Cone structure and visual pigment content in the retina of the goldfish

Abstract

Histologic examination of the retina in comet goldfish 11 cm long (nose to base of tail) reveals at least six distinct varieties of cones. