Muscle spindle ultrastructure revealed by conventional and high‐resolution scanning electron microscopy

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Abstract
Muscle spindles in the tenuissimus muscle of mature golden Syrian hamsters were examined by conventional and high-resolution scanning electron microscopy (HRSEM). For conventional SEM, entire muscles were first fixed in 2.5% buffered glutaraldehyde. Spindles were then isolated with a dissecting microscope under darkfield illumination and postfixed in 1.0% OsO4. Some spindles were treated with 8 N HCl at 60°C to clearly expose intrafusal fiber surfaces once the outer capsular sheath was mechanically disrupted. Preparation for HRSEM included aldehyde/osmium fixation and freeze-cleavage in liquid N2. The cytosol and certain cellular elements were also selectively extracted by immersion in 0.1% OsO4 for varying time intervals. In these preparations, the capsular sleeve showed a multilayered pattern of vesicle-laden cells with variant surface topography in different regions, including filopodia and small bristle-like surface-projections. An interlacing three-dimensional network of collagen fibrils intervened between the capsular lamellae. Within the spindles, sensory and fusimotor nerve endings closely adhered to the outer surfaces of intrafusal fibers. Sensory nerve terminals were enveloped by a prominent external lamina, and those that were cleaved open contained a plethora of elongated mitochondria that ran parallel with the longitudinal axis, along with vesicles, axoplasmic filaments, and lysosomes. Multiple adhesion sites between the sensory nerve membrane and the underlying sarcolemma of the intrafusal fiber were also observed in select regions. Fusimotor nerve endings were covered externally by processes of Schwann cells and their axoplasm was filled with a multitude of cellular organelles and synaptic vesicles. Dilated cisternae of the sarcoplasmic reticulum and numerous mitochondria were, in addition, observed below the postjunctional sarcolemma at the neuromuscular interface. The methodology used in this study provides a novel view of the exquisite three-dimensional architecture of this complex neuromuscular receptor.