Synaptic organization of columnar elements in the lamina of the wild type in Drosophila melanogaster

The synaptic connections within the lamina, the first of the optic neuropiles underlying the insect's compound eye, have been little studied in Drosophila melanogaster until now, despite the genetic advantages of this animal. Here we report the reconstruction through its entire depth of one of the lamina modules, or cartridges, of a female wild-type Drosophila, for which a series of EM cross sections was analysed at levels extending from the retinal basement membrane to the first optic chiasma. A complete, comprehensive catalogue of the synaptic connections of all columnar elements has been compiled from this single series, confirmed from comparisons with less completely photographed cartridges. Combinations of the 12 types of cartridge neurons form divergent multiple-contact synapses (dyads, triads, and tetrads) throughout the lamina's depth. These 12 neuron types include 11 narrow-field elements (one class of receptor terminal, R1-R6, providing input to the cartridge; two types of long visual fiber from the ommatidium, R7 and R8; five types of monopolar cell, L1-5; and three types of medulla cell--two centrifugal neurons C2 and C3, and a third, T1) as well as a wide-field intrinsic or amacrine cell. Connections within the lamina formed by L4 from two adjacent cartridges (posterodorsal and posteroventral) contribute to the matrix of connections. In addition, connections of at least one other wide-field element have also been incorporated.