FuChi: A New Chicken Biosensor Illuminates Cell Cycle Dynamics in Living Embryos

For more than a decade, biologists studying how tissues grow have relied on a clever piece of genetic engineering known as Fucci, short for fluorescent ubiquitination-based cell cycle indicator. The technology exploits the fact that two proteins, CDT1 and Geminin, are destroyed by the cell’s own degradation machinery at complementary points in the cell cycle. By fusing truncated versions of these proteins to fluorescent tags of different colours, researchers can make cells glow red in one phase and green in another, effectively turning the invisible choreography of cell division into a live light show. Now, a team led by researchers at the University of Edinburgh and collaborators across Europe and Japan has brought this capability to one of developmental biology’s most treasured model organisms: the chicken embryo. Their new tool, described in PLOS Biology, is called FuChi, and it is the first viable, stably expressing Fucci line ever created in an avian species.

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