The three phases of ovulation – filmed for the first time from start to finish

During each menstrual cycle, 15 to 30 eggs mature in a woman’s follicles: fluid-filled projections inside the ovaries. As a rule, however, only one of these follicles makes it to ovulation: the follicle ruptures and the egg migrates into the fallopian tube. There she has a maximum of 24 hours to encounter a sperm cell. If the egg is fertilized, it travels to the uterus. A new life is growing.

But what exactly happens when you ovulate? How does it decide which of the two ovaries it takes place in? And which hormones play which role?

Researchers still know little about this elementary reproductive process. Now a research team at the Max Planck Institute for Multidisciplinary Natural Sciences has managed to film it in its entirety for the first time. The researchers used a newly established live cell microscopy method. They filmed ovulation on isolated ovarian follicles from mice.

“We can distinguish between three phases,” explains Melina Schuh, head of the Meiosis department at the Max Planck Institute. First, fluid penetrates the follicles and they expand. Hyaluronic acid is essential for this process: if the researchers blocked its production in their experimental cells, the follicles expanded less and ovulation did not occur. Then, during follicle contraction, muscle cells pull the follicle together. When the research team inhibited the contraction of these muscle cells, the follicles contracted less. “Even then, no ovulation occurred,” says Christopher Thomas, co-first author who is now published in . In the third phase of ovulation, the follicle finally ruptures. Its surface bulges, opens and releases the egg. Ovulation is complete.

The follicle then closes again, forming the so-called corpus luteum, which produces the hormone progesterone. This hormone prepares the uterus for implantation of an embryo. If the egg cell is not fertilized or the fertilized egg cell does not implant, the corpus luteum regresses after 14 days. A new menstrual cycle begins.

The researchers led by Melina Schuh hope that the new method will help to better understand the many mechanisms of ovulation. “Our results show that ovulation is a remarkably robust process,” she says. “Although an external stimulus is essential to trigger ovulation, the subsequent processes occur independently of the rest of the ovary because all the information is contained within the follicle itself.”