A peculiar myofibrillar pattern in the murine muscle-tendon junction.

The muscle-tendon junctions of adult mice show a special arrangement of myofibrils. Essentially, the myofibrils are oriented in an anchor-like manner in the large finger-shaped processes of the muscle fiber, which contain "mitochondrial sacks" (Demmel et al. 1979) in a subsarcolemmal position. Furthermore, the single myofibril frequently branches out into smaller fibrils. It is suggested that due to the three-dimensional arrangement, these processes gain a high rigidity during contraction, thus maintaining their shape and preserving the "mitochondrial sacks".