Self-organization of MTOCs replaces centrosome function during acentrosomal spindle assembly in live mouse oocytes.
Chromosome segregation in mammalian oocytes is driven by a microtubule spindle lacking centrosomes. Here, we analyze centrosome-independent spindle assembly by quantitative high-resolution confocal imaging in live maturing mouse oocytes. We show that spindle assembly proceeds by the self-organization of over 80 microtubule organizing centers (MTOCs) that form de novo from a cytoplasmic microtubule network in prophase and that functionally replace centrosomes. Initially distributed throughout the ooplasm, MTOCs congress at the center of the oocyte, where they contribute to a massive, Ran-dependent increase of the number of microtubules after nuclear envelope breakdown and to the individualization of clustered chromosomes. Through progressive MTOC clustering and activation of kinesin-5, the multipolar MTOC aggregate self-organizes into a bipolar intermediate, which then elongates and thereby establishes chromosome biorientation. Finally, a stable barrel-shaped acentrosomal metaphase spindle with oscillating chromosomes and astral-like microtubules forms that surprisingly exhibits key properties of a centrosomal spindle.
Schuh
Melina
M
Gene Expression Unit, European Molecular Biology Laboratory, Meyerhofstrasse 1, D-69117 Heidelberg, Germany.
Ellenberg
Jan
J
D000818:Animals; D016764:Cell Polarity / Q000502:physiology; D002478:Cells, Cultured; D018385:Centrosome / Q000378:metabolism* / Q000502:physiology; D005260:Female; D007399:Interphase / Q000502:physiology; D051379:Mice; D008810:Mice, Inbred C57BL; D022101:Microtubule-Organizing Center / Q000378:metabolism* / Q000502:physiology; D008870:Microtubules / Q000502:physiology; D009865:Oocytes / Q000378:metabolism*; D008941:Spindle Apparatus / Q000378:metabolism*