Clathrin-coated vesicles mediate the transport of materials such as growth factors by endocytosis, exocytosis and membrane recycling (Willingham and Pastan, 1985). Coated vesicles are distributed in a variety of tissues including skeletal muscle, where they are thought to partake in both endo-and exocytosis (Benson et al., 1985; Haye et al; 1986). The presence of coated invaginations under the sarcolemma of sensory regions of intrafusal fibers in muscle spindles of 18-day gestational rats led to speculation that the development of intrafusal fibers may be influenced by neurotrophic factors released by sensory terminals and internalized via coated vesicles (Landon, 1972;Zelená & Soukup, 1973). We tested the hypothesis that coated organelles are involved in the exchange of information in the developing muscle spindle both between incoming axons and intrafusal fibers and between the different types of intrafusal fibers. If these two modes of communication do operate, then one would expect that the incidence of coated organelles would be higher 1) in developing versus adult spindles, 2) under sensory rather than motor terminals and 3) under apposed surfaces of intrafusal fibers rather than under the free sarcolemmal surface.
KeywordsMuscle Spindle Coated Vesicle Sensory Region Intrafusal Fiber Membrane Recycling
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