Chromatin- and actin-mediated mitochondrial streaming leads to patterning of mitochondrial distribution in oocytes

Mitochondria are highly dynamic organelles, and their spatiotemporal organization is strictly regulated. While it has long been recognized that mitochondria in ovulated oocytes are concentrated in the spindle hemisphere, the mechanism remains unknown. Through live cell imaging and modelling, we have discovered three features of mitochondrial dynamics in the ooplasm: (i) actin-driven cortical mitochondrial streaming is delimited to the boundary of the polarized spindle hemisphere; (ii) distinct from bulk cytoplasmic streaming, mitochondrial streaming occurs bilaterally and perpendicular to the long axis of the MII spindle and, (iii) mitochondria movement from the cytoplasm to the polarized cortex is via MYO19-mediated chromatin-associated channel around the spindle midzone. This directionality in mitochondrial streaming patterns the ooplasm of the spindle cortex, creating mitochondria-rich and mitochondria-poor regions. These features explain the establishment of the polar gradient of mitochondria in MII oocytes and may provide new insight into the spatiotemporal organization of mitochondria in cells.