Schwann cell dynamics with respect to newly formed motor-nerve terminal branches on mature (Bufo marinus) muscle fibers

A study has been made of the formation of synaptic terminals from long processes formed at the end of motor nerve branches of endplates in mature amphibian (Bufo marinus) muscle. Injection of fluorescent dyes into individual motor axons showed the full extent of their branches at single endplates. Synaptic vesicle clusters at these branches were identified with styryl dyes. Some terminal branches consisted of well separated varicosities, each possessing a cluster of functioning synaptic vesicles whilst others formed by the same axon consisted of closely spaced clusters of vesicles in a branch of approximately uniform diameter. All the varicosities gave rise to calcium transients on stimulation of their parent axon. Both types of branches sometimes possessed short processes (<5 microm long) or very long thin processes (>10 microm long) which ended in a bulb that possessed a functional synaptic vesicle cluster. These thin processes could move and form a varicosity along their length in less than 30 min.Injection of a fluorescent dye into terminal Schwann cells (TSCs) at an endplate showed that they also possessed very long thin processes (>10 microm long) which could move over relatively short times (<30 min). Injecting fluorescent dyes into both axons and their associated TSCs showed that on some occasions long TSC processes were accompanied by a long nerve terminal process and at other times they were not. It is suggested that the mature motor-nerve terminal is a dynamic structure in which the formation of processes by TSCs guides nerve terminal sprouting.