Rather than dividing to produce two identical daughter cells, an oocyte divides to produce a large egg ready for fertilization and a very small polar body. This extreme asymmetric division allows the egg to retain all of the crucial cytoplasm to support a future early embryo. This asymmetry is necessary for fertility and development, but the mechanisms required for this event are not completely understood. A recent paper describes the importance of two novel actin nucleators called Spire1 and Spire2 in mouse meiotic divisions. These Spire proteins drive the assembly of an actin network that acts as a substrate for positioning of the meiotic spindle, and promotes the cytokinetic cleavage furrow that results in polar body extrusion. Image above shows a mouse oocyte undergoing a meiotic division—chromosomes are cyan, microtubules are blue, and cortical actin is red.
BONUS!! Check out this great movie showing spindle positioning in normal (left) and Spire-deficient (right) ooctyes.
To see the cover of Current Biology for this issue, which features the above image, click here.
Pfender, S., Kuznetsov, V., Pleiser, S., Kerkhoff, E., & Schuh, M. (2011). Spire-Type Actin Nucleators Cooperate with Formin-2 to Drive Asymmetric Oocyte Division Current Biology DOI: 10.1016/j.cub.2011.04.029
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