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Even before fertilisation, there are many careful processes that allow early egg cells or oocytes, to mature. Here we see DNA chromosomes (highlighted in red) inside two starfish oocytes attempting to divide through meiosis. Usually, spindle fibres (blue) pull the cell and DNA apart into two structures of different sizes. While the smaller polar body is recycled, the larger of the two structures becomes the mature egg, keeping most of the cytoplasm and nurturing molecules contained in the original oocyte. In the bottom cell however, larger asters of spindle fibres mean the division is more symmetrical, similar to how a fertilised egg divides. Researchers blocked an important set of linked proteins, the Mos–MAPK pathway, in this oocyte, and they suggest MAPK might serve a switch between asymmetric and symmetrical division, perhaps yielding clues to similar mechanisms in human fertility.

Written by John Ankers

John Ankers

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A former researcher in systems biology, John is now an online biology tutor and professional coach supporting the wellbeing of scientists and NHS professionals. He's writing, too, of course.

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• Image from work by Ivan Avilov and colleagues
• Research Group Cytoskeletal Dynamics in Oocytes, Max Planck Institute for Multidisciplinary Sciences, Göttingen, Germany
• Image originally published with a Creative Commons Attribution 4.0 International (CC BY 4.0)
• Published in Journal of Cell Biology, October 2025