Cytological features of pig oocytes in the process of spontaneous maturation

Abstract

Aim. The study was aimed at determining the cytological changes, related to the germinal vesicle (GV) of porcine oocyte during the follicular maturation. Methods. The ovaries were classifi ed according to the estrous cycle phase, judging by the status of yellow bodies and follicles. The oocytes were isolated from each animal using dominating and atresiated follicles separately. The oocytes were used to prepare whole mounts by a specially designed express-method, which is thermal fi xation (about 95 °С) and their further transfer into the medium for clarifi cation. The whole procedure of preparing the mount from one oocyte or their group lasted for about 10 min. The microscopic studies were conducted using the phase contrast. A total of 831 oocytes were studied. Results. It was established that sow oocytes had eccentric location of GV which may be found in oocytes from early antral follicles. This condition is kept until the beginning of meiosis restoration and GV destruction. The nucleolus is also located eccentrically. Another relevant cytological change was gradual condensation of chromatin around the nucleolus, starting from separate clots on its surface and further formation of circular structures. Conclusions. Contrary to laboratory rodents, GV in porcine oocytes has eccentric localization, which is set even in early antral follicles and remains until the beginning of meiosis restoration. The process of chromatin transformation was found to be its gradual condensation around the nucleolus which may be defi ned as a nucleolus complex.