Now, that radical makeover is rewriting what scientists believe is possible during the course of ordinary animal development. A team of researchers led by Stanford University has found strong evidence that the majority of cells in this worm’s larval body are not discarded and rebuilt, nor do they simply carry over their old jobs into adulthood. Instead, most of them are actively reprogrammed—stripped of their larval identities and retooled to perform entirely new functions in the adult organism. The finding, published in Nature Communications, marks the first time extensive cellular reprogramming has been documented during normal development in an animal with a bilateral body plan, the body architecture shared by humans and most familiar animals.

“Reprogramming is a bit of an exotic fruit in developmental biology,” said Christopher Lowe, senior author of the study and a professor of biology in the Stanford School of Humanities and Sciences. “Generally, we used to think that as cells develop, they become increasingly restricted in their function. But when we start looking at more animals that go through metamorphosis, they may reveal that reprogramming is a much more common feature of development.”

For decades, biologists have been divided about what actually happens inside a larva as it metamorphoses. Roughly 80 percent of animal species undergo some form of metamorphosis—the stepped development that carries an animal from egg to larva to adult, a process familiar to anyone who has watched a caterpillar become a butterfly or a tadpole become a frog. Yet despite how common the phenomenon is, the cellular mechanics underlying it have remained stubbornly opaque. Two competing ideas dominated the literature. One held that larval cells largely die off during metamorphosis, and the adult body is constructed from newly generated cells. The other suggested continuity of function: a larval skin cell would become an adult skin cell, a larval neuron would remain an adult neuron, with each lineage simply growing and refining its original role.

The Stanford-led study suggests both models are, at least for this worm, largely wrong.

Paul Bump, then a doctoral student in Lowe’s lab at Stanford’s Hopkins Marine Station and now an assistant professor at Pomona College, led the effort to trace the fate of cells through the worm’s transformation. The team performed single-cell RNA sequencing on more than 87,000 individual cells, sampled across five developmental stages: early larvae, late larvae, metamorphosis itself, and early and late juveniles. This technique captures a molecular snapshot of each cell—essentially a readout of which genes are active at a given moment—allowing researchers to classify cells by type and to assess how closely any two cells resemble one another in their identity and function.

The results were striking. When the researchers sorted the cells into twelve broad classes—among them cartilage, immune, and skin cells—they discovered that many larval cells bore a closer molecular resemblance to each other than they did to the adult cells performing the same function. The most dramatic example involved the nervous system: larval neurons were more similar to larval gut cells than they were to the adult neurons they would ultimately give rise to. In other words, the larval neuron’s nearest molecular relative was not the adult neuron, but a completely different tissue in the same larval body. That pattern, repeated across more than half of all the cells analyzed, points to wholesale reprogramming rather than simple continuity.

There were exceptions, and they matter. Muscle cells and mesoderm-derived cells, which form parts of organs, largely retained their identities from larva to juvenile. But the breadth of transformation elsewhere in the body was unlike anything previously documented in a bilaterian during normal development.

Genetic data alone, however, could not rule out one nagging alternative: that the reprogrammed-looking adult cells were actually brand new, born after the original larval cells had perished. To settle the question, Bump used a persistent dye—a lineage tracer—to label larval cells before metamorphosis began, then followed those tagged cells through the transition. The labeled cells survived and persisted into the adult organism, providing direct visual evidence that the larval body is not dismantled and rebuilt from scratch.

“This suggested that cells were not large-scale dying; they were actually being carried over,” Lowe said. “Based on their RNA sequence data, we saw that they had become very different in cell type, which was supportive of the idea that larval cells were being reprogrammed into new fates during metamorphosis, which goes against what most of the field would have predicted.”

The evolutionary context makes the finding all the more tantalizing. Cellular reprogramming has been observed before, but only in circumstances scientists considered unusual: after injury, in animals capable of regenerating lost limbs or entire organs, and in a handful of organisms like sponges and jellyfish during development. Those animals, however, sit far from humans on the evolutionary tree. Schizocardium californicum is different. As a member of the phylum Hemichordata, the acorn worm occupies a branch considered an evolutionary link to vertebrates—the group that includes all mammals, including humans. Finding extensive reprogramming in a normal developmental program of a relative of vertebrates suggests the phenomenon may be far more woven into the fabric of animal development than anyone suspected.

