Mitosis is used for things like growth and repair. Not all cells divide. Skeletal muscle cells don’t divide. When they are damaged, the missing tissue gets filled in with scar tissue. You might be thinking, “Hey, some people grow their muscles really big!” That’s true, but they do it by increasing the size of the cells and the blood supply to the muscles, not by adding cells.

The protein fibers that contract can be added or lost depending on how much the muscle is used. Making and supporting muscles costs calories and other resources, so our bodies only invest in as many as we need.

If you think about it, it makes sense. Skeletal muscle cells are really made of a bunch of individual muscle cells that make up one long cell with lots of nuclei. They have to be strong and not pull apart. If they were dividing, they would be weak and our muscles could tear. Ouch. What kinds of cells do you think go through mitosis all the time? [Hint: repair and replace]

Skeletal muscle cells are really interesting because the actual fibers do not undergo mitosis. When you exercise, the muscle fibers grow and create more filaments which allow the muscle to do more work. These filaments are basically just long protein ropes (made of actin and myosin) that contract to force your skeleton to move. But even though the cells are growing, they are not dividing. It is important to make the distinction of skeletal muscle fiber cells, which are the striated cells that we picture when we think of muscles, because there are accessory cells assisting muscles that can divide. These cells are called satellite cells, which undergo mitosis when skeletal muscles get injured. The satellite cells then fuse with damaged skeletal muscle cells to help the repair process. Isn’t it cool that there are cells that cannot directly repair themselves? Our body can be so complex, and there is always so much more to learn about physiology. Thanks for the question!

Skeletal muscles are the muscles we use to move around everyday. When we are growing as a child, the muscle cells undergo mitosis so that our muscles can grow in proportion to our bones, height, and weight. During adulthood, though, no new muscle cells are generated. Muscle cells can be enlarged in a process called hypertrophy, like you see with people who work out a lot, but no new ones will be created. This poses a problem for doctors with patients that have damaged muscle tissue!

Muscle cells develop from myocytes, which are a kind of stem cell (specifically, a muscle stem cell). These myocytes can undergo mitosis, but that implies that the developed muscle cells themselves cannot. In this case, the answer to your question is "never".