Retinal fine structure in the European eel Anguilla anguilla. VIII. Photoreceptors of the sexually mature silver eel stage

The morphology of the photoreceptors of the sexually mature silver eel has been investigated by electron microscopy and these findings compared with observations made on the photoreceptors of the glass, yellow and sexually immature silver eel stages of the lengthy life cycle of the European eel (Anguilla anguilla). As in previous stages the photoreceptors of the sexually mature silver eel are adequately described as either rods or single cones. The rod: cone ratio at this stage is about 200: 1 which is the highest ratio of any of the stages investigated. Rods and perhaps also cones are capable of photomechanical movements. In the light-adapted stage studied, rod inner segments are the same width in the ellipsoid region as the outer segments but the rod myoid region is much thinner. Rod outer segments in the mature silver eel are slightly longer than that observed in the immature silver eel but remain much the same diameter as in previous stages. Cone outer segments taper distally and are not as wide as the inner segments. Both rods and cones display organelles in the inner segment region indicative of active protein production as it is well established that the inner segment is the synthetic center of these cells. In most cones degenerative changes were noted in the mitochondria of the ellipsoid and in the presence empty vacuoles within their cytoplasm. Cone nuclei are large and vesicular and in the light-adapted state are always located sclerad to the external limiting membrane. Rod nuclei are smaller and more electron dense and located within the outer nuclear layer. Both rods and cones still show both invaginated (ribbon) and superficial synaptic sites. In the sexually mature silver eel, therefore, the rod: cone ratio becomes higher still; the rod outer segments become somewhat longer in length and cone cell degenerative changes are even more widespread.