As a high school student (many years ago), I read a story about the death of the last dusky seaside sparrow. At that moment, I realized that an entire species had been lost, forever. I decided it would be really important, not to mention cool, to be able to help threatened species from becoming extinct, and maybe even to bring back extinct species. However, it wasn’t until I was in graduate school that I realized how the loss of genetic diversity might be reversed using deceased individuals. And not via the reanimation of a corpse—well mostly not.
The body exists as a vessel for sperm and eggs to get around, so why couldn’t we just move the gonads to a new body? As it turns out, there are stem cells contained within the testes that are responsible for the continual production of sperm and perhaps in the ovary for production of egg cells. Why is it exciting that gonads contain stem cells? Because these cells are self-renewing, meaning that when they divide, one cell remains a stem cell while the other becomes a sperm or potentially an egg.
All we have to do is keep the gonadal stem cells from a deceased individual alive, provide the conditions that promote cell division and maturation of functional gametes, and we would have the ability produce new offspring.
Although this sounds rather complicated, transferring avian gonad stem cells to a host chicken gonad, that can provide the appropriate conditions, is possible. Since these stem cells are not capable of producing a whole new individual, only more gametes, they cannot be used to clone birds.
It’s All in the Technique
Over the last few years, my students and I have been working on techniques to transfer these stem cells from the gonads of deceased birds to the gonads of chicken embryos. Our day starts when we receive a piece of gonadal tissue from necropsy.
The first step is to dissociate the gonad so none of the cells stick together. Cellular separation is important because if the stem cell is surrounded by support-cells, it may not behave as expected after being transferred into the chicken.
Next, the stem cells are dyed so we can track them in the host chicken gonad. As you can probably imagine, the injection of stem cells into a chicken embryo is tricky business. A glass needle, thinner than a human hair, is mounted on a mechanical manipulator. The manipulator is then used to very carefully insert the needle into the blood vessel of a chicken embryo and inject the stem cells from the deceased individual. This step is done when the chicken embryo is only two days old.
So far, we have tracked and located the stem cells of several different species of birds inside the host chicken gonads. Although my students and I have not yet reached our goal of making kiwis (or other threatened species) from chickens, we are well on our way.
It is exciting to think that this project of species recovery was yesterday’s science fiction, today’s science experiments, and hopefully tomorrow’s science fact!
Tom Jensen, Ph.D., Scientist, Reproductive Physiology Division.