Embryonic synapse ultrastructure in the absence of neurexin.

<p>Homozygous <i>Df(3R)Exel6191</i> mutants (abbreviated ‘<i>Df/Df’</i>) are devoid of neurexin. Synaptic vesicles, electron dense presynaptic membrane and presynaptic dense projections at active zones (AZ) are visible in both wildtype and homozygous <i>Df(3R)Exel6191</i> mutant embryonic body wall NMJs. Similarly, both WT and <i>Df(3R)Exel6191</i> mutant NMJ synapses show postsynaptic electron dense membrane (PSD). NMJ synapses in <i>Df(3R)Exel6191</i> mutants show no significant changes in the number of synaptic vesicles, diameter of AZs, or diameter of PSDs, compared to wildtype NMJ synapses. However, the electron dense postsynaptic muscle membrane in <i>Df(3R)Exel6191</i> mutants sometimes extends beyond the presynaptic specialization, or is not associated with any visible presynaptic specialization. In some cases, we serial sectioned individual synapses to confirm that apparently misaligned PSDs did not pair with an AZ in a neighboring section. 142 sections from several animals were analyzed for the active zone & PSD statistics. For quantification, measurements were made from all available sections of a synapse and then averaged together to produce a single measurement for each synapse. N (indicated in figure) represents the number of unique NMJs that were sectioned.