Coypu (Myocastor coypus bonaeriensis) Ovarian Morphology During the Oestrous Cycle

The coypu (Myocastor coypus bonariensis) is an annual polyestrous hystricomorph rodent with variable-length oestrous cycles, which ovarian histoarchitecture during this period is not fully described. This study analyses variations in ovarian morphology during the oestrous cycle, focusing on follicular development and atresia. Eighteen sexually mature virgin females aged 7–9 months, weighing 4.795 ± 0.3996 kg, were used. Daily colpocytological analyses were performed to establish the cycle duration, followed by euthanasia at different phases—prooestrus, estrus, meteostrus, and on days 5, 10, and 20 of dioestrus. Morphology and superficial appearance of the ovaries, weight, and volume at each stage were recorded, and volume increase over time was analysed. A morphological classification and size ranges every 100 μm for follicular development analysis were used. Relative area analysis determined the proportions of different ovarian components and the absolute and relative frequencies of atretic and/or luteinized structures. Our study revealed differences between prooestrus, meteostrus, and all dioestrus phases, as well as between oestrus and metoestrus, and dioestrus at 10 and 20 days. Ovarian surface exhibited variations across different oestrous cycle stages, and atretic follicles were observed in all analysed stages. These findings provide new data and contribute to the understanding of the reproductive cycle in this species.