Histomorphologic changes in expanded skeletal muscle in rats.

Tissue expansion is one of the powerful tools for various reconstructive procedures and has proven to provide more available local tissues. However, limited attention has been given to the characteristics of expanded skeletal muscle. Using a rat model (n = 41), we expanded the rat gracilis muscle and investigated the histomorphologic changes in the expanded skeletal muscle. By expansion, the gracilis muscle after 3 weeks increased 50.4 to 58.4 percent in length and 60.5 percent in width and decreased 39.0 to 42.0 percent in thickness. Histologically, the expanded muscle demonstrated a normal striation and no signs of inflammation or necrosis. The cross-sectional areas of muscle fibers indicated that expanded muscle consisted of predominantly smaller fibers. Vasculature in the expanded muscle demonstrated a longer network of arteries and a more obvious and developed arterial arcade. The average number of sarcomeres in a fiber estimated from the sarcomere length and fiber length was significantly greater (46.5 percent) in the expanded muscle. These findings suggest that the expansion of skeletal muscle is not a "stretching" process of muscle but rather a growth process of the muscle accompanied by an increase in the number of sarcomeres per fiber. Furthermore, the expanded skeletal muscle appears to preserve normal skeletal muscle architecture, vasculature, and function while undergoing the ischemic stress of expansion.