Muscles in a Dish

What am I looking at?

This is a fluorescently labeled culture of mouse muscle cells grown from stem cells, which have fused together to form so-called myotubes. The nuclei of these cells are teal (1). The remaining stem cells and partially differentiated muscle cells are green (2). The myosin protein is red (3). The areas that appear yellow are partially differentiated stem cells that have begun to express the myosin protein (4).

Biology in the background

Skeletal muscle cells are long and thin and can contain multiple nuclei. These cells fuse end to end with their neighbors to form myotubes, which then collectively form muscle fibers. When bundled together, these muscle fibers create what we commonly call a muscle.

The myosin protein (in red) is one of the key proteins involved in contracting a muscle. Multiple myosin proteins group together to form filaments (called “thick filaments”) within a muscle cell. They interlock with a similar array of so-called “thin filaments” formed by F-actin (the F stands for “filamentous”) proteins. Together, thick and thin filaments form the basic contractile unit of a muscle, known as a sarcomere.

How do muscle fibers move? Motor neurons kick off the process by sending a chemical message to the muscle cells. The muscle cells release calcium from their cellular stores. Together, these signals cause the two sets of filaments to slide against each other. Translating the chemical energy into mechanical energy (i.e., force), the myosin fibers can pull on the actin fibers, forcing the cell to contract and making the muscle fibers move.

In both mice and humans, muscle cells are large. They can be up to 100 micrometers across, or slightly wider than the width of a human hair. And they can be many times longer than that, with the longest muscle fibers reaching 12 centimeters, or about 4.5 inches.

Technique  

This image was created using fluorescence microscopy.