The chicken embryo has long been a favourite of developmental biologists for practical reasons. Eggs are inexpensive, available in large numbers, and develop outside the mother’s body, which means the embryo can be observed directly through a window cut in the shell at virtually any stage. Tissue can be grafted, genes can be manipulated, and organs can be imaged as they form. Yet despite these advantages, chicken researchers have never had access to a genetic cell cycle reporter, leaving them to infer proliferation patterns from fixed tissue stained for markers such as phospho-histone H3 or proliferating cell nuclear antigen. Such snapshots capture only a moment in time and reveal nothing about how long individual cells linger in each phase or when they commit to dividing.

Creating a Fucci chicken was not simply a matter of porting the mouse construct into a bird. The team, whose work was spearheaded by Zoe Sudderick and James Glover, designed an entirely new reporter architecture optimised for avian systems. At its core is a multicistronic construct: a single genetic cassette that produces multiple proteins from one transcript. The first component is mCerulean-tagged histone H1.0, a linker histone that is expressed in quiescent and early G1 cells and, crucially, carries a nuclear localisation signal so the fluorescence marks nuclei rather than diffuse cytoplasm. This component is joined by a self-cleaving 2A peptide to the tandem Fucci(CA)2 biosensor, a newer-generation Fucci design that improves on the original reporters in several important ways.

Those improvements matter because older Fucci systems have a well-known blind spot. The classic reporters distinguish G1 from S/G2/M, but they cannot separate cells in S phase from those in G2 or mitosis, and they fail to label cells in the earliest part of G1, when CDT1 levels have not yet accumulated sufficiently. The Fucci(CA)2 design addresses both problems, allowing researchers to assign individual nuclei to G1, S, G2, or M phases with confidence. The construct also includes epitope tags so that cell cycle states can be detected in fixed tissues by immunostaining, complementing the live fluorescence readout. Additional practical refinements help the system work reliably in the chicken, where transgenesis is technically demanding and expression levels must be robust enough to image through intact tissue.

Generating the line itself required the specialised facilities of the Roslin Institute, where the team used lentiviral delivery to introduce the construct into the chicken germline and bred founder birds to establish stable lines. The resulting FuChi chickens express the reporter constitutively, so every cell in the embryo carries the biosensor from the earliest stages of development. The authors report that the line is viable and fertile, with the reporter faithfully reporting cell cycle states both in cultured cells and in living embryos. Validation experiments confirmed that fluorescence colours matched independent measures of cell cycle phase, establishing that the system can be trusted for quantitative analysis rather than merely qualitative observation.

With the tool in hand, the researchers set out to map proliferation across developing tissues. Because the reporter labels nuclei in phase-specific colours, they could build spatial atlases of cell cycle status in intact organs, revealing where in a tissue cells were actively cycling, where they had exited the cycle, and how these patterns shifted over developmental time. Such maps are far more informative than mitotic counts alone, because they capture the full distribution of phases and can expose gradients of proliferative activity that fixed-tissue staining would miss. The team also applied the reporter to migrating cell populations, analysing how cell cycle state relates to movement during embryogenesis, a question that has been difficult to address in any vertebrate system.

Perhaps the most striking results came from live imaging of early embryos during gastrulation, the dramatic process by which a simple ball of cells organises itself into the three germ layers. Using time-lapse microscopy of FuChi embryos, the researchers tracked mesendoderm cells as they egressed from the primitive streak, the structure through which these cells ingress and then migrate to form embryonic tissues such as the prechordal plate. The imaging revealed that the transition out of S phase may itself be a key morphogenetic event in this process, suggesting that the cell cycle machinery and the movements of gastrulation are more intimately linked than previously appreciated. If cells must reach a particular cell cycle state before they can effectively leave the streak and contribute to forming structures, then proliferation timing is not merely a background process but an active participant in shaping the embryo.

