To clarify the cytological characteristics of the porcine oocytes having meiotic competence, the oocytes retrieved from small (1–2 mm in diameter) and large follicles (3–6 mm) were compared. Cumulus-lacking oocytes were also examined to distinguish the characteristics between growing and atretic follicular oocytes. Distributions of cytoskeleton and mitochondria were observed by fluorescence staining in relation to the germinal vesicle (GV) stages, classified as GV 0 and GV I - GV IV. Oocyte size (107.2 ± 0.6 μm vs. 116.9 ± 0.7 μm, p<0.05) and maturation rate (9 vs. 87%, p<0.01) were lower in the small follicular oocytes than the large follicular oocytes. GV 0 oocytes were more frequently found in the small follicles than in the large follicles (67 vs. 9%, p<0.01). Most of the GV 0 oocytes from small follicles were likely forming the cytoplasmic microtubules and microfilaments. In contrast, most of the GV oocytes from large follicles showed an even distribution of fibrous microtubules in the ooplasm. In some of the small follicular oocytes beyond the GV II stage, fluorescence intensities of micro tub ules and mitochondria decreased. These features were similar to those observed in the cumulus-lacking oocytes. The results suggest that most of the GV 0 oocytes may be developing cytoplasmic cytoskeleton, and consequently are unable to complete meiosis. Also, advanced GV stage oocytes with low density of cytoskeleton and mitochondria, especially those from small follicles, may be derived from atretic follicles.
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Vol. 25 • No. 3