Age-related changes in hepatocyte ploidy, binucleation, proliferative activity, and senescence-associated cell-cycle arrest may jointly influence liver regenerative capacity, but these features are usually evaluated separately. We developed two composite indices and examined their behavior in a long-term rat model of chronic circadian disruption. Male Wistar rats were assigned to control lighting, constant illumination (dark deprivation; DD), or constant illumination with melatonin (DD + melatonin, 10 mg/kg five times weekly) and assessed at 6, 12, 18, and 24 months. The Index of Regenerative Reserve (IRR) was defined as the proportion of binucleated hepatocytes divided by mean hepatocyte ploidy. The Integrated Index of Regenerative Status (IRS) further incorporated Ki-67 (a marker of active cell proliferation) and p21 (a cyclin-dependent kinase inhibitor associated with cell-cycle arrest) as fractional terms: IRS = IRR × (1 + Ki-67)/(1 + p21).In DD animals, IRR declined from 2.00 ± 0.42 at 6 months to 0.386 ± 0.19 at 24 months; the corresponding 24-month IRS was 0.32 ± 0.17. In DD + melatonin animals, 24-month IRR and IRS were 2.285 ± 0.41 and 2.08 ± 0.16, respectively. Exploratory receiver-operating-characteristic analysis yielded areas under the curve of 0.843 for IRR and 0.905 for IRS. These findings support IRR and IRS as experimental composite descriptors of the hepatic regenerative phenotype. External validation and direct functional regeneration assays are required before broader biological or clinical interpretation.
