The Endocrinology of Muscle Growth

The road to anabolic insight must include a biological understanding of what muscle growth actually entails. Often simplified by the term "protein synthesis': muscle growth is actually a highly complex process involving much more than just building proteins from amino acids. Muscle hypertrophy, the correct scientific term for the way we adult humans build skeletal muscle, actually requires the fusion of new cells (called satellite cells) with existing muscle fibers. Since this discovery of satellite cells in 1961, a great deal of research into the mechanisms of muscle hypertrophy has been undertaken. Scientists have come to understand that unlike normal muscle cells, these satellite cells can be regenerated throughout adult life. Furthermore, they serve not as functional units of their own, but provide some of the necessary components to repair and rebuild damaged muscle cells. These satellite cells are normally dormant, and sit resting in small indentations on the outer surface of the muscle fibers, waiting for something to trigger them into activation.

Injury or trauma will provide the stimulus necessary to activate satellite cells. Once activated, they will begin to divide, multiply, and form into myoblasts (myoblasts are essentially donor cells that express myogenic genes). This stage of hypertrophy is often referred to as satellite cell proliferation. The myoblasts will then fuse with existing muscle fibers, donating their nuclei. This stage of the process is usually called differentiation. Skeletal muscle cells are multinucleated, which means they possess many nuclei. Increasing the number of nuclei allows the cell to regulate more cytoplasm, which allows more actin and myosin, the two dominant contractile proteins in skeletal muscle, to be produced. This increases the overall cell size and protein content of the muscle cell. Incidentally, the number of nuclei in relation to cross-sectional area also helps to determine the fiber type of the cell, namely slow twitch (aerobic) or fast twitch (anaerobic)326 327. It is important to note that we are not increasing muscle cell number with muscle hypertrophy. We are only increasing cell size and protein content, even though we are using satellite cells to help accomplish this. It is possible for myoblasts to fuse together and actually form new muscle fibers.This is called muscle hyperplasia,and equates to the legitimate growth of new muscle tissue. This is, however, not the primary mechanism of muscle growth in adult life.