Retinal fine structure in the european eel Anguilla anguilla. II. Photoreceptors of the glass eel stage.

The eye of the European eel (Anguilla anguilla) undergoes marked morphological changes during its long and complex life cycle. This report describes the fine structure of the photoreceptors of the glass eel stage and forms part of a comparative study of the retina of the eel during the major stages of its life cycle. The photoreceptors of the glass eels are readily divisible into rods and cones based on morphological criteria. Rods are more numerous and much longer than cone cells, reaching to the pigment epithelial layer. In rods the inner and outer segments are of the same diameter whereas in cones, inner segments are much wider than the outer segments which taper distally. While the nuclei of rods are located at all levels within the outer nuclear layer, in the light-adapted condition at least, cone nuclei are predominantly located scleral to the external limiting membrane. The synaptic terminal of rods displays 2-3 invaginated (ribbon) synaptic sites while cones have 8-10 such sites. Both rods and cones also possess superficial synaptic sites. Apparent cellular degeneration is quite widespread amongst rods and is also occasionally noted for cone photoreceptors.