One of my favorite analogies in cell biology revolves around cupcakes and asymmetric cell division, which happen to be two of the most wonderful things in the world. If you cut a cupcake in half down the center, you have two equal pieces with both icing and cake. Or, you can cut the cupcake in half across the center, and have one piece with just icing and one piece with just cake. Today’s image is from a paper describing how a cell divides to result into two equal icing-and-cake cells.
During development, the orientation of cell division within an epithelial sheet helps to drive tissue shape changes. Symmetric cell division, during which the mitotic spindle is parallel to the plane of the sheet, leads to tissue growth and elongation, while asymmetric division, during which the spindle is perpendicular to the epithelial sheet, causes tissue thickening and stratification. Most research on mitotic spindle orientation has focused on asymmetric cell division, but a recent paper describes interesting results on how a spindle is positioned during symmetric division. Woolner and Papalopulu looked at epithelial tissue in early frog embryos to test possible mechanisms of spindle positioning in symmetric cell divisions. As seen in the image above (left), the spindle is positioned precisely in the plane of the epithelial sheet. Woolner and Papalopulu found that a basally-directed force (pushing down, into the sheet) is provided by microtubules and myosin-10, while an apically-directed force is provided by actin filaments and myosin-2. Both of these forces are required to position the spindle in the plane of the epithelium, and at its proper position along the apical-basal axis. In the middle image above, the spindle is positioned near the apical cell surface after astral microtubules were disrupted. After actin-filament disruption (right image), spindles moved toward the basal cell surface.
Woolner, S., & Papalopulu, N. (2012). Spindle Position in Symmetric Cell Divisions during Epiboly Is Controlled by Opposing and Dynamic Apicobasal Forces Developmental Cell, 22 (4), 775-787 DOI: 10.1016/j.devcel.2012.01.002