Actin Duet for Fusion

What am I looking at?

This video captures two cultured mouse skeletal muscle cells undergoing fusion. The actin protein is labeled in green, and the myosin protein is labeled in red. You can see robust, finger-like actin protrusions dynamically forming at the contact site between the cells during the cell-cell fusion process.

Biology in the Background

Mature myofibers are long and thin, and they contain multiple nuclei. In humans, a single myofiber may contain hundreds or even thousands of nuclei. The fusion of mononucleated muscle cells into multinucleated myofibers is an essential step to build skeletal muscle during development, and to repair skeletal muscle after injury. The fusion of muscle cells is driven by actin-propelled invasive protrusions, like the ones dynamically formed where the two cells meet in the video. These actin protrusions enhance the cell membrane contact surface, push the two membranes into close proximity, and increase mechanical tension at the cell-cell fusion site to help cells overcome the energy barriers required for membrane fusion. After the fusion pore opens, the protrusions at the cell contact zone dissolve, followed by striking rearrangements and integration of the actin structures of the two cells while they fuse to become one.

Mouse muscle cells like those seen here can grow up to 20 micrometers across, or roughly 3.5 times smaller than the width of a human hair.

Technique

This video is created using confocal microscopy.