Evidence for putative photoreceptor axon terminals in the medulla externa of the crayfish

Summary Within the medulla externa of the crayfish compound eye a class of axonal endings with similar characteristics to the photoreceptor terminals of the lamina ganglionaris were studied with light and electron microscopic techniques. These terminals are restricted to the superficial layers of the medulla externa and each is marked by a rod-shaped inclusion selectively impregnated with reduced silver methods.Electron microscopy of the medullary terminals confirms the presence of a rod inclusion composed of fine regularly arranged filaments. These inclusions are often closely associated with mitochondria and glycogen deposits within the endings. Synaptic contacts made by these terminals are characterized by a presynaptic ribbon density which is in contact with two or three postsynaptic elements. Often one postsynaptic element participates in more than one synaptic complex. Numerous invaginated processes, microtubules, synaptic vesicles, and ER cisternae are also present in the medullary terminals.The eighth retinula cell in the retina of the crayfish studied here resembles that previously observed by Nässel (1976). The similarity of the medullary terminals to the photoreceptor endings in the lamina suggest that they may belong to one of the eight photoreceptor cells forming an ommatidium.This work was supported in part by grants from the National Science Foundation (BNS77-15803) and National Institute of Health (NS08964)The authors wish to acknowledge the technical assistance of Ms. Georgia Hammond-Soltis