Eggs reveal their hidden architecture as researchers uncover how 200,000 mitochondria find their place
Scientists at Monash University have uncovered how mammalian eggs organise hundreds of thousands of mitochondria - the tiny energy-producing structures essential for fertilisation and early embryo development - solving a long standing question in reproductive biology.
The research team used advanced live imaging to observe, in real time, how mitochondria move within living mouse eggs. The study revealed that a mature egg contains approximately 200,000 mitochondria, most of which accumulate in the same hemisphere as the chromosomes. Although biologists have recognised this distinctive arrangement for many years, the mechanism responsible has remained unknown.
By combining live-cell imaging, experimental manipulation and innovative three-dimensional computational modelling, the researchers discovered that mitochondria are guided into position by two cooperating transport systems. One mechanism uses actin-driven cytoplasmic streaming to move mitochondria through the spindle hemisphere, while a second mechanism channels mitochondria through a narrow pathway adjacent to the chromosomes. This channel is regulated by the motor protein MYO19, directing mitochondria towards specific regions of the egg.
Mitochondria streaming into position in a mouse egg. Live imaging of a mouse egg as it transitions from meiosis I to meiosis II. After the asymmetric division that buds off a small polar body, the bulk of the mitochondria (orange) move towards the half of the cell containing the chromosomes (green). Actin (white) outlines the egg's cortex. This redistribution establishes the polarised mitochondrial pattern found in mature eggs and serves as the starting point for this study.
Published in Nature Communications, the study was led by Dr Inwon Lee and Professor John Carroll, of the Oocyte and Embryo Development Lab at the Monash Biomedicine Discovery Institute (BDI).
A key component of the project was an interdisciplinary collaboration with engineers from Monash University's Department of Mechanical and Aerospace Engineering. Associate Professor Reza Nosrati and PhD researcher Morteza Nazari developed 3D computational fluid-flow models that revealed how mitochondrial streaming patterns the egg's interior in three dimensions, creating distinct mitochondria-rich and mitochondria-poor regions.
The work opens new avenues for understanding how egg organisation influences fertilisation, meiotic division and the earliest stages of embryo development.
Read the full paper published in Nature Communications, titled Chromatin- and actin-mediated mitochondrial streaming leads to patterning of mitochondrial distribution in oocytes DOI: 10.1038/s41467-026-73192-z
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About the Monash Biomedicine Discovery Institute at Monash University
Committed to making discoveries that will relieve the future burden of disease, Monash Biomedicine Discovery Institute at Monash University brings together more than 120 internationally renowned research teams. Spanning seven discovery programs across Cancer, Cardiovascular Disease, Development and Stem Cells, Infection, Immunity, Metabolism, Diabetes and Obesity, and Neuroscience, Monash BDI is one of the largest biomedical research institutes in Australia. Our researchers are supported by world-class technology and infrastructure, and partner with industry, clinicians and researchers internationally to enhance lives through discovery.