This kind of insight illustrates why a four-phase reporter represents such a leap over earlier technology. In older systems, a cell leaving S phase and a cell entering mitosis would look identical, making it impossible to detect a phase-specific transition tied to a morphogenetic behaviour. With FuChi, the distinction is immediate and visible in living tissue. Combined with the chicken embryo’s optical accessibility, this opens the door to quantitative studies of cell cycle kinetics in contexts that would be far harder to image in mice, where development occurs in utero, or in zebrafish, where the Fucci tools available have generally been limited to the older two-colour designs.

The applications extend well beyond normal development. Because the reporter works in vitro as well as in vivo, the same line can be used to study cell cycle dynamics in cultured chicken cells, in organ growth and tissue homeostasis, and in disease processes where proliferation goes awry. The authors also point to infection responses as a promising area, since many pathogens manipulate the cell cycles of their host cells, and a live reporter would allow those manipulations to be watched directly. Cancer biology, regenerative medicine, and stem cell research all stand to benefit from a system in which the proliferative status of every cell is continuously readable in an intact, developing organism.

FuChi arrives at a moment when live imaging and quantitative developmental biology are converging, and it fills a conspicuous gap in the toolkit. The chicken embryo, with its external development, surgical accessibility, and rich experimental heritage, now possesses a cell cycle biosensor that matches or exceeds what is available in any other vertebrate model. As the authors and their colleagues at institutions including the University of Dundee and RIKEN demonstrated, pairing advanced reporter genetics with the intrinsic advantages of the avian embryo delivers a premier platform for studying how cells time their divisions as they build a body. For developmental biologists seeking to connect the cell cycle with morphogenesis, growth control, and disease, the lights have just come on.

Subject of Research: A transgenic Fucci cell cycle biosensor for tracking cell cycle phases in living chicken embryos

Article Title: FuChi is a cell cycle biosensor for tracking cell cycle dynamics during avian development

Article References: Sudderick, Z. R., Briggs, T., Mubarak, S., Van Kerckvoorde, M., Hernandez Rodriguez, A. R., Panda, S. K., Riddell, J., Batho-Samblas, C., Taylor, L., McTeir, L., Meunier, D., Findlay, A., Roberts, F. S., Raper, A., Sakaue-Sawano, A., Miyawaki, A., Rainger, J., Schoenebeck, J. J., Weijer, C. J., … Glover, J. D. (2026). FuChi is a cell cycle biosensor for tracking cell cycle dynamics during avian development. PLOS Biology, 24(10), e3004036. https://doi.org/10.1371/journal.pbio.3004036

Image Credits: AI Generated

DOI: 10.1371/journal.pbio.3004036

Keywords: Fucci, FuChi, cell cycle, chicken embryo, gastrulation, fluorescent biosensor, developmental biology, live imaging, primitive streak, mesendoderm, transgenic chicken, proliferation

The core idea is elegantly simple. The researchers designed photocleavable hashtag oligonucleotides, or PHOs, short synthetic DNA molecules conjugated to cholesterol so that they insert themselves into the fatty membranes of living cells. Each PHO carries two distinct barcode sequences, an inner and an outer tag, joined through a chemical bond that snaps apart when illuminated with ultraviolet light. Because the tags sit on the cell surface, they do not perturb the cell’s internal biochemistry, and because they are cleaved by light, they can be erased selectively wherever a researcher chooses to shine a beam.

To convert this chemistry into spatial information, the team used a digital micromirror device, the same light-engine found in many projectors, to cast user-defined patterns of ultraviolet light onto cultures of PHO-labeled cells. Cells bathed in light lose their outer tag; cells kept in shadow retain it. After several minutes of patterned exposure, the researchers dissociate the sample and run it through standard single-cell sequencing workflows, including droplet-based platforms such as 10x Genomics. In the resulting data, the ratio of outer to inner barcode reads recovered from each cell encodes how much light that cell received, and therefore where it sat in the original culture. In effect, the light pattern is stamped onto the cells as a molecular memory.

