Research by Karolinska University Hospital provides new findings on female fertility

New discoveries about follicles, the sacs of cells found in women's ovaries, may have implications for future infertility treatment.

A new study from Karolinska University Hospital has shown that there are two types of immature follicles in a woman's ovary, and they likely have different functions.

According to the researchers, only one type of follicle is involved in egg development. The other appears to be entirely dedicated to hormone secretion, a process called signaling. The discovery could have implications for how female fertility is assessed in the future and how involuntary childlessness is treated.

“Until now, we thought that all immature follicles had the same conditions to develop into mature eggs. It is important to understand why some follicles seem to choose not to mature their eggs and instead secrete hormones, and how we can take that into account in fertility treatments,” he says. Sebastian Brussel GedelofSenior Physician and Operations Manager at the Medical Unit for Gynecology and Reproductive Medicine at Karolinska University Hospital.

The researchers were also able to show that these immature follicles, which all women already have during the fetal stage, change during puberty. Previously, it was thought that they remained dormant as a reserve. Since some fertility treatments may also be relevant for children, such as freezing follicles in childhood cancer patients, it becomes important to understand whether follicles collected before puberty can mature their eggs naturally.

This study is a collaboration between the Medical Unit for Gynecology and Reproductive Medicine at Karolinska University Hospital, other Swedish university hospitals in the Sveafertil network, the Karolinska Institutet and several researchers in the Nordic countries. It is published in the journal Nature Communications.

– Studying the ovaries is a huge challenge, and close collaboration between different hospitals and universities is crucial. Through this, we have now learned important new biology about women's ovaries, he says. Paulina Damdimopoulouresearch group leader, activist in gynecology and reproductive medicine in the Middle East and researcher at Karolinska Institutet.

Ovarian cyst

• Follicles are sacs of cells found in the ovaries.

• Each follicle contains an egg cell.

• All follicles are formed early during development in the fetal ovary in an immature form.

• Immature follicles are also called “ovarian reserve.”

• Immature follicles grow toward ovulation after puberty.

• The egg leaves the follicle during ovulation.

• The word follicle comes from the Latin word for small sac.

• The follicle is also called the egg sac.

Karolinska University Hospital is one of the largest hospitals in Europe, and together with Karolinska Institutet we lead medical development in Sweden. For us, healthcare, research and education are equally important parts of our work to expand and improve people’s lives. Visit the Karolinska University Hospital website: www.karolinska.se