High voltage electron microscopy of the optic neuropile of the housefly, Musca domestica.

Synaptic cartridges of the first optic neuropile (lamina ganglionaris) of the housefly were examined by high voltage electron microscopy (HVEM). Stereo pairs (from thick, i.e., 0.25 mum, sections viewed at 1,000 kV) provided a three dimensional representation of cartridge neurons and clearly revealed the lateral spread, bifurcation and some functional associations of Type I (L1, L2) monopolar interneurons. Slightly proximal to cartridge neck level, pairs of retinular (R) axons made contact with each other and it appeared that R processes projected through the cleft between the Type I interneurons. No junctional modifications were seen between contiguous R axon terminals. The speculation was made that functional contact might exist between neighboring R axons prior to their extensive synapses with principal first order interneurons. Such alleged coupling between R axons would account for several electrophysiological findings from other laboratories. Modifications in EM technique applicable for HVEM were detailed. The value of obtaining thick serial sections and the use of the HVEM in expediting three dimensional reconstructions of neuropile were demonstrated.