The precision of this stamping is tunable. Because photocleavage scales monotonically with light dose, a smoothly graded illumination pattern produces a continuous gradient of barcode cleavage across the sample, allowing cells to be assigned positions along an axis rather than sorted into coarse bins. Sequential rounds of labeling push resolution further: by applying different photomasks in succession with distinct PHO species, each carrying its own fluorophore and barcode, the team generated combinatorial codes that distinguish multiple spatial quadrants within a single dish. A refined second-generation tag, held in place by bridging oligonucleotides that prevent cleaved fragments from washing away, preserves earlier labeling patterns through subsequent rounds, expanding the encoding capacity exponentially with each additional exposure.

Critical to any method that douses living cells with ultraviolet light is demonstrating that the light itself does not distort the biology being measured. The researchers addressed this directly. Comparing transcriptomes of illuminated and non-illuminated cells, they found correlations approaching unity, with principal component analysis showing no separation between the two groups and differential expression testing revealing essentially no genes significantly altered by saturating UV exposure. They also showed that PHOs linger on cell membranes with a half-life of roughly nine hours, defining a practical window for live-cell experiments, and that the method works equally well on fixed samples, broadening its applicability to precious clinical specimens that cannot be kept alive.

Where scSTAMP-seq truly distinguishes itself is in its modularity. Rather than reading out gene expression alone, the team extended the approach to scSTAMP-MAT-seq, which jointly profiles messenger RNA, DNA accessibility, and DNA methylation from the same individual cell, all while retaining the spatial barcode. The workflow combines the light-stamped tags with a methyltransferase that marks open chromatin and a methylation-sensitive restriction enzyme that fragments the genome at methylated sites. Benchmarking showed that the multiomic version recovers tens of thousands of accessible GpC sites and endogenous methylated CpG sites per cell, comparable to the non-spatial predecessor, meaning spatial resolution came at little cost to data quality.

To showcase the method’s power, the researchers applied it to two-dimensional human gastruloids, micropatterned colonies of induced pluripotent stem cells that recapitulate the spatial patterning of the early embryo. During gastrulation, the embryonic disc organizes itself into domains fated to become different germ layers, and understanding how position and gene regulation intertwine during this process is a central question in developmental biology. Using a radial light gradient that decayed nonlinearly from the colony edge toward the center, the team encoded continuous positional information across each gastruloid and then read out the transcriptome of thousands of individual cells.

The resulting spatial maps revealed that canonical lineage markers such as CDX2, SOX17, EOMES, and DNMT3B each occupied characteristic radial positions, reproducing the epiblast-like, primitive streak-like, and extraembryonic-like domains seen in real gastrulating embryos. More strikingly, when the researchers grouped genes by their spatial expression profiles and examined the behavior of chromatin regulators, the enzymes and binding proteins that chemically modify and interpret histones and DNA, they found that these regulators themselves were spatially patterned. Histone editors, histone readers, DNA modifiers, ATP-dependent remodelers, and Polycomb group proteins each showed distinct distributions across the radial axis, suggesting that the epigenetic machinery governing cell fate is itself organized by position within the emerging tissue.

This observation speaks to a long-standing challenge in genomics. Studies from the ENCODE project and numerous single-cell atlases have catalogued gene expression and epigenetic states across cell types, but disentangling whether epigenetic differences drive spatial organization or merely reflect it requires measurements that capture both simultaneously in an intact spatial context. By coupling spatial barcoding with joint multiomic readout, scSTAMP-seq offers a way to ask these questions directly, correlating chromatin state, DNA methylation, and transcription as functions of position within a developing or diseased tissue.

The practical accessibility of the method may prove as influential as its scientific results. Unlike purpose-built spatial transcriptomics instruments that require specialized arrays or imaging systems, scSTAMP-seq piggybacks on standard single-cell sequencing platforms and a digitally addressable light source found in many microscopy facilities. The team has deposited its sequencing data in the Gene Expression Omnibus and released its analysis code on GitHub, lowering the barrier for other laboratories to adopt the approach. With a patent application pending and demonstrated compatibility with both live and fixed cells, plate-based and droplet-based workflows, and transcriptomic plus epigenomic readouts, light-stamped barcoding stands to bring spatial reasoning to single-cell genomics labs that could never justify a dedicated spatial platform, potentially reshaping how developmental biology, neuroscience, and cancer research map the molecular landscapes of complex tissues.

