Mitochondria aggregation patterns during human oocyte maturation

Animal studies have demonstrated that mitochondrial distribution within the oocyte is closely associated with nuclear and cytoplasmic maturation. Aberrant mitochondrial localisation patterns have been linked to incomplete cytoplasmic maturation and reduced developmental competence. The aim of this study was to characterise the dynamics of mitochondrial distribution in human oocytes across different stages of maturation. Using discarded oocytes from patients undergoing intracytoplasmic sperm injection (ICSI) we analysed the concentration of mitochondria and their distribution patterns. Homogeneously diffuse, centrally located, and peripheral distributions were detected in mature metaphase II (MII), immature metaphase I (MI) and immature germinal vesicle (GV) oocytes, respectively. Of the MII oocytes, 61% presented an evenly diffuse pattern and of the GV oocytes, 53% presented a peripheral distribution. The mean mitochondrial density increased significantly from 278,501 ± 65,989 area units in GV oocytes to 352,340 ± 37,942 in MII oocytes (p = 0.003). For cytoplasmic occupancy, which followed a non-normal distribution, the median percentage increased from 73.7% (IQR: 66.9%-78.7%) in GV oocytes to 100.0% (IQR: 84.8%-100.0%) in MII oocytes (p < 0.001). Upon maturation, human oocytes demonstrate an increase in mitochondria density and cytoplasm occupancy. Importantly, the mitochondria tend to be located mainly at the periphery in immature oocytes, achieving a more diffuse homogeneous pattern as the oocyte matures. This pattern of distribution may play an important role in cytoplasmic maturation of human oocytes and their further development.