Maturation-related changes in bovine oocytes under different culture conditions

Abstract

Immature bovine cumulus-oocyte complexes collected from ovaries of slaughtered animals were cultured for different lengths of time in two buffer systems, one against air and one against 5% CO 2 in air, to determine a proper maturation time for subsequent in vitro fertilization and to investigate the possibility of using an air-buffer system for a routine in vitro maturation scheme. Maturation-related changes in the nucleus and cytoplasm of the oocytes were followed to monitor this simplified routine procedure. Cumulus cells started to expand after 12 h of incubation, when all oocytes were in metaphase I. Although the first polar body started to be extruded after 18 h of incubation, the extrusion rate increased during a period of up to 20 h of culture in 5% CO 2 in air and 24 h of culture in air. However, rates of maturation after 20, 24 and 26 h of incubation did not differ significantly among oocytes regardless of the culture conditions. The reorganization of cytoplasmic organelles followed a stage-related pattern, as was evident from the examination of semithin and ultrathin sections from selected oocytes. A significant proportion of the bovine oocytes had completed cytoplasmic and nuclear maturation after 20 h of culture, whether or not additional CO 2 was present in the culture atmosphere.