Subject of Research: A light-encoded barcoding method for spatially resolved single-cell transcriptomics and epigenomics

Article Title: Photolabile oligonucleotides with topological light gradients enable spatially resolved single-cell transcriptomics and epigenomics

Article References: Piscopio, R. A., Chialastri, A., Wang, C., Godzik, M., Heom, K. A., Wang, W., Li, L. J., Saxena, N., Wilson, M. Z., & Dey, S. S. (2026). Photolabile oligonucleotides with topological light gradients enable spatially resolved single-cell transcriptomics and epigenomics. Nature Biotechnology. https://doi.org/10.1038/s41587-026-03328-5

Image Credits: AI Generated

DOI: 10.1038/s41587-026-03328-5

Keywords: scSTAMP-seq, spatial transcriptomics, single-cell sequencing, photocleavable oligonucleotides, epigenomics, DNA methylation, chromatin accessibility, gastrulation, human gastruloids, digital micromirror device, multiomics, Nature Biotechnology

The team combined lineage tracing in mouse embryos with directed differentiation of human pluripotent stem cells to follow the developmental trajectories of neural ectoderm progenitors. Their findings reveal that two progenitor populations, which they designate anterior neural ectoderm and posterior neural ectoderm, arise side by side at the same moment in embryogenesis. The anterior neural ectoderm is committed to generating the forebrain and midbrain, while the posterior neural ectoderm is destined to produce the hindbrain. These are not cells that gradually acquire regional identity as the brain takes shape; they are lineage restricted from the outset, carrying fundamentally different developmental programs encoded in their chromatin.

Lineage tracing experiments in mice provided the crucial in vivo evidence. Using a Gbx2-CreER reporter system, the researchers labeled posterior neural ectoderm cells at embryonic day 7.0 and tracked their descendants. The labeled cells gave rise exclusively to hindbrain structures, never to forebrain or midbrain. Conversely, when individual Sox2-positive neural ectoderm progenitors were marked with fluorescent reporters at embryonic day 7.5 and followed to embryonic day 9.5, the resulting cell clusters occupied either the forebrain-midbrain domain or the hindbrain, but never spanned both. The analysis of 494 cell clusters from 16 embryos showed a clean partition, with no single progenitor contributing to both anterior and posterior brain regions.

This partitioning challenges the classical view, rooted in experiments dating back to the 1950s, that a common neural ectoderm progenitor generates the entire central nervous system and that regional identity is imposed later by external signaling gradients. The new data suggest that the forebrain, midbrain and hindbrain are already separated at the level of progenitor identity, before the neural tube has even formed. The brain, in other words, is not sculpted from a uniform clay but assembled from two pre-patterned building blocks that arrive with their fates largely predetermined.

To understand how this early commitment is molecularly encoded, the researchers turned to human pluripotent stem cells. They developed differentiation protocols that reliably generate anterior neural ectoderm-like cells and posterior neural ectoderm-like cells in vitro. When these two populations were challenged with forebrain-, midbrain- or hindbrain-inducing signals, they responded in strikingly different ways. Anterior neural ectoderm cells readily adopted forebrain and midbrain fates but resisted hindbrain conversion. Posterior neural ectoderm cells did the opposite, efficiently producing hindbrain progenitors while remaining refractory to anterior cues. This asymmetry was not a matter of subtle bias; it reflected deep lineage commitment that persisted even when cells were exposed to strongly opposing differentiation signals.

