Abstract
BackgroundWe tested the hypothesis that organelles in bovine oocytes undergo changes in number and spatial distribution in a manner specific for phase of follicle development.MethodsCumulus-oocyte-complexes were collected from Hereford heifers by ultrasound-guided follicle aspiration from dominant follicles in the growing phase (n = 5; Day 0 = ovulation), static phase (n = 5), regressing phase (n = 7) of Wave 1 and from preovulatory follicles (n = 5). Oocytes were processed and transmission electron micrographs of ooplasm representing peripheral, perinuclear and central regions were evaluated using standard stereological methods.ResultsThe number of mitochondria and volume occupied by lipid droplets was higher (P < 0.03) in oocytes from regressing follicles (193.0 ± 10.4/1000 μm3 and 3.5 ± 0.7 %) than growing and preovulatory stages (118.7 ± 14.4/1000 μm3 and 1.1 ± 0.3 %; 150.5 ± 28.7/1000 μm3 and 1.6 ± 0.2 %, respectively). Oocytes from growing, static and preovulatory follicles had >70 % mitochondria in the peripheral regions whereas oocytes from regressing follicles had an even distribution. Oocytes from growing follicles had more lipid droplets in peripheral region than in central region (86.9 vs. 13.1 %). Percent surface area of mitochondria in contact with lipid droplets increased from growing (2.3 %) to static, regressing or preovulatory follicle stage (8.9, 6.1 and 6.2 %). The amount, size and distribution of other organelles did not differ among phases (P > 0.11).ConclusionsOur hypothesis was supported in that mitochondrial number increased and translocation occurred from a peripheral to an even distribution as follicles entered the regressing phase. In addition, lipid droplets underwent spatial reorganization from a peripheral to an even distribution during the growing phase and mitochondria-lipid contact area increased with follicle maturation.
Highlights
We tested the hypothesis that organelles in bovine oocytes undergo changes in number and spatial distribution in a manner specific for phase of follicle development
Qualitative morphological observations The oocytes presented a series of typical membranebounded organelles and lipid droplets (Fig. 2)
We postulate that the oocytes from static phase follicle were morphologically similar to those from preovulatory phase follicles as they had mitochondria in the peripheral region and increased contacts between mitochondrial and lipid droplet compared to other phases
Summary
We tested the hypothesis that organelles in bovine oocytes undergo changes in number and spatial distribution in a manner specific for phase of follicle development. Bovine oocytes achieve the competence to sustain fertilization and initial embryonic development to the blastocyst stage by the time the surrounding follicles reach 2–3 mm [1, 2]. Oocyte processes most likely influenced during follicular selection and dominance are cytoplasmic events such as changes to metabolic pathways, organelle structure and function, and their interactions with each other. Mitochondria and the smooth endoplasmic reticulum (SER) are the primary source of Ca2+ oscillations that govern events such as nuclear maturation, fertilization and activation of embryonic development [10, 11]. The events of cytoplasmic development before the LH surge during follicular dominance, are less studied. The results of an electron microscopy-based qualitative study suggest that structural changes with respect to different organelles occur in the dominant follicle before the LH surge [12]
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