Follicle-oocyte atresia and temporal taphonomy in cold-stored domestic cat ovaries.

In vitro oocyte maturation followed by in vitro fertilization (IVM/IVF) success in the domestic cat remains inferior to commonly studied livestock or laboratory species. The objectives here were (1) to histologically assess atresia status of freshly excised follicle/oocyte complexes, and (2) to evaluate taphonomic change (deterioration after excision) of these complexes after ovarian cold storage for up to 48 h. After excision of 50 ovarian pairs, one ovary was preserved immediately and the other stored in phosphate buffered saline (4 degrees C) for 4, 8, 12, 24, or 48 h before fixation and examination. Ovaries were classified as luteal if prominent corpora lutea (CL) were present or as follicular if antral follicles and no CL were present. Two classes of follicle-oocyte complexes (preantral and antral) were microscopically evaluated. Of the 2,280 complexes examined, 64.3% demonstrated clear evidence of slight to severe degeneration, with various stages being described and photographed for the first time. There was no histological evidence indicating distinctive morphological differences between oocytes recovered from follicular versus luteal donors. Storage of whole ovaries in cold saline inhibited taphonomic changes for 48 h after excision. In summary, there is marked variability in the number and quality of follicle populations in cat ovaries. A high percentage of full-sized follicular oocytes are undergoing atresia at any given time. However, additional gross degeneration as a result of cold-storage appears modest for up to 48 h. Nonetheless, this high level of natural atresia in the cat likely contributes to comparatively lower IVM/IVF success than in other species.