KANSAS CITY, MO -- Most cells rely on structural tethers to position chromosomes in preparation for cell division. Not so oocytes. Instead, a powerful intracellular stream pushes chromosomes far-off the center in preparation for the highly asymmetric cell division that completes oocyte maturation upon fertilization of the egg, report researchers at the Stowers Institute for Medical Research.
Their findings illustrate how oocytes repurposed a dynamic cellular mechanism capable of generating considerable intracellular forces and widely used by migrating cells to propel them forward, to set the stage for asymmetric cell division the kind of cell divisions that generate two different daughter cells. It might also lead to improvements in the selection criteria used to choose the most promising oocytes for in-vitro fertilization.
As a mammalian egg develops, it undergoes two highly asymmetric cell divisions, known as meiosis I and II. During each of these divisions, the cytoplasm divides unequally, giving rise to a large egg and two polar bodies that are much smaller than the developing oocyte. To achieve this uneven distribution of the cytoplasm, the meiotic spindlethe structure that separates the chromosomes into daughter cellshas to be positioned close to the so-called cortical cap, the region where the polar body will form.
"Conventional thinking predicted some sort of physical tether that moors the meiotic spindle at the cortical cap," says Rong Li, Ph.D., Stowers investigator and senior author of the study published in the August 28, 2011, advance online edition of Nature Cell Biology. "It came quite as a surprise that, instead, a continuous intracellular flow pushes the spindle into the correct position and keeps it there."
Earlier studies had ruled out microtubules, which help position the spindle during mitotic cell divisions, as potential tethers while older studies had hinted at actin as a possible candidate. Ac
|Contact: Gina Kirchweger|
Stowers Institute for Medical Research