Skeletal muscle in the pineal gland of the bat, Rhinopoma microphyllum: an ultrastructural investigation.

The unusual occurrence of skeletal muscle in the pineal gland of the bat Rhinopoma microphyllum is described. Individual muscle fibres were centrally placed and were incompletely encapsulated by a labyrinthine network of widened spaces filled with electron-dense material. The 2.4 micron long sarcomeres consisted of well developed A, I, H, M, and Z bands. A few myofibrils were oriented transversely within the same fibre. T system components, mitochondria and glycogen appeared normally distributed. Unmyelinated nerve bundles impinged upon the fibre but neuromuscular junctions were not observed. Several possible sources of origin for this tissue in the pineal gland are reviewed.