In 1870, German anatomist Wilhelm Waldeyer described a peculiar network of tissue connected to the ovary. It was small, easily overlooked and apparently without any obvious purpose. Early editions of Gray's Anatomy included illustrations of it, but as the understanding of reproductive biology advanced, the structure gradually disappeared from modern medical textbooks.
For more than a century, the rete ovarii remained something of an anatomical afterthought. Scientists knew it existed, yet it was frequently dismissed as a functionless remnant of embryonic development.
That assumption is now being challenged by researchers at Duke University, whose investigations suggest that the overlooked structure may be involved in transporting biological substances towards the ovary and maintaining its internal environment.
The findings, published in the journal eLife on March 19, 2025, offer an unusual reminder that even familiar parts of mammalian anatomy can still hold important unanswered questions.
The research was conducted by Dilara N. Anbarci, Jennifer McKey, Daniel S. Levic, Michel Bagnat and Blanche Capel, working across Duke University and the University of Colorado Anschutz Medical Campus. Rather than studying human ovaries, the scientists examined mice, following the development of the rete ovarii from embryonic life into adulthood.
Using advanced confocal microscopy and molecular markers, the team reconstructed the structure in three dimensions. What earlier anatomical studies had struggled to visualise emerged as an organised network comprising three distinct regions.
One region extends into the ovary itself, another forms a network of tubes outside it, and a connecting region links the two. Importantly, the researchers confirmed that this arrangement persists into adulthood, rather than disappearing after development.
But the most revealing evidence came from what happened inside those tubes.
The researchers injected fluorescent tracers into the outer tubular region and watched as the material travelled towards the ovary. The experiment demonstrated that the rete ovarii provides a pathway through which fluid and its contents can move in a particular direction.
Further investigation revealed that cells lining the tubes possess tiny hair-like projections called cilia and molecular machinery associated with secretion. Through mass spectrometry, the scientists identified proteins within the tubular fluid, including molecules potentially involved in maintaining ovarian tissues.
One particularly interesting candidate was IGFBP2, a protein that interacts with insulin-like growth factors, which participate in various processes of cellular growth and development. Its presence raises questions about whether the rete ovarii could influence signalling within the ovary.
The researchers also discovered that the tubular network is closely surrounded by blood vessels and immune cells called macrophages. Nerve fibres make direct contact with portions of the structure.
This arrangement led the team to propose that the rete ovarii might serve as a sensory appendage, potentially responding to signals from surrounding tissues while transporting substances towards the ovary.
The implications are intriguing because ovarian function depends on considerably more than the production of eggs. Hormonal signalling, tissue maintenance, follicle development and interactions between different cell populations must be coordinated throughout reproductive life.
However, the scientists have not demonstrated that the rete ovarii regulates fertility, controls hormone production or directly affects egg maturation. Its precise functions in living animals remain to be established, and equivalent activity has not been demonstrated in humans.
As an accompanying scientific commentary in eLife explains, the findings encourage researchers to reconsider a structure that received relatively little attention despite its long history in anatomical literature.
Its rediscovery raises a question that extends beyond reproductive biology: how many structures have been dismissed as evolutionary leftovers simply because scientists had not yet learned how to examine them?
