Rete Ovarii: The 'New Organ' That's Been Known Since 1870

“Scientists just discovered a new organ in the female body.” It’s a great headline, and it has been doing the rounds again this autumn. The structure it describes is real, and the research behind it is genuinely interesting. But almost every word of that first sentence is wrong.

The structure is called the rete ovarii, Latin for “net of the ovary.” It isn’t new. It was first described in 1870, it appeared in early editions of Gray’s Anatomy, and it has been sitting next to the ovaries of humans and many other mammals the whole time. What’s new is that scientists have finally started asking what it does, and the early answers, in mice, are surprising.

A structure that was written off, not missed

The rete ovarii is a small network of tubes attached to the ovary. It forms from the same embryonic tissue as the rete testis in males, and for most of the 20th century it was treated as a leftover from development: present, but pointless. Because no one could agree on a function, it gradually disappeared from textbook drawings of the female reproductive tract. The researchers who revived it describe it as having become “a mysterious orphan structure.”

It isn’t a mouse quirk, either. The same structure has been documented in guinea pigs, cows, cats, sheep, pigs, deer mice, camels, dogs, monkeys and humans. Personally, I think that’s the detail that should have raised eyebrows decades ago. Evolution doesn’t usually hang on to the same structure across that many species unless it’s doing something.

How it was rediscovered

Part of the reason the rete ovarii got ignored is how it was studied. Older researchers cut the ovary into thin slices and looked at them one at a time. Seen that way, a long, coiled tube looks like a scatter of unconnected dots, and it’s hard to tell whether you’re looking at one structure or many.

That changed when developmental biologist Jennifer McKey, then at Duke University, began imaging entire mouse ovaries in 3D. She kept noticing a curving, horseshoe-shaped structure at the base of the ovary, went back through the old anatomical literature, and matched it to the forgotten rete ovarii. Her team published the rediscovery in 2022.

Mapped whole, the rete ovarii turned out to have three distinct regions: a fine network of cell cords inside the ovary, a long coiled tube outside it that ends in a swollen tip, and a connecting section that joins the two. All three persist from development into adulthood.

Figure: Anbarci et al., eLife 2025, CC BY 4.0

What the new research actually found

The study behind the latest wave of headlines was published in the journal eLife in March 2025 by Dilara Anbarci, McKey and colleagues at Duke. Its central experiment is simple to describe. The team injected a fluorescent dye, made of molecules too large to seep passively through tissue, into the swollen tip of the external tube in week-old mice. Within 15 minutes, the dye had flowed through the rete ovarii and spread into the ovary. When they injected dye into the ovary instead, it stayed put and never flowed back.

In other words, fluid moves one way: from the rete ovarii into the ovary. When the researchers analysed that fluid, they found it contained secreted proteins, some of them potentially important to how the ovary works. The tube is also wrapped in smooth muscle that could squeeze it, threaded with blood vessels, surrounded by immune cells, and directly touched by nerve fibres.

An earlier paper from the same group, published in 2024, found that some genes in the rete ovarii switch on when estrogen levels peak. Put together, Anbarci’s interpretation is that the structure might work something like an antenna for the ovary, picking up signals from the body and sending supplies in response.

What we don’t know yet

Here’s where the viral version runs ahead of the science. Everything above comes from mice, and most of the key experiments were done in very young animals. Nobody has yet shown what happens to fertility if the rete ovarii is removed or switched off, which is the test that would prove it matters.

Serge Nef, a developmental biologist at the University of Geneva who wasn’t involved in the work, told Science News the experiments show “there’s something going on,” but called it “just the beginning of the story.” It is also still unknown whether the human rete ovarii behaves the same way, although single-cell studies have identified its cells in human fetal gonads.

So “scientists discovered a new organ” is wrong, and “scientists proved a forgotten organ controls fertility” would be wrong too. The accurate version is less punchy but more interesting: a structure everyone wrote off for a century turns out, at least in mice, to be connected, active and sending something into the ovary.

I don’t think the real story needs exaggerating. It’s a useful reminder that “we know everything about the body” is never quite true, and that some of what we think we know is just what nobody bothered to check. The rete ovarii was never hidden. It was ignored, and that is a different kind of discovery.

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