Dynamic changes in mitochondrial distribution in human oocytes during meiotic maturation

The change of mitochondrial distribution in human oocytes during meiotic maturation was assessed using 223 human oocytes donated from patients undergoing fertility treatment between June 2013 and February 2016.

Live cell images of fluorescence-labelled mitochondria in human oocytes were analysed to investigate dynamic changes in mitochondrial distribution during meiotic maturation using a confocal microscope combined with an incubator in the presence or absence of colchicine and cytochalasin B, inhibitors for tubulin and actin filament, respectively. Subcellular distribution of mitochondria in human oocytes was also assessed at various stages using a transmission electron microscope (TEM).

Live cell imaging analysis revealed that the mitochondria-occupied cytoplasmic area decreased from 83 to 77 % of the total cytoplasmic area around 6 h before germinal vesicle breakdown (GVBD) and that mitochondria accumulated preferentially close to the perinuclear region. Then, the mitochondria-distributed area rapidly increased to 85 % of total cytoplasm at the time of GVBD. On the other hand, there was no significant change in mitochondrial distribution before and after polar body extrusion. Such changes in mitochondrial localization were affected differently by colchicine and cytochalasin B. Most of mitochondria in the cytoplasm formed cluster-like aggregates before GVBD while they distributed homogeneously after GVBD.

Most mitochondria localized predominantly in the non-cortical region of the cytoplasm of GV stage-oocytes, while the mitochondria-occupied area decreased transiently before GVBD and increased rapidly to occupy the entire area of the cytoplasm at GVBD by some cytoskeleton-dependent mechanism.