The problem the researchers set out to solve is one of fragmentation. Liver aging involves several distinct cellular processes that are usually measured in isolation: the ploidy of hepatocytes, meaning how many copies of the genome each liver cell carries; the frequency of binucleated cells, which carry two nuclei; the proliferative activity of the tissue; and the accumulation of senescent cells that have permanently exited the cell cycle. Each of these features tells part of the story, but none of them alone describes the overall regenerative phenotype of the organ. The team’s answer was to combine them into single numerical descriptors that can be tracked over time and compared across experimental groups.

The first of these measures is the Index of Regenerative Reserve, abbreviated IRR. It is defined as the proportion of binucleated hepatocytes divided by the mean hepatocyte ploidy. The logic behind this ratio reflects the biology of the liver. Binucleated hepatocytes are generally associated with a state in which cells have completed cell-cycle events without dividing, and their abundance has been linked to regenerative flexibility. Mean ploidy, meanwhile, rises as hepatocytes undergo repeated rounds of genome duplication without cell division, a process known as polyploidization that becomes more prominent with age and chronic injury. By placing the proportion of binucleated cells over the average ploidy, the index captures the balance between these two faces of hepatocyte cell-cycle adaptation in a single value.

The second measure, the Integrated Index of Regenerative Status, or IRS, builds on the first by adding two molecular markers as fractional terms. The formula is IRS equals IRR multiplied by the quantity one plus Ki-67, divided by the quantity one plus p21. Ki-67 is a well-established marker of active cell proliferation, so a higher value indicates that more hepatocytes are actively cycling. p21 is a cyclin-dependent kinase inhibitor that enforces cell-cycle arrest and is widely used as a marker of cellular senescence. By multiplying the regenerative reserve by a proliferation-weighted numerator and a senescence-weighted denominator, the IRS is designed to fall when proliferative activity wanes or senescent arrest rises, and to hold steady or increase when the tissue retains its capacity to renew.

To test how these indices behave, the researchers turned to a model of accelerated aging that is unusual among liver studies: chronic circadian disruption. Male Wistar rats were assigned to three lighting conditions. A control group experienced normal light-dark cycles. A second group was kept under constant illumination, a condition the authors describe as dark deprivation, abbreviated DD. A third group experienced the same constant illumination but received melatonin at a dose of 10 milligrams per kilogram of body weight, five times weekly. Animals in all groups were assessed at four time points: 6, 12, 18, and 24 months of age, spanning a substantial fraction of the rat lifespan.

The rationale for this design rests on a growing body of evidence that light at night and disrupted circadian rhythms do more than disturb sleep. Previous work has shown that circadian disruption induced by light-at-night exposure can accelerate aging and promote tumorigenesis in rats, and the circadian clock itself has been shown to regulate hepatic polyploidy through the Mkp1-Erk1/2 signaling pathway. Constant illumination effectively deprives animals of the darkness that signals the pineal gland to produce melatonin, a hormone with antioxidant properties and broad influence over circadian physiology. The third group, in which melatonin was replaced exogenously, allowed the team to ask whether restoring the hormone could protect the liver’s regenerative profile.

The results were striking. In the dark-deprived animals, the Index of Regenerative Reserve fell from 2.00 plus or minus 0.42 at 6 months to 0.386 plus or minus 0.19 at 24 months, a decline of roughly eighty percent over the course of the study. The corresponding Integrated Index of Regenerative Status at 24 months was 0.32 plus or minus 0.17. These numbers depict a liver whose regenerative reserve is progressively eroded under conditions of chronic light exposure, consistent with the idea that circadian disruption drives pro-senescent remodeling of hepatic tissue.

In the animals that received melatonin alongside constant illumination, the picture was entirely different. At 24 months, the IRR in this group stood at 2.285 plus or minus 0.41 and the IRS at 2.08 plus or minus 0.16, values comparable to or even slightly above the starting levels seen in younger animals. In other words, exogenous melatonin appeared to preserve the composite regenerative profile of the liver across nearly the entire observation period, despite the animals being exposed to the same light regime as the unprotected group. The authors note that this finding aligns with their earlier work describing chronic light-induced desynchronosis as a model of accelerated metabolic aging in rats and the preventive role of melatonin in that context.

To explore whether the indices could discriminate between groups in a statistically meaningful way, the team performed an exploratory receiver-operating-characteristic analysis, a method borrowed from diagnostic testing that evaluates how well a continuous measure separates two conditions. The analysis yielded areas under the curve of 0.843 for the IRR and 0.905 for the IRS. An area under the curve of 1.0 represents perfect discrimination, while 0.5 represents chance performance, so values in the mid-0.8 to low-0.9 range indicate substantial, though not perfect, separation between the aging profiles of the dark-deprived and melatonin-treated animals. The IRS, which integrates molecular information on top of the structural measures, showed the stronger discriminatory performance.

The authors are careful about what these numbers do and do not establish. They present the IRR and IRS as experimental composite descriptors of the hepatic regenerative phenotype, and they state explicitly that external validation and direct functional regeneration assays are required before broader biological or clinical interpretation. The indices were developed and tested in a single animal model, and the receiver-operating-characteristic analysis was exploratory rather than confirmatory. Still, the approach addresses a real gap. Human studies have shown that hepatocyte expression of the senescence marker p21 is linked to fibrosis and adverse liver-related outcomes in alcohol-related liver disease, and that early liver regeneration after hepatectomy varies with age, yet clinicians lack integrated quantitative tools to gauge regenerative potential. If validated in independent models and eventually in human tissue, composite indices of this kind could offer a way to monitor how aging, circadian disruption, and interventions such as melatonin reshape the liver’s capacity to renew itself, turning a collection of scattered cellular measurements into a single readable signal of regenerative health.

Subject of Research: Quantitative composite indices of liver regenerative potential in a rat model of accelerated aging under circadian disruption and melatonin treatment

Article Title: Quantitative assessment of liver regenerative potential during accelerated aging: regenerative reserve and regenerative status indices

Article References: Areshidze, D. A., Kozlova, M. A., Anurkina, A. I., & Moskalev, A. (2026). Quantitative assessment of liver regenerative potential during accelerated aging: regenerative reserve and regenerative status indices. Biogerontology, 27(6), Article 179. https://doi.org/10.1007/s10522-026-10527-9

Image Credits: AI Generated

DOI: 10.1007/s10522-026-10527-9

Keywords: liver aging, hepatocyte polyploidy, binucleated hepatocytes, cellular senescence, melatonin, circadian disruption, regenerative capacity, Ki-67, p21, Biogerontology, Quantitative, assessment