Proportions of slow myosin heavy chain‐positive fibers in muscle spindles and adjoining extrafusal fascicles, and the positioning of spindles relative to these fascicles

Chicken leg muscles were examined to calculate the percentages of slow myosin heavy chain (MHC)‐positive fibers in spindles and in adjacent extrafusal fascicles, and to clarify how the encapsulated portions of muscle spindles are positioned relative to these fascicles. Unlike mammals, in chicken leg muscles slow‐twitch MHC and slow‐tonic MHC are expressed in intrafusal fibers and in extrafusal fibers, suggesting a close developmental connection between the two fiber populations. In 8‐week‐old muscles the proportions of slow MHC‐positive extrafusal fibers that ringed muscle spindles ranged from 0–100%. In contrast, proportions of slow MHC‐positive intrafusal fibers in spindles ranged from 0–57%. Similar proportions in fiber type composition between intrafusal fibers and surrounding extrafusal fibers were apparent at embryonic days 15 and 16, demonstrating early divergence of extrafusal and intrafusal fibers. Muscle spindles were rarely located within single fascicles. Instead, they were commonly placed where several fascicles converged. The frequent extrafascicular location of spindles suggests migration of intrafusal myoblasts from developing clusters of extrafusal fibers toward the interstitium, perhaps along a neurotrophic gradient established by sensory axons that are advancing in the connective tissue matrix that separates adjoining fascicles. J. Morphol. 242:157–165, 1999. © 1999 Wiley‐Liss, Inc.