Nucleoli are dense, rounded structures visible inside the nucleus of virtually every eukaryotic cell, and they function primarily as ribosome factories. Each human cell can contain one to several nucleoli, depending on the cell type and how metabolically active it is. Despite lacking any surrounding membrane, nucleoli maintain a remarkably organized internal architecture through the physics of liquid phase separation, and research over the past decade has revealed that their roles stretch well beyond ribosome production into stress sensing, gene regulation, aging, and disease.
What Nucleoli Are Made Of and How They Stay Organized
A nucleolus forms around specific chromosomal regions called nucleolar organizer regions, which carry clusters of the genes encoding ribosomal RNA. In humans, these clusters sit on five different chromosomes. Active transcription of those genes draws in hundreds of proteins and RNA molecules, and the resulting concentration of material gives rise to a structure you can see under a basic light microscope as a dark spot within the nucleus.
What makes nucleoli structurally unusual is that they hold together without a lipid membrane. Instead, they self-organize through liquid-liquid phase separation, a process by which certain protein and RNA mixtures spontaneously separate from the surrounding liquid, much the way oil droplets form in water. Work combining in vivo imaging with in vitro reconstitution showed that the nucleolus is not a single uniform droplet but a set of coexisting, immiscible liquid phases arranged in layers. The layering is driven by differences in surface tension encoded in the amino-acid sequences of the component proteins.1Cell. Spatial Organization of a Multiphase Liquid Organelle in Interphase Cells This concept of the nucleolus as a multilayered biomolecular condensate has become central to understanding how the initial steps of ribosome assembly are organized spatially.2PubMed. The nucleolus as a multiphase liquid condensate
In a typical mammalian nucleolus, three subcompartments are recognized. The innermost is the fibrillar center, where the ribosomal RNA genes sit and transcription begins. Surrounding it is the dense fibrillar component, where early processing and chemical modification of the newly made RNA takes place. The outermost layer, called the granular component, is where the maturing ribosomal subunits accumulate before export to the cytoplasm. Super-resolution microscopy has shown that even within the granular component, the distribution of key molecules is not uniform. Ribosomal RNA and the protein SURF6 cluster together in zones that are spatially distinct from zones enriched in nucleophosmin (NPM1), creating sub-phases within the granular component itself.3Molecular Cell. Multiphase organization of the nucleolar granular component underlies ribosome subunit assembly In vitro reconstitution experiments support the idea that these sub-phases form a core-shell arrangement, with a SURF6/rRNA-rich core surrounded by an NPM1-rich shell, and that the handoff between these zones facilitates an assembly-line-like export of finished ribosomal subunits.4bioRxiv. Granular component sub-phases direct ribosome biogenesis in the nucleolus
Building Ribosomes Step by Step
The nucleolus exists because cells need enormous quantities of ribosomes. A rapidly growing mammalian cell can produce on the order of thousands of ribosomes per minute, and the nucleolus is where the process starts. RNA Polymerase I transcribes the ribosomal RNA genes into a long precursor transcript.5PubMed Central. Basic mechanisms in RNA polymerase I transcription of the ribosomal RNA genes That precursor then undergoes an elaborate series of cuts, folds, and chemical modifications before yielding the mature ribosomal RNA molecules that form the structural and catalytic core of each ribosome.
Much of this processing depends on small nucleolar ribonucleoproteins, molecular complexes that guide site-specific chemical modifications of the ribosomal RNA. These complexes direct two main types of modification: the addition of a methyl group to certain sugar residues and the conversion of specific uridine bases to pseudouridine.6PubMed Central. snoRNPs: Functions in Ribosome Biogenesis These modifications are not cosmetic; they stabilize the final ribosome’s structure and influence how accurately it reads genetic instructions.7PubMed Central. Tuning the ribosome: The influence of rRNA modification on eukaryotic ribosome biogenesis and function Ribosomal proteins, manufactured in the cytoplasm, are imported back into the nucleolus where they join the maturing ribosomal RNA. The partially assembled subunits are then exported to the cytoplasm to become functional ribosomes.
