Presynaptic active zones at neuromuscular junctions of larval frogs

Freeze-fracturing presynaptic membranes at tadpole neuromuscular junctions display small clusters of large P-face particles, including short double linear arrays. Short pairs of double particle rows are randomly oriented at some junctions. At others, presynaptic membranes are crossed at regular intervals by long pairs of double rows indistinguishable from those characterizing the active zones of adult amphibian neuromuscular junctions. Formation of double particles rows, pairing of the double rows, and transverse alignments of the pairs are shown to be independent processes.