Polar Body

Asymmetric division:  Polar bodies serve to eliminate one half of the diploid chromosome set produced by meiotic division in the egg, leaving behind a haploid oocyte. To produce the polar bodies, the cell must divide asymmetrically, which is fueled by furrowing (formation of a trench) near a particular point on the cell membrane.  (from wikipedia)

Image: Immunofluorescence micrograph of mouse oocyte during the first meiotic division. Microtubule and cortical actin are labeled blue and red, respectively, while DNA is stained cyan. The image was recorded on a scanning confocal (Zeiss LSM 710) with a 63x 1.2 NA water immersion objective. The diameter of the oocyte is 75 micron. Learn more in Pfender et al. (2011). (Image from cell.com-link)

For a human or mouse oocyte to mature into a fertile egg, it must first extrude half its chromosomes into a small cell, called the polar body. Recently, Pfender and colleagues identified the actin nucleators Spire as key factors for this extreme asymmetric division in mouse oocytes. Spire1 and Spire2 help assemble an actin network that positions the spindle asymmetrically. These factors then promote extrusion of the polar body by assembling a cleavage furrow.

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