An ultrastructure investigation of intrafusal muscle fibers in myotonic dystrophy.

Ultrastructure features of muscle spindles from two patients with myotonic dystrophy are described. Intrafusal muscle fibers exhibit extensive splitting with nuclear bag fibers affected more so than nuclear chain fibers. No sensory endings are present on nuclear chain fibers nor on one nuclear bag fiber throughout the equatorial and myotube regions. Small motor end plates are evident on various segments of split intrafusal fibers in the polar region and some of these extend into the myotube region. Satellite cells are numerous on both nuclear bag and nuclear chain fibers. These frequently occupy the cleft space between segments of split intrafusal fibers. The myotonic dystrophy muscle spindle ultrastructure features seem to closely resemble the appearance of developing mammalian muscle spindles as illustrated with opossum fetal tissue.