A cell’s plasma membrane protects and contains the contents of the cell, but is also flexible and fluid enough to allow the many events that take place at or involve the membrane, such as cell migration and changes in cell shape. A recent paper presents a mechanism for how a plasma membrane can accomplish both structure and flexibility. Kapustina and colleagues monitored rounded cells and the periodic membrane protrusions they make, and found compression (folding) and dilation (unfolding) of the plasma membrane and underlying actin cortex during protrusion events. This compression-dilation mechanism allows the cell to make rapid changes in cell shape, and can produce amoeboid-like migration movements under certain conditions. The electron microscopy images above show a cell fixed during membrane oscillations. Membrane folding appears as finger-like or round projections. (The yellow box shows the position of the higher magnification image on the right; red arrows point to dense cortical regions.)
BONUS!! Beautiful movie of F-actin (green) and myosin (red) during membrane protrusions is below. More movies of these membrane dynamics can be seen here.
Kapustina, M., Elston, T., & Jacobson, K. (2013). Compression and dilation of the membrane-cortex layer generates rapid changes in cell shape originally published in the Journal of Cell Biology, 200 (1), 95-108 DOI: 10.1083/jcb.201204157