Electron microscope observations on human fetal striated muscle.

The superior rectus oculi muscle from human fetuses of 5, 9.2, 12 and 24 cm crown--rump length (of ages estimated to be 10, 12, 15 and 23 weeks respectively) have been examined by electron microscopy. "Myotube satellite cells" closely associated with myotubes and myocytes were present in all specimens, but their relative numbers declined with advancing age. Some were small with scanty cytoplasm containing few organelles. Others were rich in organelles, including Golgi apparatus, granular endoplasmic reticulum, comma and dumb-bell shaped dense bodies and centriole or basal body: these cells were numerous in the three smaller specimens but almost absent from the largest. Seemingly active "myotube satellite cells" often extended cytoplasmic processes beyond their confining basal laminae into the endomysial space to contact freelying cells of similar appearance, as well as axon-associated Schwann cells, often to form an extensive network. These "myotube satellite cells" resembled Schwann cells in all respects save association with axons, and it is suggested that they are, indeed, Schwann cells so disposed as to promote axonal growth towards differentiating, but as yet uninnervated, myocytes. Neuromuscular contacts were increasingly numerous with advancing age, usually where several or many axonal terminals contacted a relatively mature (myofilament-rich) muscle cell. Immature myotubes seldom made contact with axonal terminals, even when a closely adjacent (and "coupled") mature muscle cell did so. A sequence of axonal growth and retraction has been proposed which reconciles accounts of early but temporary polyneuronal innervation with commonly accepted ideas regarding the scattered distribution of the muscle components of motor units.