Unlike other cells in mammals, eggs lack centrosomes—crucial stabilizing structures that organize strandlike proteins called spindles, which pull chromosomes apart during cell division. How eggs form without centrosomes—a long-standing mystery—was solved in August by biologists at the European Molecular Biology Laboratory (EMBL) in Heidelberg, Germany.
To create an egg, a progenitor cell called an oocyte divides into two daughter cells: a hulking egg cell and a wimpy polar body. The oocyte’s chromosomes must be carefully sorted so that a representative half goes to each daughter cell. If not, the egg can end up with too many or too few chromosomes, leading to infertility or developmental disorders like Down syndrome.
“Why is the division of egg cells—which is so important at the start of animal life—why is that not very reliable?” asks Jan Ellenberg, unit coordinator and senior scientist at EMBL.
Using a powerful microscope to observe mouse oocytes as they split, Ellenberg’s group found that the spindles assembled into two coherent structures, one for the future egg and one for the future polar body.At first, spindles appeared throughout the cell in a sort of mesh. Next, they began to attract each other, forming around 80 different organizing centers. After gathering into a big blob around the chromosomes, the many microtubule organizing centers then began to repel each other. The tug-of-war as the spindles attracted and repelled one another eventually gave rise—over several hours (compared with the 10 to 15 minutes it takes other body cells to divide)—to two distinct structures, yanking the chromosomes to opposite poles.
The team also noticed that during fertilization, the many organizing centers disassembled, re-creating the mesh throughout. This flexibility might help explain why eggs use such an unusual mechanism: Microtubule organizing centers are also critical for bringing together the egg and sperm nuclei after fertilization.
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