Sea Star Ovaries Hint At Ancient Roots Of Human Fertility

Sea Stars May Hold the Secret to How Fertility Evolved

Scientists have long known how the brain and ovaries work together to control reproduction in mammals. Less clear is how far back that machinery reaches into animal evolution. A new study points to an unexpected source of answers: a spiny, five-armed creature that has drifted across ocean floors for hundreds of millions of years.

Researchers studying the bat star, Patiria miniata, found its ovary contains cell types and signaling systems that closely resemble those in the human female reproductive system, including molecular echoes of the hormonal chain that controls ovulation. Sea stars have no centralized brain, hypothalamus, or pituitary gland. Yet neurons inside their ovaries carry molecular signatures resembling parts of the reproductive signaling system found in vertebrates.

Published in Nature Communications, the findings suggest the machinery for egg production, and possibly its hormonal control, is older than scientists had appreciated, potentially predating the split between the lineages that gave rise to sea stars and humans.

Sea Star Ovaries Yield a Detailed Cell Map

Sea stars are not an obvious choice for reproductive biology research. But the bat star has a quality that makes it useful for the job: it continuously produces new eggs throughout adult life, unlike humans, born with a fixed egg supply that declines with age. That difference could open new questions about reproductive aging.

A team at the Marine Biological Laboratory in Woods Hole, Massachusetts, used single-cell RNA sequencing, a technique that works like taking a census of every cell in the ovary, to read individual cells’ genetic activity. Paired with 3D imaging, this let scientists map which cell types exist, where they sit, and how they signal each other.

From three female sea stars, the team cataloged 35,877 cells sorted into 27 groups across nine categories: egg cells at different stages, supporting cells that nurture those eggs, immune cells, muscle cells, cells lining the ovary wall, and neurons.

Follicle Cells Mirror Human Fertility Machinery

Among the most arresting findings was the molecular resemblance between sea star follicle cells and their mammalian counterparts. In the human ovary, granulosa cells and theca cells nurture eggs and produce hormones that drive the reproductive cycle. One group of sea star follicle cells expresses many of the same genes as mammalian granulosa cells, including foxl2, a master regulator of granulosa cell identity. A second group resembled theca cells, with genes linked to steroid hormone production. Compared against published ovarian studies from fruit flies, sea urchins, zebrafish, mice, and human fetal tissue, one sea star follicle cell group consistently aligned with granulosa or follicle populations in every case.

A simple explanation, the authors write, is that granulosa-like cells may have originated before the evolutionary split, more than 500 million years ago, between the lineage that produced sea stars and the one that produced vertebrates, including humans. They present this as an inference, noting that broader species sampling is needed to rule out convergent evolution, in which similar cell types evolve independently rather than from a shared ancestor.

Sea Star Neurons Do the Brain’s Job Without One

In mammals, egg development and ovulation follow a chain of hormonal commands that begins in the brain. The hypothalamus releases GnRH, which tells the pituitary gland to release hormones that travel to the ovaries and trigger egg maturation. Sea stars have neither structure.

Instead, neurons living inside the ovary itself appear to be doing some of that work. They express genes for GnRH and other signaling molecules that regulate the reproductive hormone axis in mammals, including serotonin and dopamine. In cross-species comparisons, they lined up most closely with the pituitary cells that release hormones driving ovulation.

Receptors for many of these molecules turned up in the sea star’s clusters of early egg-producing cells, suggesting the neurons and egg progenitors communicate directly, with no need for a brain to coordinate the process. This internal ovarian nervous system, the authors propose, may be doing locally what the brain and pituitary gland do in vertebrates, with reproductive control starting as a self-contained system within the gonad before being taken over by the brain and dedicated glands as animal bodies grew more elaborate. For now, that remains a hypothesis, not a proven timeline.

Dividing Cells Suggest a Lifelong Egg Supply

Researchers also found evidence consistent with a population of stem-like cells that could help explain how sea stars keep producing new eggs, a question long debated in reproductive biology. Early egg cells formed clusters showing signs of active division: a chemical marker that labels cells copying their DNA showed those clusters were replicating, while fully formed eggs were not.

These clusters also expressed genes tied to maintaining an uncommitted, stem-cell-like state. The germline gene ddx4 was absent from the clusters but active in eggs that had begun to mature, suggesting these cells have not yet committed to becoming eggs. 3D imaging reinforced this: early clusters sat toward the outer edges of the ovarian lobe, while fully grown eggs sat deeper inside, a layout that matches a production line moving from the periphery inward.

Taken together, the results position the sea star as a record of reproductive biology’s deep history. The underlying logic of egg production (specialized supporting cells, hormonal signaling, and stem cell-driven renewal) appears to have been assembled early in evolution and carried forward across hundreds of millions of years. Some cellular features long tied to the mammalian ovary may have older roots than scientists expected.

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Disclaimer: This article summarizes findings from a peer-reviewed study and is intended for general informational purposes. It is not medical advice. Several conclusions described here, including the proposed evolutionary origins and the intrinsic ovarian neuroendocrine system, are presented by the study’s authors as hypotheses or inferences rather than established facts, and further research is needed to confirm them.

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