Flow cytometric evaluation of canine follicular cell apoptosis during the oestrous cycle

Apoptosis is the cellular mechanism of ovarian follicular atresia in mammals; the aim of this study was to examine the apoptosis-related cyclic changes in follicular cells of different-sized antral follicles throughout the oestrous cycle in canines. Ovaries were collected from 26 adult female dogs (1-4 years) following routine ovariohysterectomy. Antral follicles were classified as small, medium or large antral or preovulatory. The percentage of apoptotic cells was determined flow cytometrically using the terminal deoxynucleotidyl transferase-mediated deoxyuridine triphosphate (dUTP) DNA nick end labelling (TUNEL) assay. Apoptosis rate was quantified as the percentage of TUNEL-positive cells on a logarithmic scale. Percentages of TUNEL-positive cells obtained in the flow cytometric assay were compared among oestrous phases and follicular sizes using analysis of variance. Apoptotic follicles were observed in all types of canine follicles in different cycle phases and stages of development, possibly corresponding to the physiological process of the oestrous cycle. Both the oestrous phase and follicular size significantly influenced the apoptosis rate (p < .05). Apoptosis rate increased significantly (p < .05) as follicular development progressed. Apoptosis rate was the highest in large follicles during the oestrous phase (9.2%; p < .05) and the lowest in small follicles during the anestrus period (1.8%; p < .05). In conclusion, our results demonstrate significant differences in the apoptosis rate during the oestrous cycle related to follicle development in the canine ovary. Furthermore, flow cytometry using the TUNEL assay was found to be an effective method for detecting apoptosis in canine follicles.