Interneuronal and glial-neuronal gap junctions in the lamina ganglionaris of the compound eye of the housefly, Musca domestica.

The cell-body layer of the lamina ganglionaris of the housefly, Musca domestica, contains the perikarya of five types of monopolar interneuron (L1-L5) along with their enveloping neuroglia (Strausfeld 1971). We confirm previous reports (Trujillo-Cenóz 1965; Boschek 1971) that monopolar cell bodies in the lamina form three structural classes: Class I, Class II, and midget monopolar cells. Class-I cells (L1 and L2) have large (8-15 microns) often crescent-shaped cell bodies, much perinuclear cytoplasm and deep glial invaginations. Class-II cells (L3 and L4) have smaller perikarya (4-8 microns) with little perinuclear cytoplasm and no glial invaginations. The 'midget' monopolar cell (L5) resides at the base of the cell-body layer and has a cub-shaped cell body. Though embedded within a reticulum of satellite glia, the L1-L4 monopolar perikarya and their immediately proximal neurites frequently oppose each other directly. Typical arthropod (beta-type) gap junctions are routinely observed at these interfaces. These junctions can span up to 0.8 micron with an intercellular space of 2-4 nm. The surrounding nonspecialized interspace is 12-20 nm. Freeze-fracture replicas of monopolar appositions confirm the presence of beta-type gap junctions, i.e., circular plaques (0.15-0.7 micron diam.) of large (10-15 nm) E-face particles. Gap junctions are present between Class I somata and their proximal neurites, between Class I and Class II somata and proximal neurites, and between Class II somata. Intercartridge coupling may exist between such monopolar somata. The cell body and proximal neurite of L5 were not examined. We also find that Class I and Class II somata are extensively linked to their satellite glia via gap junctions. The gap width and nonjunctional interspace between neuron and glia are the same as those found between neurons. The particular arrangement and morphology of lamina monopolar neurons suggest that coupling or low resistance pathways between functionally distinct neurons and between neuron and glia are probably related to the metabolic requirements of the "nuclear" layer and may play a role in wide field signal averaging and light adaptation.