DIFFERENTIAL ULTRASTRUCTURE OF SYNAPTIC TERMINALS ON VENTRAL LONGITUDINAL ABDOMINAL MUSCLES IN DROSOPHILA LARVAE

The innervation of ventral longitudinal abdominal muscles (muscles 6, 7, 12, and 13) of third-instar Drosophila larvae was investigated with Nomarski, confocal, and electron microscopy to define the ultrastruct ural features of synapse-bearing terminals. As shown by previous worke rs, muscles 6 and 7 receive in most abdominal segments ''Type I'' endi ngs, which are restricted in distribution and possess relatively promi nent periodic terminal enlargements (''boutons''); whereas muscles 12 and 13 have in addition ''Type II'' terminals, which are more widely d istributed and have smaller ''boutons.'' Serial sectioning of the Type I innervation of muscles 6 and 7 showed that two axons with distincti ve endings contribute to it. One axon (termed Axon 1) has somewhat lar ger boutons, containing numerous synapses and presynaptic dense bodies (putative active zones for transmitter release). This axon also has m ore numerous intraterminal mitochondria, and a profuse subsynaptic ret iculum around or under the synaptic boutons. The second axon (Axon 2) provides somewhat smaller boutons, with fewer synapses and dense bodie s per bouton, fewer intraterminal mitochondria, and less-developed sub synaptic reticulum. Both axons contain clear synaptic vesicles, with o ccasional large dense vesicles. Approximately 800 synapses are provide d by Axon 1 to muscles 6 and 7, and approximately 250 synapses are pro vided by Axon 2. In muscles 12 and 13, endings with predominantly clea r synaptic vesicles, generally similar to the Type I endings of muscle s 6 and 7, were found, along with another type of ending containing pr edominantly dense-cored vesicles, with small clusters of clear synapti c vesicles. This second type of ending was found most frequently in mu scle 12, and probably corresponds to a subset of the ''Type II'' endin gs seen in the light microscope. Type I endings are thought to generat e the 'fast' and 'slow' junctional potentials seen in electrophysiolog ical recordings, whereas the physiological actions of Type II endings are presently not known. (C) 1993 John Wiley & Sons, Inc.