A tiny fish provides big answers: the first full-body map of gene activity in an adult vertebrate

Whitehead Institute researchers have used a recently-discovered tiny, transparent fish species to create the first comprehensive whole-body single-cell atlas of an adult vertebrate. This offers a new resource for studying brain regeneration, cellular diversity, and how different tissues function together.

CAMBRIDGE, Mass., Oct. 29, 2026 /PRNewswire/ -- Viewed from above, Danionella cerebrum offers a rare sight in a vertebrate: its brain, visible through the top of its head. Since the species was adopted for neuroscience research in 2018 and formally described in 2021, the tiny, transparent fish has drawn rapidly growing interest for its unique biology.

Whitehead Institute researchers recognized another opportunity in the tiny fish: it brought whole-animal single-cell profiling within reach, allowing the team to create the first comprehensive map of cell types and gene activity across an adult vertebrate. That perspective could help reveal how cells in different tissues coordinate their responses—and how changes in one part of the body influence others.

In a study published in Science on October 29, the team profiled more than 600,000 cells from across the fish to build the whole-body map of which genes are active in which cell, known as a single-cell transcriptome atlas. A cell's transcriptome is the full set of RNA molecules it has made from its DNA at a given time, providing a snapshot of which genes the cell is using.

Together, the data provide a molecular reference for the cell types found throughout Danionella, the patterns of gene activity that distinguish them, and where many of them are located. The atlas also gives researchers a baseline for exploring how injury, aging, drugs, or other changes affect cells across the whole animal. 

"Once you have a spatially resolved atlas, you know what cell types exist, but you also know the neighbors of those cell types in this context," says Kutay Deniz Atabay, who led the work as a postdoctoral researcher in the lab of Whitehead Institute Member Peter Reddien. "This gives you access at multiple different levels of complexity, from molecular to cellular to systems."

Cell atlases have become increasingly detailed, but extending them across a whole adult vertebrate has been difficult for a simple reason: size. Profiling the cellular diversity of larger animals comprehensively is technically demanding and expensive. Danionella offered a unique way around that problem. Its miniature body—barely one centimeter long—makes the experiment possible, while its close relationship to zebrafish allows researchers to draw on genetic tools developed for one of biology's most widely used model organisms.

Atabay first became interested in Danionella in early 2019, while searching for a small, transparent animal in which he could watch the nervous system regenerate. Its miniature size and accessible brain made it especially promising for that work, and the researchers found that the fish has broad regenerative capacity, including in the brain and fins. Reddien's lab, meanwhile, had already seen the value of a comprehensive whole-animal cell atlas through its work on planarians, regenerative flatworms whose small size had made such an atlas possible. Together, those interests led to a larger question.

"Deniz and I had the thought: Would it be possible to get a cell-type transcriptome atlas for a complete vertebrate?" said Reddien, who is also a professor of biology at Massachusetts Institute of Technology and an Investigator with the Howard Hughes Medical Institute. "Here's an animal where we'd have a chance."

To build the atlas, the researchers used single-cell RNA sequencing to measure gene activity in individual cells collected from across the brain, spinal cord, eyes, internal organs, fins, head, and trunk. They paired those data with spatial methods that mapped cell types and patterns of gene activity back to their locations in tissues. The resulting atlas was extensive enough to capture even exceptionally rare cell populations.

Looking across the animal revealed patterns that can be difficult to see when tissues are studied separately. In the nervous system, neurons from the same region often had more similar patterns of gene activity—even when they used different neurotransmitters, the chemicals neurons use to communicate—than when compared to neurons using the same neurotransmitter in different regions. Similar patterns have been observed in other vertebrates, including mammals, suggesting that regional and developmental identity may be an important organizing feature of vertebrate nervous systems.

The atlas also revealed signs that adult cells retain a molecular memory of where they belong in the body. Fibroblasts, a type of connective-tissue cell, maintained region-specific activity of genes best known for establishing different parts of the body during development. Cells in the pectoral and pelvic fins, for example, retained gene programs associated with forelimb and hindlimb development in other vertebrates. What these persistent signals do in adult Danionella remains unknown, but they may help tissues maintain or rebuild the appropriate structures. In planarians, positional information provides molecular guidance that helps regenerating tissues rebuild the correct structures.

The research also used the atlas as a reference for studying Danionella's remarkable regenerative abilities. After injuring the telencephalon, a region of the forebrain, the missing neurons regenerated and the researchers tracing a progression from an early wound and immune response to neural stem-cell activity and the production and maturation of new neurons.

The team has made the atlas publicly available through DigiFish, an interactive resource for exploring cell types and gene activity throughout the animal. Paired with Danionella's transparency and growing experimental toolkit, the atlas can help researchers identify specific cells and genes to study or manipulate and then follow their activity in the animal.

For Reddien, the advance presents numerous, diverse opportunities for new science. "There's been this era of genome sequencing, where you get the complete list of all the genes that it takes to make an organism," he says. "The transcriptomes of cells tell you how that genome is being utilized in the different cell types. To have the transcriptome of every cell type seems like a big turning point in how one would investigate the biology of an organism."

SOURCE Whitehead Institute