Science1 publisher2 min readPublished
Mouse ovaries keep about 14 percent of their eggs activated at every age
Across a full mouse reproductive lifespan, the share of eggs that had begun growing stayed flat. The Barcelona group takes that as a sign the ovary tracks its own supply, and says it does not yet know the signal.
The Scientist · Science desk
What happened
- High-resolution microscopy and machine counting located every oocyte in 100 mouse ovaries covering the reproductive lifespan, more than 85,000 in all, in work published in Nature Aging.
- At every age sampled, about 14 percent of the oocytes in a mouse ovary had been activated into growth, and that share did not change as the animals aged.
- According to the Barcelona Institute of Science and Technology, the result recasts the ovary as an actively monitored system that counts its own supply.
- Individual mice that Boke compared to identical twins raised in the same conditions differed three- to fourfold in how many oocytes they had.
- Mice are born with 5,000 oocytes, about two hundred times fewer than the million a human infant starts with, which sets the size of the translation gap.
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Why it matters
- constraint With no candidate signal named, the finding shapes how the organ is modelled and offers nothing yet to aim a drug or a hormone at.
- contradiction One census supports two readings that pull apart: activation behaves as though something governs it, while the initial reserve size behaves as though chance set it.
- capability A per-oocyte count with positions gives later experiments a measurable baseline, so a suspected regulator can be removed and the activated share checked for movement.
- exposure If the spread carries over to women and runs wider, two patients of the same age could sit far apart on any clinical count of remaining eggs.
If oocytes left the dormant pool at a fixed absolute rate, the activated share would climb as the reserve shrank. A flat share means the number entering growth falls in proportion to what is left [5]. Elvan Boke, senior author and a group leader at the Centre for Genomic Regulation in Barcelona, gave one candidate explanation to Discover. "It could be that every oocyte themself contributes to a signal that allows the ovary, or perhaps the organism more broadly, to sense the size of the remaining reserve," she said [10]. She also said: "We don't yet know what that signal might be or how this regulation works" [11].
In humans the pool falls from about 400,000 at puberty to an estimated 1,000 after menopause, a drop of roughly 99.75 percent [12][14][2]. "It's not that we don't have enough oocytes," Boke told Discover [15]. "We don't know, for example, whether those that are left are all bad quality, or why they didn't activate" [16].
Mice ovulate every four to five days against a human cycle of about 28 days, roughly six times as often [13][4]. Boke said the fixed fraction might also apply to human health, but that this remains unclear [20]; the team does expect it to hold across other mammals [8].
The three- to fourfold spread between individual mice was already there before puberty, which puts its origin early in development [18]. "This was really surprising because it means that there's an important stochastic component in determining the initial size of the ovarian reserve," Boke said [19]. So the starting number looks like chance and the activated share looks governed. Neither has yet been shown to be adjustable: the published work counted and placed oocytes [4], and the regulating signal, hormone or otherwise, is unidentified [7]. Boke put the clinical route in conditional terms. "If we understand why oocytes are being lost without ever being ovulated, we may eventually be able to ask whether these processes can be modulated," she said [22]. The stated long-range aim is delaying menopause and extending the fertility window [23].
"We still know remarkably little about how the ovary controls this process," Boke said [21].
What to watch
- Identification of the proposed counting signal, and whether removing it moves the activated fraction in mice.
- Whether the same fixed fraction turns up in another mammal, including human ovarian tissue, as the Barcelona team expects.
- Whether the pre-pubertal three- to fourfold spread in reserve size can be traced to a specific developmental event.