Re: Muscle Cells

This question, whether muscles grow through hypertrophy (increased cell size) or hyperplasia (increased cell number), has been debated for years, and has only recently been answered adequately. The answer is: normal exercise causes hypertrophy, whereas overexertion, or any exercise that injures the muscles, results in both hypertrophy and hyperplasia.

First, some background on muscle cells. During development, muscles are formed from aggregation and proliferation of pre-myoblasts (migratory pre-muscle cells) to form "pre-muscle masses." After forming these masses, the pre- myoblasts turn on several of the genes required to form muscles, becoming myoblasts (pre-muscle cells). The myoblasts then fuse together to form giant, multinucleated cells called myotubes. The myotubes continue to grow, but only through the addition of more myoblasts, until each contains many, many nuclei. Eventually, the myotubes accumulate enough contractile proteins to build myofibrils (the biochemical motors that make muscles work) which completely fill the myotubes, pushing the nuclei to the edges of the tube. Once this machinery is complete, the myotubes become myofibers, which are the "muscle cells" in adult muscles. The myofibers continue to grow, though - by increasing the numbers of myofibrils in each cell, the myofibers can grow to many times their original size. But, with multiple nuclei and essentially solid cytoplasms, it is impossible for myofibers to divide through mitosis (even more so, since the nuclei lost their ability replicate before the myotubes fused). So the only way to get more myofibers is to fuse more myoblasts, but all of the myoblasts were used by the embryo to form the original muscles.

That's not entirely true. The exception is a small population of satellite cells which lie dormant next to the myofibers. They lie dormant until something happens that destroys part of the nearby muscle. The body's response to the injury activates the satellite cells which reproduce very quickly and then differentiate into myoblasts which fuse to form new myotubes, and eventually myofibers, at the site of injury to replace the muscle that was damaged. However, this only occurs when there is injury. Since normal exercise doesn't (or at least, shouldn't) involve damaging muscles, the satellite cells are never activated, so no new muscle cells are produced.

It is possible to exercise in such a way as to induce sufficient injury to activate the satellite cells, by doing stressful, violent routines. However, muscle injury, like any other injury, produces scarring, inflammation, and other unappealing effects. So, stressful exercise may produce more muscle cells, but it also increases the amount of connective tissue in the muscle, making it less efficient, so the muscle becomes bigger without getting stronger.