Biology of Reproduction, Vol 23, 988-993, Copyright © 1980 by Society for the Study of Reproduction

Maturation of Rat Oocytes in vitro Related to the in vivo Stages of the 4- and 5-Day Estrous Cycle: Morning and Evening Cultures

¹ Southern Illinois University, School of Medicine, Carbondale, Illinois 62901

The capacity of rat oocytes to mature in vitro was studied in relationship to the stages of the 4- and 5-day estrous cycles and to the time of day. Antral oocytes obtained from animals on regular 4- and 5-day cycles were placed in culture under standard conditions either in the morning or in the evening and examined with a dissecting microscope 10 h later for the stage of maturation, i.e., intact nucleus or germinal vesicle (GV), germainal vesicle breakdown (GVB), or extruded polar body (PB). Three relationships were studied: oocyte maturation was compared 1) between days of the same cycle for each AM and PM culture period (day-to-day changes without regard to diurnal changes); 2) between AM and PM cultures within each cycle length (diurnal variations); and 3) between similar days of the 4-and 5-day estrous cycles (cycle length dependency). There were no differences in the total number of oocytes maturing (GVB + PB) or in the percentage of immature oocytes at the end of the incubation period. PB formation varied from low values on estrus and metestrus to peak values on diestrus and proestrus in both cycles when oocytes were placed in culture in the morning. GVB formation was the reverse of PB formation. The differences between days within the same cycle generally were not evident when oocyte culture was initiated in the evening. When morning and evening maturation rates were compared in the 4-day cycle (diurnal variations), PB formation was higher in the morning of diestrus compared with the evening of metestrus or diestrus and higher in the morning of proestrus than in the evening of diestrus or proestrus. A similar but less pronounced rhythm in PB formation was evident in the 5-day cycle in which PB formation was higher in the evening as opposed to the morning of metestrus. GVB also exhibited 12 h fluctuations and was the opposite of PB formation in both cycles. When morning and evening PB formation and GVB were contrasted for comparable days of the two cycles, the two cycles had a similar pattern of maturation prior to proestrus but diverged within 2 days after the gonadotropic surge. The results support the conclusion that rat oocyte maturation in vitro varies with the stage and length of the estrous cycle and with the time of day.

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ACKNOWLEDGMENTS The author’s research was supported by Southern Illinois University. The author wishes to thank Dr. Lynn G. Nequin for encouragement and helpful suggestions during the course of the experiments.