Analysis of a multiple-contact synapse missing a normally obligatory postsynaptic neuron: an electron microscopic study in the compound eye of Musca domestica.

In the compound eye of the adult female fly Musca domestica, photoreceptors form populations of multiple-contact output synapses, stereotypic in their architecture and in the identity of the four postsynaptic elements. Two postsynaptic elements, always originating from monopolar interneurons L1 and L2, lie side by side beneath the elongated presynaptic bar. Beneath each end of the bar, a further postsynaptic contact is located. These contacts most often are two processes either of amacrine cells or of epithelial glial cells. Monopolar cell L3 may be postsynaptic as well, with either an amacrine or a glial process completing the tetrad. To learn more about the factors determining connectivity and synaptic architecture, a three-dimensional reconstruction of serial electron microscopic sections was used to analyze a population of photoreceptor synapses at which one of the normally obligatory postsynaptic neurons, L1, was missing. In this abnormal case, the synapses make the normal four postsynaptic contacts in only 39% of the cases, otherwise making three (39%) or two (22%) contacts. Specificity of connectivity is preserved faithfully except that beta processes of T1 cells were postsynaptic at 2% of the synapses, where they do not normally contribute. In contrast to normal synapses, where amacrine and glial cell processes are mutual exclusive, such pairings could coexist in the aberrant synapse (27% of all synapses). All postsynaptic cells contributed the normal number of processes to a synaptic site, except for three synapses each with a supernumerary amacrine cell process. The postsynaptic cells therefore may be involved in regulating the number of their contacts made to a synaptic site.