Scientists have engineered fully functional rabbit penises, replacing damaged penis spongy tissue for the first time. The penises stay erect, and ejaculate, and the rabbit proved much more sexually active than normal male rabbits.  (Source: PNAS/Wake Forest University)

Anthony Atala, M.D.  (Source: Winston-Salem Journal)
Rabbit recipients of artificially grown penises are eager to copulate with female partners

We've seen all sorts of tissue engineering examples -- from bone, to brain tissue, to nerves, to vascular tissue, and even muscles -- but a new study from the Wake Forest University’s Institute of Regenerative Medicine has surpassed all of these in pure strangeness.  The study used advanced tissue regrowth techniques to create and endow lab rabbits with fully functional replacement penises.

The research was led by the institute's director, Anthony Atala, M.D., who is most famous for devising a cell seeding technique that involves spraying cells harvested from an applicable tissue onto a collagen matrix, led the research.  Under Dr. Atala's scheme the developing tissue is bathed in a nourishing serum than keeps the tissue in a chemical environment similar to the human body, and at a similar temperature to that of the human body.  Growth factors and other beneficial compounds are seeded into the tissue beforehand, to encourage the cells to divide and populate the new tissue.

Most cells contain a wealth of information about how to grow and form their local tissue.  Once in a receptive scaffold (like the collagen), they're sometimes able to grow and form new tissues, if they're exposed to the proper chemicals and physical conditions.  By combining one or more tissue types and encouraging the growth of blood vessels, organs can be formed.

Dr. Atala's team had already regrown and implanted seven human male bladders, a significant success.  Ten years later, the patients are still showing good function.  However, the penis proved a much more elusive and complex tissue to grow.  Growing the outer skin in theory wouldn't be overly challenging, but the inner spongy tissue, called corpus cavernosa, proved to be a much stiffer challenge due to the complex mix of cell types needed.

Past artificial penises grown at Wake Forest were taken off the drawing board after failing to stay erect when implanted into rabbits with a piece of their spongy tissue removed.  After close to 18 years of failed attempts, the researchers tried a different angle, removing the entire spongy tissue (not just a segment) and using different growth factors on a complex mix of cells, including smooth muscle cells and endothelial cells -- the cells needed to form the arteries needed to bring blood to the penis's spongy tissues, allowing it to become erect.

The result was a resounding success.  Writes the research team in their paper on the accomplishment, "This technology has considerable potential for patients requiring penile construction."

The resulting penises were identical to their natural kin in response to electrical and chemical stimuli.  And the recipient rabbits proved eager to copulate, with eight of the 12 rabbits with implants achieving ejaculation and four becoming fathers.  While rabbits normally like to procreate over the long term, the recipients proved unusually randy, attempting to procreate much faster than normal male rabbits.

Describes the team, "Most control rabbits did not attempt copulation after introduction to their female partners.  All rabbits with bioengineered neocorpora attempted copulation within one minute of introduction."

The study was reportedly published in the journal Proceedings of the National Academy of Sciences (PNAS) on Monday, though a quick glance at the November 9 early edition did not show the study. 

The research proves eerily similar to the frequently on-point animated show South Park's episode "Eek a Penis", which aired last year and involved the character Mr./Mrs. Garrison having his/her penis regrown on a lab rat.  Despite the success, though, such more advanced sex change operations remain out of reach, as scientists have yet to engineer functional testicles from body tissues.  Still, for some with damaged penis tissue, this study may provide new hope.

The research team next plans to start trying to implant regrown human penis tissue.  States Dr. Atala, "We're going to be doing that experimentally at our center."

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