Mature Domestic Cat Oocyte Does Not Express a Cortical Granule-Free Domain1

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Abstract
A premature release of cortical granules (CG), found in the cortex of unfertilized oocytes, and the resulting formation of a CG-free domain (CGFD) over the metaphase II spindle are associated with nuclear maturation in the hamster and mouse. The objectives of our study were to characterize and compare the distribution of CG in immature, in vitro-matured, and in vivo-matured domestic cat oocytes while determining if a CGFD is formed that may be useful as a marker for stage and normalcy of oocyte maturation. Immature, intrafollicular oocytes were collected from ovaries obtained from local veterinary clinics, and a portion of these oocytes were matured in vitro. In vivo-matured, metaphase II oocytes were flushed from the oviducts of gonadotropin-treated, ovariohysterectomized cats. CG were visualized by Lens culinaris agglutinin-biotin/Texas red-strepavidin fluorescence, routine transmission electron microscopy, and Lens culinaris agglutinin-biotin/gold-labeled strepavidin transmission electron microscopy. No CGFD was detected in any domestic cat oocyte. Immature, in vitro-matured, and in vivo-matured oocytes had similar, uniform distributions of CG throughout the entire cortical region as measured by fluorescence microscopy. In vivo-matured oocytes were further examined by transmission electron microscopy, which confirmed the lack of a CGFD. All oocytes contained CG having a mean diameter of 0.28 +/- 0.03 mum/granule and a mean density of 51.5 +/- 13.0 CG/100 mum of plasma membrane. These results 1) demonstrate a uniformity in CG characteristics and distribution in domestic cat oocytes at various developmental stages, and 2) indicate that premature CG release does not occur and therefore, that localized cortical granule exocytosis cannot be used as a marker for oocyte maturation in this species.