Because ribosome production is so resource-intensive, it is tightly coupled to the cell’s energy status and growth signals. Kinase signaling pathways, particularly those involving mTOR, link nutrient availability to the rate of ribosomal RNA transcription. In activated T cells, for example, T cell receptor signaling, mTORC1 activity, and ribosomal biosynthesis in the nucleolus all regulate each other during the burst of cell division that follows immune activation.8PubMed. Reciprocal regulation of mTORC1 signaling and ribosomal biosynthesis determines cell cycle progression in activated T cells This bidirectional regulatory network ensures that the cell does not waste energy making ribosomes it does not need, and does not starve of ribosomes when demand spikes.9Molecular Cell. Cytoplasmic-nucleolar interconnections: mechanisms and roles in health and disease
Disappearing and Reappearing Every Cell Division
One of the more dramatic things about nucleoli is that they vanish at the start of cell division and reassemble afterward. As a cell enters mitosis, its nucleoli disassemble. This disassembly proceeds through two distinct phases: an initial slow, reversible preparatory phase, followed by a rapid irreversible dissolution that happens roughly in concert with the breakdown of the nuclear envelope.10PubMed Central. Multiscale biophysical analysis of nucleolus disassembly during mitosis During this period, ribosomal RNA transcription halts, and the nucleolar components disperse throughout the dividing cell.
Reassembly during late mitosis is equally choreographed. It depends on transcription and processing machinery that was inactivated rather than destroyed during prophase, as well as on partially processed precursor ribosomal RNAs that the cell carries through division. These leftover RNAs serve as seeds, nucleating small structures called prenucleolar bodies that organize the dynamics of nucleolar rebuilding. The whole cycle is governed by the balance between specific kinases that drive disassembly and phosphatases that enable reassembly.11PubMed Central. Assembly and disassembly of the nucleolus during the cell cycle The fact that new nucleoli can form from scratch every few hours underscores how fundamentally dynamic these structures are, despite appearing stable under the microscope during interphase.
Stress Sensing and the Link to p53
Beyond building ribosomes, nucleoli act as cellular stress sensors. When something disrupts ribosome production, whether it is DNA damage, nutrient deprivation, or certain drugs, the nucleolus does not simply stall. It triggers an alarm. Ribosomal proteins that can no longer be incorporated into new ribosomes are released from the nucleolus and bind to MDM2, a protein whose normal job is to tag the tumor suppressor p53 for destruction. By tying up MDM2, the freed ribosomal proteins allow p53 to accumulate and activate cell-cycle arrest or programmed cell death.12PubMed Central. RP-MDM2-p53 Pathway: Linking Ribosomal Biogenesis and Tumor Surveillance This pathway turns the nucleolus into a kind of trip wire: if ribosome assembly goes wrong, the cell gets a stop signal before anything worse happens.
The nucleolus also participates in stress responses that go beyond p53. Cellular stress triggers a dynamic exchange of regulatory proteins in and out of the nucleolus. Some proteins are actively sequestered inside the nucleolus during stress, which can regulate gene expression, influence cell-death pathways, and modulate autophagy.13Trends in Cell Biology. Nucleoli and NF-κB: an emerging crosstalk – Section: Nucleolar Sequestration of NF-κB Proteins Regulates Cell Growth and Death The nucleolus, in other words, is not just a passive factory; it is an active participant in deciding the cell’s fate under adversity.
Nucleolar Changes in Cancer
Pathologists have known for well over a century that tumor cells tend to have large, prominent, and often numerous nucleoli. The reason is straightforward: cancer cells grow and divide aggressively, and that requires a heavy investment in ribosomes. Several powerful oncoproteins, including MYC, RAS, and the mTOR pathway, are known to ramp up ribosome production, while tumor suppressors like p53, PTEN, and RB1 normally act as brakes on the process.14Trends in Biochemical Sciences. Nucleoli and Disease When those brakes fail, ribosome biogenesis accelerates and nucleoli swell. Increases in nucleolar size or number are now regarded as clinically relevant markers of poor prognosis in several tumor types.
