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University of Wisconsin2, Madison 53706
Abstract
Three experiments were conducted to characterize the follicular oocytes of bovine ovaries and to assess the oocyte's ability to mature in vitro. Experiment 1 was designed to describe the oocytes from surface follicles of early luteal, luteal or follicular stage ovaries. Oocytes were also categorized on the basis of follicle size (1 to 3 or > 3 mm). Three classification schemes with numerical values assigned were developed to characterize investments of the oocyte, state of the ooplasm and stage of nuclear development. Characteristics typical of oocytes from nonatretic, vesicular follicles were given values of one. Characteristics with higher values were considered to be signs of degeneration. Oocytes from nine cows per stage were evaluated. A higher proportion of nondegenerate investments was found in follicles 1 to 3 mm in size (P<.01). No other size or stage differences were present for investment or ooplasm. There was a greater incidence of intact chromatin present in the follicular stage (P<.01) and in follicles >3 mm in size (P<.01). Investment and ooplasm ratings were correlated (r = .41, P>.05) as was ooplasm and chromatin (r = .54, P<.05). The ability of oocytes within estrous stage and follicle size to undergo the first meiotic division in vitro was tested in experiment 2. Oocytes from 15 cows per stage were rated on investment and ooplasm, then cultured 24 to 26 hours. After culture, the oocytes were scored on the stage of maturation reached. Oocytes from follicles > 3 mm had a higher proportion of degenerate investment and ooplasm configurations (P<.05). These two features also differed among stages of the estrous cycle, the highest proportion of oocytes with degenerate features being found in the follicular stage. Stage differences in the ability of oocytes to mature in vitro were present (P<.01) and corresponded to those found for investment and ooplasm. Investment and ooplasm appearance were correlated (r = .27, P< 05). The extremes in investment and ooplasm appearance were the only obvious indicators of an oocyte's potential to mature in vitro. This was shown in experiment 3 where only 44% of the oocytes lacking investments matured in vitro as compared to 71% with intact investments (P<.01).
1 Research supported by the College of Agricultural and Life Sciences, University of Wisconsin, Madison 53706; Oscar Mayer and Co., Madison; Public Health Service Training Grant No. 5-TO1-HD-001-4-10 from the National Institute of Child Health and Human Development; and Grant No. 630-0505B from the Ford Foundation.
2 Department of Meat and Animal Science Paper No. 703.
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