For years, scientists have sought to create a human artificial ovary, restoring fertility in patients without other options. The first cellular map of a human ovary, recently developed at the University of Michigan, Ann Arbor, represents a big leap forward in that quest.
"You cannot build something if you don't have the blueprint," said biomedical engineer Ariella Shikanov, PhD, associate professor at University of Michigan, who helped create what she and colleagues call an atlas of the ovary. "By creating a map or an atlas, we can now follow what nature created and engineer the building blocks of an ovary — and build a nature-like structure."
So far, the concept of an artificial ovary has been successful only in mice, with the development of a 3D-printed prosthetic ovary that enabled sterilized mice to have pups. Researchers hope that artificial human ovary technology could someday help women left infertile after cancer treatment, as well as patients who don't respond to fertility treatments and those with premature ovarian failure.
But Shikanov believes this research will go even further, providing a valuable resource to scientists studying diseases and other conditions related to the ovary.
"Whenever people think about the ovary, if they think about it at all, they usually think about fertility," said Shikanov. The ovary is so much more.
Besides producing and carrying a woman's unfertilized eggs during her lifetime, the ovary is also responsible for endocrine function — the production of estrogen and progesterone, which in addition to supporting reproductive health, help maintain a woman's cardiovascular, bone, and mental health.
"We don't really understand everything that is happening in the ovary yet," Shikanov said. "But we know it is an important organ."
Mapping the Ovary
Because people don't typically donate their ovaries, there are not many available for research, especially from younger reproductive age women, said Shikanov. So, the scientists set out to build a resource. They described their work in Science Advances.
To create their atlas, the researchers studied two premenopausal donor ovaries, profiling 18,000 genes in 257 regions. From three additional donor ovaries, they also generated single-cell RNA sequencing data for 21,198 cells.
"We identified four major cell types and four immune cell subtypes in the ovary," said Shikanov. Taking samples from different areas of the ovary revealed distinct gene activities for oocytes, theca cells, and granulosa cells — expanding scientists' understanding of the molecular programs driving ovarian follicle development.
What's unique about their work is the focus on both single cell and spatial analysis, said study coauthor Jun Z. Li, associate chair of the University of Michigan's Department of Computational Medicine and Bioinformatics. Specifically, they used a relatively new method called spatial transcriptomics, which allows them to see which genes are being activated and where.
"We are constructing the spatial arrangement of the cells in the ovary," said Li. "This spatial analysis is like saying, 'Let me look at where you are and who your neighbor is.'"
Their findings are built on other genetic and cellular research in the field, Li noted. Biomedical engineers in other areas of medicine are applying similar technologies to other organs including the heart, the breast, and bone — part of a larger project called the Human Cell Atlas.
Advancing Women's Health Research
Historically, women's health research has been underfunded and underrepresented, but the authors believe their atlas of the ovary is a significant step forward.
"There are a lot of biological questions that we don't know the answers to about the ovary," said Shikanov.
One of the biggest mysteries is why so many eggs never become fertilizable. Each human female is born with about one to two million ovarian follicles. Each follicle carries one immature egg. Around puberty, two thirds of these follicles die off. And most that are left never develop into fertilizable eggs.
"The majority of these follicles either just grow and secrete hormones or undergo atresia," Shikanov said. "One question that we wanted to understand is, what determines an egg that can grow, ovulate, and become a fertilizable egg and potentially develop into a new human being from one that does not?"
Another big question researchers have is, what's happening with other types of cells in the ovary — the supporting cells that produce endocrine hormones? Where are they located and what proteins and RNA are they making? Their research begins to unravel some of these questions and lays a foundation for future studies.
"We wanted to analyze the transcriptional signatures from specific regions and then do bioinformatical analysis and really combine structure, function, and transcriptional signatures," Shikanov said.
Knowing the transcriptional signatures can help researchers understand disease mechanisms and then go on to develop treatments for these diseases.
Winifred Mak, MD, PhD, a reproductive endocrinologist and infertility specialist at Dell Medical School, University of Texas, Austin, studies cancer fertility preservation. "For me, it is interesting to see that there are so many different clusters of cells in the ovary that have been identified by this study that we were not necessarily aware of before," said Mak, who is not involved in the new research. "Also, the identification of new genes not previously studied in the human ovary."
What's Next
Dozens of scientists who study reproductive health are already reaching out to the researchers about their work, Shikanov said.
"We get contacted almost every day from researchers all around the world asking for data sets or asking for details from this paper," she said, "from people who study ovarian cancer, for example."
Mak said having a map of a normal ovary could also help researchers who study premature ovarian insufficiency — why the ovary sometimes goes into premature menopause — and polycystic ovarian syndrome.
Another big area of research interest is ovarian aging. "Women live so much longer now, but we still reach menopause at the age of 50," Shikanov said. "So, there are efforts going toward understanding ovarian aging and maybe preventing it to extend ovarian longevity."
Mak said it will enable scientists to "look at different age women and see what genes change across the reproductive lifespan."
The atlas may also eventually lead to treatments that help restore fertility in individuals who had and were treated for cancer as children, people who undergo sex transitions, and those whose reproductive organs have been impacted by trauma in conflict settings or accidents, Li said.
The applications are numerous and exciting, Shikanov said. "Our atlas is like a benchmark. Now researchers can collect ovaries from individuals with these diseases and conditions and try to compare what's different."