The frequent occurrence of closely packed intrafusal myotubes during the early postnatal development of muscle spindles in the Chinese hamster.
The formation of intrafusal muscle fibers of developing muscle spindles in the tenuissimus muscle of the Chinese hamster was examined by electron microscopy from birth to 7 days postnatal. At birth, at least two nuclear bag myotubes with a small aggregation of nuclei were already recognizable within a spindle capsule, establishing side-to-side contact with each other without the intercalation of a basal lamina. By the third postnatal day, most muscle spindles contained a full adult set of four intrafusal myotubes effected by the formation of nuclear chain myotubes due to the fusion of myoblasts. The intrafusal myotubes were usually enclosed in a common basal lamina and closely apposed without an interposing basal lamina, thus forming a muscle bundle. Sensory endings were thereby confined to the outer surface of the muscle bundle. After the third day, closely packed intrafusal myotubes generally separated into independent muscle fibers which were innervated by sensory endings and ensheathed in attenuated inner capsule cells. Pairings of fibers and sensory cross terminals between fibers were frequently found in the equatorial region. These phenomena suggest that the incomplete separation of the bundled myotubes at an early stage of postnatal development may persist into adulthood.
Department of Anatomy, Ehime University School of Medicine, Japan.
J
J
Desaki
D000818:Animals; D000831:Animals, Newborn / Q000254:growth & development*; D002467:Cell Nucleus / Q000648:ultrastructure; D006224:Cricetinae; D003412:Cricetulus; D008854:Microscopy, Electron; D024510:Muscle Development; D009470:Muscle Spindles / Q000648:ultrastructure*; D009132:Muscles / Q000294:innervation / Q000648:ultrastructure