Ultrastructural features of the retina and optic nerve of Strombus luhuanus,a marine gastropod

The retina and optic nerve of Strombus luhuanus were examined by transmission electron microscopy in order to provide an ultrastructural basis for their electrophysiological responses, described elsewhere. The retina exhibits a distinct rhabdomeric layer and layers of cell nuclei and neuropile. These layers are comprised predominantly of three cell types that can be readily distinguished on the basis of their shape, their nuclei and cytoplasmic inclusions such as vesicles and filaments. One type of cell, apparently a photoreceptor that depolarizes in response to photic stimulation, possesses a long distal segment with microvilli; such distal segments comprise the bulk of the rhabdomeric layer. A second cell type, which appears to be supportive in function, contains a bundle of tightly packed tonofilaments that extend across the retina from the capsule to the vitreous body; this cell is quite narrow except in the region near the rhabdomeric layer, where it is expanded and wraps around the other cell types. A third type of cell possesses many short microvilli that project from its apical end into the rhabdomeric layer; it may be a second type of photoreceptor or another type of neuron. The retina also contains bundles of cilia that appear to project from a possible fourth type of cell. The layer of neuropile contains numerous processes that exhibit a variety of vesicle types and structures generally associated with synapses; these appear to play a role in mediating inhibitory and excitatory interactions between the retinal neurons. The optic nerve exhibits two populations of fiber distinguishable on the basis of mean diameter. Fibers in these two populations apparently yield “on” and “off” discharges in response to photic stimulation of the eye.