Synaptogenesis in the first optic neuropile of the fly's visual system

The developmental transformation of the chief afferent class of fly photoreceptor synapse has been examined from serial electron micrographs of animals fixed at 74, 81, 94 and 100% pupal development (100% pupal development being defined by the time of normal adult eclosion). Animals were selected both by their age and the conformity of their eye coloration to standards for each stage. Two animals were analysed from each stage, one in greater detail than the other. For the first, the exact coordinates of the cartridges (the synaptic columns of the first optic neuropile) from which the analyses were made were mapped and selected to be within the same region of the eye field at all stages. From all animals a portion of one or two cartridges was analysed from series of up to 100 sections and the synapse populations (greater than 80) were analysed for their fine structure and postsynaptic composition. Adult synapses are confirmed as tetrads, with two of the four postsynaptic elements invariably from two monopolar interneurons L1 and L2, one from each. The two others are usually from alpha processes of the same amacrine cell. Synapses appear during the last half of pupal development, with no obvious asynchrony of ultrastructural maturation and in parallel with those of at least one of the other synaptic classes present (which were otherwise not studied). Many adult features of synaptic ultrastructure emerge late, only by 94% pupal development. These include adult numbers of synaptic vesicles, the complete form of the presynaptic ribbon with platform and the postsynaptic cisternae of L1/L2. Prior to 94% the synapses are smaller with postsynaptic elements having a less regular geometry and with postsynaptic densities which are subsequently lost (alpha processes) or replaced by cisternae (L1/L2). At the presynaptic sites of the younger animals (74%, 81%) dyads and triads of postsynaptic elements coexist with tetrads, those of older animals having, on average, more postsynaptic processes per synapse. It is suggested that individual synapses assemble piecemeal, element by element.