The molecular basis of this commitment was revealed through OmniATAC-sequencing, a technique that maps open, accessible regions of chromatin where regulatory elements are poised for activation. The researchers found that day-2 anterior and posterior neural ectoderm cells harbored dramatically different chromatin landscapes. Regions associated with forebrain and midbrain genes were accessible in anterior neural ectoderm but closed in posterior neural ectoderm, while hindbrain-associated regulatory elements showed the reverse pattern. These diverging chromatin states foreshadowed the eventual regional identities of the cells, indicating that lineage commitment is written into the epigenome well before morphological differences become visible.

Beyond resolving a fundamental question about brain origins, the work carries significant practical implications. The researchers demonstrated that their posterior neural ectoderm protocol could be extended to generate hindbrain rhombomere 5/6-specific motor neurons from human pluripotent stem cells, a neuronal subtype that has historically been difficult to produce in vitro. These motor neurons exhibited electrophysiological properties consistent with mature neurons, including voltage-dependent sodium and potassium currents and the ability to fire action potentials. The ability to generate specific hindbrain neuronal populations on demand opens new avenues for modeling motor neuron diseases, testing drugs and developing cell-based therapies.

The study also revealed that the dual-progenitor architecture of the brain may be far older than mammals themselves. By examining embryos from hemichordates, a group of marine invertebrates that diverged from the vertebrate lineage approximately 550 million years ago, the researchers found evidence of analogous anterior and posterior ectodermal domains. This evolutionary conservation suggests that the fundamental strategy of building a nervous system from two parallel lineages was established in the common ancestor of all deuterostomes, the superphylum that includes hemichordates, echinoderms and vertebrates. The brain, in this view, is not a vertebrate innovation but an ancient composite structure whose basic blueprint predates the emergence of backbones by hundreds of millions of years.

The implications of this work extend into multiple domains of neuroscience and regenerative medicine. If the brain is indeed a composite of two lineage-restricted progenitors, then understanding the signals that specify anterior versus posterior neural ectoderm becomes critical for generating specific brain regions in vitro. The researchers showed that modulating WNT signaling, a pathway long known to pattern the anterior-posterior axis, could direct anterior neural ectoderm cells toward more posterior fates within the forebrain-midbrain spectrum. However, once cells had committed to the anterior or posterior lineage, the barriers between them proved largely insurmountable, reinforcing the idea that these are fundamentally distinct developmental programs rather than points along a continuous gradient.

As the field grapples with the implications of this revised model, the study stands as a powerful reminder that some of the most basic assumptions in developmental biology remain open to revision. The idea that the brain arises from a single homogeneous progenitor pool has been a cornerstone of neural development textbooks for decades. The demonstration that two parallel lineages, each with its own chromatin signature and developmental potential, contribute to the mammalian brain forces a rethinking of how the central nervous system is organized at its deepest level. It also provides a new framework for understanding congenital brain malformations, which may arise not from defects in a general neural progenitor but from specific disruptions to one of these two ancient lineages.

Subject of Research: Two parallel neural ectoderm progenitors that generate the forebrain, midbrain and hindbrain during embryonic brain development

Article Title: Two parallel neural ectoderm progenitors contribute to the developing brain

Article References: Jokhai, R. T., Dundes, C. E., Ahsan, H. S., Kang, R. S., Salomon-Shulman, R. E. A., Rajan, A., Kim, Y. S., Stanton, L. J., Xu, C., Do, S., McDonald, B. D., Andrade López, J. M., Urrutia, H. A., Greenfeld, H., Wong, A., Qu, Y., Petkovic, A. S., Miao, Y., Garcia, K. C., … Loh, K. M. (2026). Two parallel neural ectoderm progenitors contribute to the developing brain. Nature Neuroscience. https://doi.org/10.1038/s41593-026-02433-7

Image Credits: AI Generated

DOI: 10.1038/s41593-026-02433-7

Keywords: neural ectoderm, brain development, lineage tracing, gastrulation, forebrain, hindbrain, chromatin, pluripotent stem cells, motor neurons, evolutionary conservation, WNT signaling, neural progenitors