Ultrastructural study of a blood-muscle spindle barrier after systemic administration of horseradish peroxidase.

The permeability of the normal muscle spindle capsule to the entrance of an exogenous protein tracer was assessed by the use of horseradish peroxidase (HRP). Adult mice were injected intravenously with a solution of HRP (MW 40,000). After varying intervals, ranging from two to 240 minutes, animals were perfused aortically with fixative, and anterior tibialis muscles were removed, processed, and examined. The results were evaluated by electron microscopic cytochemical techniques, and attention was directed to the movement of tracer and its relationship to the outer and inner capsule of the muscle spindle. HRP was first demonstrated in cytoplasmic vesicles crossing the continuous capillary endothelium supplying the muscle spindle, and was then detected in the interstitial space surrounding the outer capsule. In polar regions, HRP had entered the periaxial space by two minutes, and it was seen in cytoplasmic vesicles of the attenuated outer capsule. Later, HRP abutted the sarcolemma of intrafusal fibers, and by 12.5 minutes there was evidence of tracer in T-tubules of these muscle cells. Whereas at no time was tracer observed traversing any of the numerous intracellular junctions of the capsule, it appeared that polar regions were leaky and open-ended at their distal portions. The sensory equatorial zone was considerably less permeable to the entrance of tracer. Whereas HRP was visible intially in cytoplasmic vesicles of the subjacent capillary endothelium, it was not until 12.5 minutes that racer could be demonstrated within some vesicles of equatorial outer capsule cells. At later times, a small amount of HRP was observed in the equatorial periaxial space, where it was phagocytosed and finally sequestered by cells of the inner capsule as membrane-bound lysosomal deposits. Consequently, at no time was tracer incorported into either intrafusal fibers or their sensory nerve terminals. Thus, in contract to polar regions, the muscle spindle capsule in the equatorial zone appears to be effective in preventing the indiscriminate penetration of HRP from the bloodstream. This suggests dissimilar paths of tracer movement from the microvasculature into sensory and non-sensory regions of the muscle spindle.