Muscle spindle morphology in the tenuissimus muscle of the golden syrian hamster

The tenuissimus is a long, thin hindlimb skeletal muscle which in hamsters contains about 200 extrafusal muscle fibers. Embedded in this richly innervated muscle is a continuous array of 16-20 closely packed muscle spindles suggesting that it may play a role in hindlimb proprioception. This high spindle density also makes the muscle ideal for the isolation and harvesting of these sensory receptors. In this correlative light and electron microscopic study, freshly frozen specimens were first prepared for serial microscopic analysis. Camera lucida reconstruction of spindle distribution showed a close proximity to the main artery and nerve in the central core of the muscle. Oxidative enzyme and myosin ATPase staining profiles were examined in both the intrafusal and extrafusal fiber populations. Type I and type II extrafusal fibers were present in even numbers and were distributed evenly throughout muscle cross-sections. Enzyme staining varied along the lengths of the three intrafusal fiber types. The fine structure of spindles was examined using transmission (TEM), conventional scanning (SEM), and high resolution scanning electron microscopy (HRSEM). For conventional SEM, isolated spindles were first fixed in 2.5% buffered glutaraldehyde, followed by 1% osmication, and mechanical disruption of the outer capsule under the dissecting microscope. Preparation for HRSEM included aldehyde/osmium fixation and freeze-cleavage of entire tenuissimus muscles in liquid N₂. Selective extraction of the cytosol with 0.1% OsO4 permitted the visualization of numerous intracellular structures. In these specimens, the capsular sleeve showed a multilayered pattern of vesicle-laden cells with variant surface topography in certain locations. Punctate sensory nerve endings adhered intimately to the surfaces of underlying intrafusal fibers in the equatorial and juxtaequatorial regions. By TEM and HRSEM these endings appeared crescent-shaped and were enveloped by external laminae. Each profile contained a plethora of mitochondria and cytoskeletal organelles. The methodology used in this study provides, for the first time, a three-dimensional view of the exquisite cytological architecture of this neuromuscular receptor.Medicine, Faculty ofGraduat