Some aspects of the structural organization of the intermediate retina of dipterans

the intermediate retina can be divided into three main layers: (a) the fenestrated or multiple chiasmata region; (b) the unipolar cell soma layer; and (c) the external plexiform layer. The fenestrated layer contains numerous tracheae and photoreceptor axons running toward the plexiform layer. Two types of neurons (type I and type II) have been identified within the unipolar cell soma layer. Type I neurons have plasmatic folds and send out long cytoplasmic branches; type II neurons are pear-shaped and have a smooth surface. They lack folds and cytoplasmic branches. Type I neurons are the second-order neurons of the fly visual system. At the boundary of the plexiform layer they send out collaterals which form a plexus. The external plexiform layer consists of numerous discrete units—the optical cartridges—separated by tall epithelium-like glial cells. Each cartridge is formed by six photoreceptor axons which surround the prolongations of two type I neurons. The two trunks of the latter make contacts by intimate apposition. These second-order prolongations send out collateral branches which penetrate between the crown of photoreceptor axons and synapse with them. Synapses also occur between the photoreceptor axons and the second-order main trunk. Synaptic loci are characterized by the presence of a small synaptic ribbon located within the photoreceptor axon. This ribbon lies parallel to the photoreceptor axon membrane and is commonly connected to it by means of a short dense band. The synaptic cleft is about 150 A wide. Two collaterals of the second-order fibers are frequently found synapsing on the same small patch of the photoreceptor membrane. By means of the serial sectioning technique it has been possible to demonstrate that each second-order fiber establishes synaptic contact with each one of the six photoreceptor axons. The photoreceptor axons and the second-order neurons seem to be under control of centrifugal fibers. Two of these fibers can be found in relation with each photoreceptor axon and the same fibers make contacts with the second-order prolongations. Synaptic ribbons occur within the centrifugal fibers, suggesting that they play the role of presynaptic elements. Three different types of neuroglial cells are present in the intermediate retina: (a) satellite glial cells which ensheath the unipolar cell bodies; (b) tall neuroglial cells (“epithelial cells” of Cajal) which lie between the optical cartridges; and (c) the marginal glial cells which envelop the second-order prolongations as soon as they leave the optical cartridges. The “epithelial cells” of Cajal send out slender prolongations, the capitate projections, which end by means of an enlarged mass (1000 A in diameter) within small invaginations of the photoreceptor axon membrane. Interposed between the glial membrane and the photoreceptor membrane, lies, in these invaginations, an amorphous substance 100 A thick. Considered as a whole, each optical cartridge appears as a multiple synapse unit ensuring that information conveyed by each of the six photoreceptors will be funneled in each of the two second-order fibers. These units are under central control by means of the centrifugal fibers.