the animals' penile sheath, organized tissue with vessel structures began to form, they reported this week in the Proceedings of the National Academy of Sciences.
When the researchers subsequently exposed the rabbits to females, all tried to mate, eight of the 12 males ejaculated sperm, and four of the females became pregnant and had normal litters. "These results are encouraging," Atala said. "They indicate the possibility of using laboratory-engineered tissue in men who require reconstructive procedures. A lack of erectile tissue currently prevents us from restoring sexual function to these patients." There is reason to believe, he added, that a penis implanted in a baby would grow as the child does, suggesting that the technique also could be used to correct congenital defects. He said the team's next step will be to grow human tissue in the lab.
-- Thomas H. Maugh II
Photo: Rabbits, like this one, normally don't have problems reproducing. But, since their physiology is very similar to that of humans, North Carolina researchers have used them in research on artificial penises. Credit: Franck Robichon / EPA