Getting to that discovery was not easy. Schizocardium californicum is not a standard laboratory organism, and the team had to adapt genetic tools and techniques designed for other, better-studied animals to make the analysis possible. But for Lowe’s lab, that difficulty was precisely the point. The group specializes in “non-model” marine organisms, betting that unusual animals will reveal not just their own developmental secrets but broader truths about the evolutionary history of animal life.

The gap they aim to fill is enormous. Most model organisms—mice, zebrafish, fruit flies, and the like—are direct developers: they hatch as miniature versions of their adult selves and grow steadily into maturity. These animals are easier to keep and study in the lab, and they are genetically closer to humans, which is why research has clustered around them. But direct developers are the minority. The majority of animal species pass through a larval stage and undergo metamorphosis, and developmental biology has comparatively little to say about what happens at the cellular level during that passage.

Fittingly, the worm’s own family history helped sharpen the contrast. Schizocardium californicum has a well-studied cousin, Saccoglossus kowalevskii, sometimes called the Virginia acorn worm, which is a direct developer. When the Virginia acorn worm hatches, it already has the worm-like body plan it will keep for life. Its California relative, by contrast, begins existence as a larva that looks nothing like its adult form—and, according to the new study, its interior is transformed as thoroughly as its exterior.

“You can watch this process of metamorphosis and see physically how things radically change, but this morphological transformation is also mirrored by a massive change in cellular components,” Lowe said.

The implications stretch well beyond acorn worms. If cells routinely abandon old identities and adopt new ones during ordinary development in a vertebrate relative, then cellular reprogramming may need to be reclassified from an exotic exception into a mainstream feature of how animals are built. It could also refine how scientists think about regeneration, wound healing, and even the rules that normally keep adult cells locked into their specialized roles.

The research received support from a Chan Zuckerberg Biohub Intercampus Research Award, the National Science Foundation, a Myers Trust Award, and a Haderlie Memorial Award. Additional Stanford co-authors include Laurent Formery, a former postdoctoral scholar, and Lauren Lubeck, a doctoral student in Lowe’s lab, along with researchers affiliated with Baylor College of Medicine, Chan Zuckerberg Biohub in San Francisco, Johns Hopkins University, the Stowers Institute for Medical Research, and the University of California, Berkeley.

For now, the floating head of the Pacific has delivered its verdict: during metamorphosis, the body you inherit may be entirely your own.

The choroid plexus, a specialized tissue located within the brain’s ventricular system, is best known for producing and regulating cerebrospinal fluid. CSF serves multiple fundamental roles, including cushioning the brain, providing essential nutrients, and maintaining homeostasis within the central nervous system. However, this new research unveils that the choroid plexus’s secretory activities extend far beyond mere fluid production, actively modulating the CSF proteome through sophisticated apocrine secretion processes.

Apocrine secretion is a form of exocytosis characterized by the release of membrane-bound vesicles containing complex molecular cargo. Unlike classical secretion pathways, apocrine secretion enables the transfer of larger and more diverse cellular components into the extracellular space. By employing advanced proteomic analyses, live imaging, and molecular biology techniques, the study’s authors demonstrate that the choroid plexus utilizes this unconventional secretory mechanism to selectively enrich CSF with a suite of proteins critical for neural development.

Such proteins appear to orchestrate the intricate dance of neurogenesis, synaptogenesis, and cellular migration that underpins brain maturation. The timing and composition of these secreted factors are tightly regulated, suggesting a developmental “program” guiding the choroid plexus to fine-tune the CSF milieu in response to the brain’s evolving needs. These findings imply that the choroid plexus functions not just as a passive CSF factory, but as a dynamic signaling hub coordinating brain growth.

Proteomic profiling revealed that the CSF of developing mice contains proteins previously unassociated with choroid plexus activity, including growth factors, immune modulators, and extracellular matrix components. Intriguingly, many of these proteins have established roles in neural differentiation and vascular development, hinting at cross-talk between the choroid plexus, cerebrovasculature, and neural progenitor populations. This complex molecular ballet is essential for establishing the proper architectural and functional groundwork that supports cognitive capacities later in life.

To unravel the cellular machinery behind apocrine secretion, the researchers employed high-resolution microscopy to observe the choroid plexus epithelium in action. They uncovered vesicular structures budding from the apical surface of epithelial cells, laden with cargo destined for release into the CSF. Molecular characterization of these vesicles identified unique protein markers and lipid compositions, confirming their apocrine origin. This approach illuminated the exquisite control exerted by choroid plexus cells over what is secreted and when.

