Canine uterine fibrosis associated with age and pyometra

This study investigated whether aging and uterine pathology drive fibrosis in the canine uterus by quantifying collagen deposition in endometrial and myometrial compartments. Uterine samples from 55 small-sized, intact, nulliparous bitches were classified into five groups according to age and pathological conditions. Healthy group divided into juvenile, young adult, old adult, and the pathological group included cystic endometrial hyperplasia (CEH), and pyometra. Fibrosis was assessed using Masson’s trichrome staining; collagen proportion was quantified with ImageJ and analysed in SAS. Across healthy age groups, collagen proportions in the endometrial stroma, circular myometrium, and longitudinal myometrium did not differ significantly (P > 0.05). In contrast, dogs with pyometra showed a significantly higher collagen proportion in the circular myometrium than in healthy old adult group with the same age (P < 0.05). No significant differences were detected in the endometrial stroma or longitudinal myometrium in pathological groups compared to healthy old adult group (P > 0.05). These findings indicate that physiological aging does not drive uterine fibrotic remodeling in dogs. The absence of significant collagen accumulation across all uterine layers may reflects species-specific reproductive physiology, as the canine uterus undergoes infrequent estrous cycles and prolonged anestrus, resulting in limited cumulative epithelial turnover and minimal stromal activation with age. Moreover, healthy aging lacks the inflammatory stimuli necessary to trigger fibroblast proliferation, explaining why fibrotic changes did not emerge in either the endometrium or myometrium. In comparison, fibrosis arises as a pathology-driven and layer-specific response in pyometra, where intense inflammation and elevated pro-fibrotic cytokines stimulate fibroblast activation and collagen accumulation, particularly within the circular myometrium. Further studies should investigate qualitative extracellular matrix changes and alternative molecular or biomechanical indicators to identify more sensitive biomarkers of uterine aging in dogs.