The relationship between cancer and the nucleolus is not just about size. Evidence suggests that overproduction of ribosomes, decreased ribosome biogenesis, and qualitative changes in nucleolar function can all contribute to the development of cancer.15PubMed Central. New Insights into Connection of Nucleolar Functions and Cancer This insight has opened a therapeutic angle. Small molecule drugs that selectively block RNA Polymerase I and disrupt nucleolar function have been developed. These drugs selectively kill tumor cells in animal models while largely sparing normal cells, exploiting the fact that many cancer cells are more dependent on high rates of ribosome production than healthy cells are. Some of these compounds have progressed to clinical trials.16PubMed. Targeting the nucleolus for cancer intervention The strategy essentially weaponizes the nucleolar stress pathway described above: by jamming the ribosome assembly line in cancer cells, these drugs activate p53 and push cancer cells toward death without the DNA-damaging side effects of conventional chemotherapy.17PubMed. Targeting RNA polymerase I transcription and the nucleolus for cancer therapy
Nucleolar Dysfunction in Neurodegeneration and Rare Genetic Diseases
Disrupted nucleolar function is not unique to cancer. A group of conditions collectively called ribosomopathies arise from genetic mutations that impair ribosome assembly. Diamond-Blackfan anemia, for example, results from mutations in genes encoding ribosomal proteins and is characterized by severe anemia. Other ribosomopathies, including Treacher Collins syndrome and dyskeratosis congenita, each produce a distinct clinical picture depending on which step of ribosome biogenesis is affected.18Blood. Ribosomopathies: human disorders of ribosome dysfunction These diseases demonstrate how exquisitely sensitive certain tissues are to even modest reductions in ribosome output.
Neurodegenerative diseases have their own nucleolar story. In amyotrophic lateral sclerosis caused by the C9orf72 repeat expansion, motor neurons in the spinal cord show shrunken nucleoli. This shrinkage appears before the classic pathological hallmarks of the disease, such as TDP-43 mislocalization, suggesting that nucleolar stress may be an early upstream event rather than a late consequence.19PubMed Central. Nucleolar stress in C9orf72 and sporadic ALS spinal motor neurons precedes TDP-43 mislocalization The same nucleolar shrinkage was also observed in sporadic ALS cases with no C9orf72 mutation, hinting that nucleolar stress could be a more general feature of motor neuron degeneration.
The picture in frontotemporal lobar degeneration linked to C9orf72 is more complicated. Neurons in the frontal cortex of these patients also have smaller-than-normal nucleoli overall. But neurons that contain certain toxic dipeptide repeat protein inclusions show enlarged nucleoli compared to neighboring neurons without those inclusions, a pattern researchers describe as bidirectional nucleolar stress.20PubMed Central. Bidirectional nucleolar dysfunction in C9orf72 frontotemporal lobar degeneration That suggests multiple distinct toxic mechanisms are at work within the same disease, some shrinking nucleoli and others swelling them.
Small Nucleoli and Long Life
While large nucleoli are associated with cancer, the opposite seems to be linked to longevity. A study across multiple species found that long-lived organisms and long-lived individuals within a species consistently have smaller nucleoli. In the roundworm C. elegans, animals representing several distinct longevity pathways all shared this trait: small nucleoli, lower expression of ribosomal RNA and ribosomal proteins, and reduced levels of the nucleolar protein fibrillarin. Among genetically identical wild-type worms, individual nucleolar size was highly predictive of how long each animal would live. The finding held in fruit flies, mice, and even in human muscle biopsies from people who had undergone dietary restriction combined with exercise.21Nature Communications. Small nucleoli are a cellular hallmark of longevity
The implication is that dialing down ribosome production, whether through genetic changes or caloric restriction, extends lifespan and is accompanied by a physically smaller nucleolus. This makes intuitive sense: ribosome biogenesis is one of the most energy-consuming activities in a cell, and reducing it may free resources for maintenance and repair. The researchers proposed that small nucleoli could serve as a conserved cellular hallmark of metabolic health and longevity across species.
