First Whole-Body Single-Cell Atlas of an Adult Vertebrate Published

Researchers at the Whitehead Institute have profiled more than 600,000 cells to build the first comprehensive whole-body single-cell atlas of an adult vertebrate, the roughly 12 mm transparent fish Danionella cerebrum. The paper, published in Science and led by first author Kutay Deniz Atabay, PhD, and senior author Peter Reddien, PhD, is now publicly browsable through the interactive DigiFish resource at digifish.wi.mit.edu.

What does the atlas contain?

The team combined regionally stratified single-cell RNA sequencing with whole-animal spatial transcriptomics — specifically multiplexed high-throughput fluorescent in situ hybridization (MERFISH) and hybridization chain reaction fluorescent in situ hybridization (HCR-FISH) — to build, validate and explore a body-wide transcriptome atlas. Cells came from the brain, spinal cord, eyes, internal organs, fins, head and trunk.

The result is a molecular reference covering which genes are active in which cell type, the gene-expression patterns that distinguish those types, and where many of them sit within the animal. The atlas captured even exceptionally rare cell populations, and the authors write that it "yields a comprehensive resource for myriad questions in biology and neuroscience."

For lab planners, the resource offers a baseline for measuring how injury, aging, drugs or other perturbations affect cells across an entire animal, rather than one dissected tissue at a time.

Why this fish and not a mouse?

Size has been the standing barrier. "The molecular mapping of cell transcriptomes across the entire animal anatomy of an adult vertebrate has not been achieved, largely because most vertebrates are too large for comprehensive molecular access," the investigators noted. At roughly one centimeter, Danionella makes organism-wide profiling feasible.

The species carries practical advantages beyond scale. It has the smallest vertebrate brain on record — about 650,000 neurons within 0.6 mm³, roughly four times the size of the Drosophila brain yet containing all major vertebrate brain regions. The brain is visible through the top of the head, and the authors argue the fish "is arguably unparalleled for noninvasive live whole-brain and whole-body imaging at single-cell resolution in an adult vertebrate."

The species was adopted for neuroscience research only in 2018 and formally described in 2021, but it is closely related to zebrafish, so researchers can transfer an expansive molecular toolkit from one of biology's standard model organisms. Its genome is approximately half the size of the zebrafish genome.

What did the data reveal?

Two biological findings stand out. First, in the nervous system, neurons from the same region tended to show more similar gene-activity patterns than neurons using the same neurotransmitter in different regions. Similar patterns appear in mammals, suggesting regional and developmental identity organizes vertebrate nervous systems.

Second, adult fibroblasts retained region-specific activity of genes best known for laying out body parts during development. Cells in the pectoral and pelvic fins, for example, held gene programs associated with forelimb and hindlimb development in other vertebrates. What these persistent positional signals do in the adult fish remains unknown, though in planarians such information guides regenerating tissue to rebuild correct structures.

The team also applied the atlas to regeneration. After injuring the telencephalon, a forebrain region, they traced the missing neurons regenerating through "a temporally ordered molecular cascade, involving signatures of wound response and immune cell recruitment, radial glia proliferation, neurogenesis, and neuronal maturation."

How will researchers use it?

"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," Atabay said. "This gives you access at multiple different levels of complexity, from molecular to cellular to systems."

Reddien, also an MIT professor of biology and a Howard Hughes Medical Institute investigator, frames the advance as a turning point: where genome sequencing gave the complete gene list, cell transcriptomes show how that genome is used in each cell type.

The authors conclude that the findings and resources position D. cerebrum as a model for systems neuroscience, stem cell biology and regeneration, biomechanics, collective behavior and evolutionary biology — a list that suggests DigiFish traffic will grow well beyond the regeneration community.