Figure 1

Muscle cell regeneration: A: Normal muscle cell with myonuclei and satellite cells; B: Damaged muscle cell. Muscle injury causes inflammatory response and mononucleated cells are mobilized; C: Necrosis occurs in the damaged site. Macrophages invade the damaged tissue for cleaning up cellular debris. Satellite cells are activated; D: Activated satellite cells proliferate, differentiate and fuse to each other or with existing damaged muscle fibers; E: The regenerated new muscle cell in smaller caliber with centrally-located myonuclei and renewed satellite cells. The figure is modified from the textbook of MYOLOGY by Engel and Franzini-Armstrong. Nurse cell formation: F: Invasion of Trichinella larva causes dissolution and complete loss of myofibrillar organization; G: Satellite cells are activated. Basophilic transformation occurs in the infected muscle cell. A septum is formed to limit damaged area; H: Activated satellite cells proliferate, differentiate and fuse to each other or with the infected muscle cell, which provides eosinophilic cytoplasm. The infected muscle cell dedifferentiates, reenters cell cycle and arrests at G2/M. There are many hypertrophy nuclei; I and J: The eosinophilic cytoplasm (which is provided by satellite cells) increases in volume and the basophilic cytoplasm (which originates from infected muscle cell) decreases in volume; K: The mature nurse cell is formed. The cytoplasm of nurse cell is eosinophilic.