The ultrastructure of retinula cell endings in the compound eye of the crayfish

Summary The retinula cell axons entering the synaptic region of the optic lamina in the crayfish form large expanded bag-like terminals which are organized with other neural elements into structural units called cartridges. The cytoplasm of the terminals contains synaptic and coated vesicles, ER cisternae, clusters of tubular elements, and mitochondria. Several mitochondria are often found associated with a single large rod-shaped inclusion present within each terminal. The rod-like formation could be demonstrated in both light and EM material, it is composed of 85–95 Å filaments and averages I m in width and 6.5 m in length.The terminal synaptic contacts are characterized by a bar-shaped presynaptic density and three postsynaptic elements. Some synaptic vesicles appear aligned along the bar density which measures approximately 800 Å in width and 0.75 m in length. Each terminal synapse has three postsynaptic elements which have an electron-dense fringe along their membrane bordering the synaptic cleft. From the planes of section through this contact a composite reconstruction is presented.Also present along the central border of the terminals are numerous small invaginated processes, some of which extend almost to the middle of the terminal. No membrane specializations were found along these processes and they have been tentatively identified as neuronal.