Synaptic vesicles and microtubules in frog motor endplates.

Motor endplates of the cutaneous pectoris skeletal muscle of the frog have been examined by electron microscopy using a new technique. This involves pretreatment with an albumin solution, followed by fixation with 4% unbuffered tetroxide. A small proportion of the endplate axonal ramifications show microtubules clothed in synaptic vesicles and focused on the presynaptic membrane, in particular on the active zones. The microtubules run in the presynaptic cytoplasm either parallel to or across the active zones. These two sets of microtubules cross each other at the active zones, which lie opposite the dips in the post-junctional folds. The possibility that the microtubules are involved in the translocation of synaptic vesicles to the active zone is discussed.