The 3-D printed bioprosthetic ovary, as it's termed, is "the holy grail of bioengineering for regenerative medicine," said Teresa K. Woodruff, a reproductive scientist and director of the Women's Health Research Institute at Northwestern University Feinberg School of Medicine.
"The long-term goal for this is cancer patients," Woodruff said. She hopes this new research, a collaboration with other scientists including Ramille Shah, an assistant professor of materials science and engineering at McCormick School of Engineering, will result in a way to restore fertility in women who underwent life-preserving cancer treatments that rendered them sterile.
Ovaries are essential to the female reproductive system. Not only do these glandular organs produce hormones, when healthy, they release at least one egg each month for possible fertilization.
The basic unit of the ovary is the follicle, each of which contains a single egg surrounded by the cells that make the hormones estrogen and progesterone.
"Every month, there is this really remarkable event," Woodruff said. "The ovary breaks down under hormonal control and allows the oocyte, the female egg, to move from the inside to the outside, and it moves into the fallopian tube, where it is fertilized."
For an artificial ovary to be functional, it would need to facilitate ovulation.
Faced with this task, Shah and her colleagues created a 3-D printed structure -- essentially a scaffold on which to embed follicles -- made out of hydrogels, a material that is 99% water with a little polymer in it to give it strength. Shah and her colleagues also endowed their scaffolding material with pores in which follicles could be placed.
"What that does is, it provides follicles with the space to grow," Shah said. "But also it allows space for blood vessels to infiltrate the scaffold without degrading the material."
Once seeded with follicles, the structure was transplanted into mice whose ovaries had been removed. From there, the follicles matured, and the mice ovulated. After mating, their eggs were fertilized, and they gave birth to at least two pups each.
Within the bodies of each mouse, explained Woodruff, the structure had become "a functionalized soft-organ transplant." The recipient mice actually coordinated development of ovarian tissues in their bodies, she said, because the flow of their blood through the pores of this structure helped transform it into a functional bioprosthetic.
Shah is hoping to refine the work, beginning with the pores themselves.
"This is our bioprosthetic ovary version 1," she said, admitting it was a pleasant surprise when it worked on the first try. If you look at a cross-section of an ovary, she explained, you would see varying sizes and shapes of the many pores it contains. This is so it can house follicles that are at different maturity stages.
"You have to house both the small ones and the large ones, and you have to have an environment that can provide cross-talk between these follicles, because that's how the natural ovary signals for only specific ones to ovulate," Shah said. While some follicles are instructed to ovulate, others receive signals from the ovary to stay dormant or to mature.
Compared with the 3-D printed scaffold, the ovary has "a lot more sophisticated architecture when it comes to varying pore sizes and pore shapes," said Shah, who hopes to replicate this in the bioprosthetic ovary, creating version 2.
Future applications of 3-D printing
Dr. Anthony Atala,
the director of the Wake Forest Institute for Regenerative Medicine, said the nice thing about the new study is that the researchers proved that their 3-D printed bioprosthesis could include follicles and preserve the functionality. Atala was not involved in the new research.
This has never been done before for this particular organ, he said, though other structures, including cartilage, muscle and bone, have been 3-D printed and implanted into patients.
Still, in this case, the research passed the ultimate functional test: The mice actually became impregnated and gave birth, Atala said.
The strategy with 3-D printing is to take more advanced tissues that have been engineered by hand and re-create them with 3-D printing, he said.
"The printing gives you scalability, because you can scale a technology up instead of making them by hand, it gives you reproducibility, because you can print them in the same manner every time, and it gives you precision, because it can precisely place the cells where you need them," Atala said. "It will eventually decrease the cost of the production of these technologies, because you're automating the process."
Although it's difficult to know how quickly these technologies can be translated for human use, Shah said researchers are hoping a human implant will be made within five years, though it is not likely to be a full functioning ovary replacement right off the bat. Instead, human trials might begin with a structure that substitutes for only the hormone production function of an ovary.
Future dreams include making a durable transplant for pediatric cancer patients. Before beginning treatment, these young patients would undergo a separate procedure to that end.
Young patients need "to have ovarian function to go through puberty and to have long-term endocrine health not only for fertility but also for the estrogens, which are important for bone health and cardiovascular health," Woodruff said. "We would remove either one whole ovary, or biopsy an ovary, and that's cryopreserved for that patient's later use."
If the whole ovary is removed, why not simply re-implant the cryopreserved organ when cancer treatments end? The problem is that the tissues surrounding the follicles contain cancer cells, Woodruff said.
"We don't want to reintroduce that cancer to a patient," she noted. So by removing the follicles and not transplanting the rest the ovary, instead using a bioprosthetic ovary, "we're hoping to minimize the likelihood of transferring disease."
In the end, the collaboration was "very fruitful," even if questions remain, Shah said: "How long does the actual implant survive? Does it survive after multiple different cycles? Can it lead to multiple different litters?"
"There's a lot of different knobs we have yet to turn in order to really make a long-lasting bioprosthetic ovary," she said.