Danionella cerebrum Atlas Maps More Than 600,000 Cells Across an Adult Vertebrate

A fish about 12 millimeters long has given researchers a view of gene activity across an entire adult vertebrate. Whitehead Institute scientists mapped more than 600,000 cells in Danionella cerebrum, pairing single-cell RNA sequencing with spatial methods to show both which genes cells express and where those cells sit in the animal. The resulting atlas offers a reference for investigating how tissues work, how they respond to injury and how their activity relates across the body.

The resource is available through DigiFish, an interactive site for exploring cell types and gene activity. Researchers can use it to identify cells and candidate genes for further study, then take advantage of the fish’s transparency and growing experimental toolkit to investigate them in a living animal.

A whole-body map in a small vertebrate

Whole-animal cell atlases have been difficult to build in adult vertebrates: most are too large for researchers to profile their cellular diversity at this scale. Danionella’s minute size makes that work more practical. Its relationship to zebrafish also gives researchers access to molecular tools developed for a widely used model organism. The atlas brings these advantages together, offering a body-wide molecular reference rather than a collection of tissue-specific snapshots.

The team sampled the brain, spinal cord, eyes, internal organs, fins, head and trunk. Single-cell RNA sequencing measured gene activity in individual cells, while spatial techniques placed cell identities and expression patterns back into the tissue. The methods included MERFISH and hybridization chain reaction fluorescent in situ hybridization (HCR-FISH), alongside regionally stratified single-cell sequencing.

This combination matters because a cell’s identity includes more than the genes it expresses. Its location and neighboring cells can help explain how it fits into a tissue or a larger system. Kutay Deniz Atabay, who led the work as a postdoctoral researcher in the lab of Whitehead Institute Member Peter Reddien, described that added context: “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.” He said the resource gives researchers access to questions at “multiple different levels of complexity, from molecular to cellular to systems.”

Atabay is first author of the study, and Reddien is senior and corresponding author. Their paper, “Whole-body single-cell atlas of an adult vertebrate, Danionella cerebrum,” appeared in Science.

Why Danionella is suited to the work

The species is among the world’s smallest vertebrates, measuring about 12 millimeters. Its brain is also exceptionally small, with roughly 650,000 neurons in a volume of 0.6 cubic millimeters, according to the study. Despite its size, the brain contains all the major vertebrate brain regions. Viewed from above, the fish’s brain can be seen through its head, an unusual feature that makes noninvasive imaging possible.

The researchers describe D. cerebrum as particularly suited to imaging the adult brain and body at single-cell resolution. Its transparency, paired with an atlas that identifies cells and their locations, creates a way to connect molecular observations with activity in a living animal. The species was adopted for neuroscience research in 2018 and formally described in 2021; interest has since grown around its distinctive biology and experimental potential.

Atabay began exploring the fish in early 2019 while looking for a small, transparent animal in which to study nervous system regeneration. Researchers found that Danionella can regenerate several tissues, including parts of the brain and fins. Reddien’s lab had already built whole-animal atlases in planarians, small flatworms known for regeneration. Those lines of work prompted a question: could the same organism-wide approach be applied to a vertebrate?

“Here’s an animal where we’d have a chance,” Reddien said of the idea he and Atabay developed. Reddien is also a professor of biology at the Massachusetts Institute of Technology and an investigator with the Howard Hughes Medical Institute.

Patterns that emerge across tissues

The breadth of the dataset let the researchers identify rare cell populations and compare patterns across regions. In the nervous system, neurons from the same brain region often shared more similar gene activity with one another than with neurons in other regions that used the same neurotransmitter. This pattern has also been seen in other vertebrates, including mammals. It points to regional identity as an important feature of how vertebrate neurons are organized, alongside the chemical signals they use to communicate.

The atlas also shows that adult cells can retain molecular signals associated with their position in the body. Fibroblasts, which help form connective tissue, expressed region-specific activity in genes best known for their roles in establishing body regions during development. Cells in the pectoral and pelvic fins, for example, retained gene programs associated with forelimb and hindlimb development in other vertebrates.

The role of these persistent signals in adult Danionella is still unclear. They may help tissues maintain their identities or rebuild the right structures after damage, but the atlas does not establish that function. In planarians, by comparison, positional information helps guide regeneration. The parallel gives researchers a concrete question to pursue in a vertebrate with substantial regenerative capacity.

A reference for studying regeneration

The team used the atlas to examine what happens after injury to the telencephalon, a region of the forebrain. Missing neurons regenerated, and the researchers traced a sequence of molecular activity: an early wound response and immune cell recruitment, followed by radial glia proliferation, the generation of new neurons and their maturation.

That progression gives future studies a baseline for asking when particular cell populations become active and how their responses relate. More broadly, the atlas can help researchers compare changes across tissues after injury, aging, drug exposure or other conditions. Its value lies in connecting those changes to a map of cell types and locations throughout the animal.

Reddien sees the resource as a step beyond knowing an organism’s gene inventory. “The transcriptomes of cells tell you how that genome is being utilized in the different cell types,” he said. Mapping expression across cell types, he added, could mark “a big turning point in how one would investigate the biology of an organism.”

The study’s authors point to possible applications in systems neuroscience, stem cell biology and regeneration, biomechanics, collective behavior and evolutionary biology. The atlas does not answer those fields’ questions on its own; it gives researchers a shared reference for framing them and choosing what to examine next. As work on Danionella expands, the combination of whole-body cell maps, accessible imaging and regenerative biology should make it possible to follow those questions across scales, from individual genes to interactions among tissues.