Quantification and localization of cortical granules during oogenesis in the mouse

Abstract

Although the important subplasmalemmal localization of cortical granules (CG) has been documented, little is known about the timing and mechanism of granule translocation to the oocyte cortex. A fluorescent CG probe, confocal microscopy, and image analysis were used to obtain kinetic, spatial, and temporal information about CG migration during mouse oogenesis. The mean CG density in the cortex increased exponentially, from 3 to 14 CG/100 μm2, during oocyte growth from 40-50 μm to 70-80 μm in diameter, respectively. Full-grown, ≥ 80-μm, germinal vesicle-intact oocytes had a density of 30- 35 CG/100 μm2. During growth from 40-50 μm to ≥ 80 μm, the mean total number of CG in the cortex per oocyte increased from < 500 to > 6000. When analyzed in terms of the stage of germinal vesicle chromatin organization, the mean CG density increased from 3 to 21 CG/100 μm2 from stage I to early stage IV, respectively. In 50-60-μm oocytes,there was a crescent-shaped area of perinuclear staining containing both granules and Golgi apparatus-like structures, which were also more sparsely distributed in the subcortical cytoplasm. It is likely that these were sites of CG production from which translocation takes place; they were not observed in ovulated metaphase II eggs. This study demonstrates that CG number in the cortex increases continually during mouse oocyte growth (in contrast to that in several other species), that there is a greater than twofold increase in CG number during the final phase of growth, and that subcortical production ceases after ovulation.