The advancement of science is a funny thing. We may know a lot about alpaca sex, but when it comes to how human babies are made, there are still gaps in our knowledge. Now, biologists at the Max Planck Institute for Multidisciplinary Studies have filled in a big blank about one of the earliest phases of the reproductive cycle.
Most of the process of reproduction is fairly straightforward, and if you don’t know the basics, you may be overdue for a very uncomfortable talk with your parents. But what happens before eggs begin their trek through the fallopian tubes has been mysterious, even to biologists. The eggs are tiny—only 0.005 inches (0.12 millimeters) across—making them difficult to observe inside the body. Since they are released quickly from one of two ovaries (with no way to predict which), there has never been a clear view of an egg being released. The Max Planck scientists devised a method to witness this stage, an advance that could one day result in new fertility treatments.
Each month an #egg is ovulated from the #ovary, starting the journey of reproduction. But how does ovulation occur?🥚
Our latest work in @NatureCellBio led by @LastChrisThomas and @Tabea_Marx, describes the control of #ovulation using live imaging.https://t.co/EYjwLfdAdy (1/7) pic.twitter.com/69QcE673UL
— Schuh Lab (@SchuhLab) October 16, 2024
In all mammals, including humans, each egg is housed in a small, fluid-filled sac within the ovaries called an ovarian follicle. During a fertile cycle, upwards of 30 eggs mature, but only the best-developed follicles actually rupture and release the egg. The biologists noted in the journal Nature Cell Biology that while genetic studies, animal dissections, and cultured samples have provided insights into this process, nothing compares to video footage for studying biological processes.
To finally get a look at how an egg is released from its follicle, the researchers grabbed ovarian tissue from some mice that had been genetically engineered to produce eggs that are easy to see on camera. They placed the tissue samples in dishes on powerful camera lenses and triggered ovulation by exposing the ovarian cells to two key hormones involved in reproduction and development.
What they witnessed was a multistep process that involved an interplay of muscle contractions and the release of very specific chemicals, ultimately producing an egg. “We can distinguish three phases,” said Melina Schuh, a director at the Max Planck Institute, who worked on the study, in a statement. “The follicle expands, contracts, and finally releases the egg.”
During the expansion process, cells inside the follicle, called cumulus cells, produced a chemical called hyaluronic acid which flowed into the follicles. When the biologists altered the tissue to prevent hyaluronic acid from being produced, the follicles’ growth was stunted, and there was no ovulation.
Once the follicle had grown large enough, muscle tissue within it began to contract. Eventually, the follicle’s surface burst open—so, in a way, the famous scene from Alien isn’t that far off as a metaphor for reproduction. In fact, video of a follicle bursting looks more like something from another extraterrestrial-themed movie: Starship Troopers, in which the gigantic bugs launched glowing projectiles from their butts.
“When the follicle ruptures, which happens in the third phase, the egg is released and ovulation is complete,” said Tabea Lilian Marx, a doctoral student at the institute, who also worked on the study. “The surface of the follicle bulges outward and eventually ruptures, releasing the follicular fluid, the cumulus cells, and, finally, the egg.”
In the study, the team expressed excitement at what their technique for watching ovulation could mean for the study of fertility. Future research could focus on what happens when ovarian follicles are exposed to different chemicals and drugs. Given the precarious state of fertility treatment in the United States, that can only be a good thing.
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