Photoreceptor Fine Structure in the Northern Pike (Esox lucius)

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Abstract
The morphology of the retinal photoreceptors has been studied in the adult northern pike (Esox lucius) by light and electron microscopy. Three photoreceptor types are seen: rods, single cones, and twin cones. The rod photoreceptors are the most numerous. They are elongate cells basically similar to other vertebrate rod cells described. Twin cones are the more numerous of the two cone types present and are formed by two cone cells lying in close apposition along their inner segments, with a membrane specialization where the two cells are apposed. Each member of the twin cone is morphologically similar to the other and to the single cone. The cone photoreceptors display in addition to the regular outer segment, an accessory outer segment. The ellipsoid of the cone cell inner segment is unusually large. The synaptic region of the cone cells is larger and more elaborate than that of the rods. Synapses in both rods and cones, however, appear to be formed by both superficial and invaginated contacts with bipolar and horizontal cells. The single and double cones are arranged in a well-defined, repeating mosaic pattern.

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