The developmental implications of these findings are profound. Disruptions in CSF composition during critical windows of brain maturation are increasingly implicated in neurodevelopmental disorders such as autism, schizophrenia, and hydrocephalus. By elucidating the precise biological processes underpinning CSF proteome establishment, this study paves the way for targeted therapeutic strategies aiming to correct or compensate for dysfunctional choroid plexus secretion.

Further experiments involving genetically modified mouse models demonstrated that perturbing key genes involved in apocrine secretion led to aberrant CSF protein profiles and measurable defects in brain architecture. These models showcased decreased neuronal proliferation and altered synaptic connectivity patterns, emphasizing how vital the choroid plexus’s secretory output is for normal neurodevelopment. The causal links drawn in this work elevate apocrine secretion from a peripheral curiosity to a central player in brain health.

The researchers also addressed the temporal dynamics of CSF proteome changes, noting that distinct developmental stages are characterized by unique secretory signatures from the choroid plexus. Early embryonic brain favors factors promoting progenitor cell expansion, while later stages see a surge in proteins supporting differentiation and synaptic network formation. Such temporal specificity underscores the adaptive nature of choroid plexus secretion in meeting the evolving requirements of the developing brain.

Beyond developmental biology, the study’s implications extend into aging and neurodegeneration. The choroid plexus remains active throughout life, and alterations in its secretory programs may contribute to age-related cognitive decline and neuroinflammatory states. By understanding how choroid plexus apocrine secretion sculpts CSF composition across the lifespan, scientists might unlock new biomarkers or intervention points to combat debilitating brain disorders.

The methodological innovations harnessed in this study, including state-of-the-art mass spectrometry and single-cell transcriptomics, set a new standard in the field. The ability to correlate live cellular behaviors with proteomic snapshots brings unprecedented resolution to our view of brain fluid biology. Such integrated approaches are critical for disentangling the multilayered regulatory networks that maintain neural homeostasis.

Interestingly, this research also challenges longstanding dogmas about the blood-brain barrier and choroid plexus interfaces. The revelation that choroid plexus epithelial cells export large protein complexes via apocrine vesicles suggests selective gateways that complement classical barrier functions. This nuanced view prompts a reconsideration of how molecules are trafficked between blood, brain, and CSF compartments, offering fertile grounds for future exploration.

Moreover, the discoveries have potential translational impact in neurosurgical and pharmacological fields. Understanding the choroid plexus’s secretory routes could inform targeted drug delivery systems, enabling therapies to harness or modulate CSF content effectively. This could be particularly transformative for treating pediatric brain disorders where developmental timing is crucial.

The study’s authors call for expanded investigation into human choroid plexus biology, recognizing that mouse models, while invaluable, offer only a preliminary glimpse into the complexity of human brain fluid regulation. Human brain development presents additional layers of sophistication, including prolonged maturation periods and more intricate cellular architectures that may diversify choroid plexus functions.

In summary, this landmark study redefines our conception of the choroid plexus from a passive CSF producer to an active architect of the brain’s internal chemical landscape. Through apocrine secretion, it delicately sculpts the proteomic environment in the CSF, orchestrating developmental processes essential for building a functional and resilient brain. This insight opens exciting avenues for basic science research and clinical innovation, heralding a new era in neurobiology.

As we move forward, deciphering the full repertoire of choroid plexus-derived factors and their targets within the brain will remain a priority. Integrating these findings with neural circuit mapping and behavioral analyses promises to clarify how molecular changes manifest as cognitive outcomes. The choroid plexus, long overlooked, is emerging as a catalytic hub shaping brain health from the earliest stages of life.

This discovery invigorates the scientific community’s appreciation for the multifaceted nature of brain fluid biology. With continued interdisciplinary research, we stand on the cusp of unraveling the mysteries of how the brain’s internal environment is crafted, maintained, and altered throughout life, offering hope for novel therapies to safeguard mental health and cognitive vitality.

Subject of Research: Role of choroid plexus apocrine secretion in shaping cerebrospinal fluid proteome during mouse brain development

Article Title: Choroid plexus apocrine secretion shapes CSF proteome during mouse brain development

Article References:
Courtney, Y., Head, J.P., Dani, N. et al. Choroid plexus apocrine secretion shapes CSF proteome during mouse brain development. Nat Neurosci (2025). https://doi.org/10.1038/s41593-025-01972-9

Image Credits: AI Generated