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Effect of the quality of the cumulus–oocyte complex in the domestic cat on the ability of oocytes to mature, fertilize and develop into blastocysts in vitro

Immature cumulus–oocyte complexes (COCs) were recovered from freshly excised domestic cat ovaries and graded at a magnification of x 40 for the condition of the cumulus oophorus of the oocyte cytoplasm. Grade I and II COCs were those with a uniformly dark cytoplasm and a readily identifiable, eccentrically located germinal vesicle. Grade I COCs had five or more cumulus oophorus cell layers, whereas grade II complements had less than five cell layers. Grade III and IV COCs were those undergoing progressive stages of oocyte cytoplasmic deterioration indicated by transparency or mosaic fragmentation and partial-to-complete loss of cumulus oophorus cells. In Expt 1, 699 oocytes were cultured for maturation and fertilization in vitro. More (P < 0.05) oocytes from grade I COCs matured (59.3%) and fertilized (29.7%) than from all other grades. Maturation and fertilization success did not differ (P > 0.05) for grade II (32.4, 11.6%, respectively) and grade III (21.9, 5.1%) oocytes, but these values were superior (P < 0.05) to those of grade IV (5.1, 1.4%). In Expt 2, 1040 COCs were graded, cultured for maturation and then inseminated. Of grade I oocytes, 24.4% developed into blastocysts compared with only 5.3% of grade II oocytes (P < 0.05). In general, oocytes from grade III and IV COCs were incapable of cleaving or growing in vitro. Of the 1739 COCs collected for both experiments, 12.3% met grade I criteria, the only category that provided consistent maturation, fertilization and development to blastocyst stage in vitro. In summary, a highly heterogeneous population of cumulus–oocyte complexes can be separated in the cat on the basis of grossly apparent morphological characteristics that, in turn, reflect functional differences in the ability of oocytes to mature, fertilize and develop in vitro.