How Viruses Hijack the Nucleolus
Many viruses, including RNA viruses that replicate in the cytoplasm and have no obvious business in the nucleus, interact with the nucleolus during infection. Viral proteins can be targeted to the nucleolus, nucleolar proteins can be drawn out into the cytoplasm, and non-nucleolar host proteins can be rerouted into the nucleolar compartment.22PubMed Central. Manipulation of Cellular Processes via Nucleolus Hijaking in the Course of Viral Infection in Mammals The viruses co-opt nucleolar proteins to help with their own replication, transcription, and particle assembly.23PubMed Central. RNA viruses: hijacking the dynamic nucleolus
The consequences for the host cell are broad. Viral manipulation of the nucleolus can arrest the cell cycle, alter the rate of ribosome production (either ramping it up to produce viral proteins or shutting it down to suppress immune responses), trigger or block programmed cell death, and reshape signaling cascades involved in the stress response. Understanding these interactions has become a growing area of virology, partly because disrupting the virus-nucleolus connection could offer new therapeutic targets.
Evolution of the Three-Compartment Nucleolus
Nucleoli exist across all eukaryotes, but they are not all built the same way. The core machinery for ribosome maturation traces back to archaea, with additional contributions from bacterial and uniquely eukaryotic proteins layered on over evolutionary time.24PubMed. Insights into the evolution of the nucleolus by an analysis of its protein domain repertoire One interesting structural difference is that many organisms, including yeast, plants, and fish, have bipartite nucleoli with only two distinct compartments rather than the three seen in mammals and birds. The third compartment, the fibrillar center, appears to have emerged at the evolutionary transition between cold-blooded and warm-blooded vertebrates, possibly linked to a substantial increase in the size of the non-coding spacer regions between ribosomal RNA genes.25PubMed. Birth of a nucleolus: the evolution of nucleolar compartments
Genetic hybrids provide another window into nucleolar biology through a phenomenon called nucleolar dominance. When two species are crossed, the ribosomal RNA genes inherited from one parent are often selectively silenced while those from the other parent remain active. This silencing involves DNA methylation and has been documented in both plant and animal hybrids.26PubMed Central. Nucleolar dominance and ribosomal RNA gene silencing In Spartina grasses, for example, the ribosomal RNA genes from one parental species dominate in homoploid hybrids, and this dominance persists even after the hybrid undergoes genome doubling to become an allopolyploid, with the silenced genes remaining heavily methylated across populations.27PubMed Central. Nucleolar dominance arises in Spartina homoploid hybrids and persists after allopolyploidization The exact rules governing which parent’s genes “win” are still being worked out, but the phenomenon underscores how tightly ribosomal RNA gene expression is regulated and how quickly epigenetic control mechanisms can reshape it.
New Ways to Watch Nucleoli in Living Cells
A practical challenge in nucleolar research has been studying these structures in real time without killing the cell. Recent advances in imaging are changing that. One group developed a short signal peptide sequence, dubbed a “nucleolar beacon,” that reliably localizes to nucleoli across different mammalian cell types. Combining this beacon with proximity-labeling technology allowed them to capture and identify the proteins residing in or near the nucleolus in living cells.28PubMed Central. Nucleolar beacon for monitoring nucleolar morphology and proteomics in living cells Another approach uses automated fluorescence microscopy paired with machine-learning image analysis to track how specific proteins shuttle between the nucleolus and the surrounding nucleoplasm when cells are placed under oxidative stress.29Nucleic Acids Research. Monitoring nucleolar-nucleoplasmic protein shuttling in living cells by high-content microscopy and automated image analysis
Even the chemistry of nucleolar imaging probes has improved. New deep-red fluorescent dyes based on benzo[c,d]indole-quinoline scaffolds brighten dramatically when they bind RNA, with over 100-fold increases in fluorescence signal upon binding. Their longer emission wavelength gives them better tissue penetration and makes them easier to combine with other fluorescent labels in the same experiment.30PubMed. Benzo[c,d]Indole-Quinoline-Based Deep-Red Emissive Probes for Live-Cell Imaging of Nucleolar RNA These tools matter beyond the lab bench. Measuring nucleolar size and protein composition in patient-derived cells could eventually become a way to track disease progression or drug responses, given how intimately nucleolar changes are tied to cancer, neurodegeneration, and metabolic health.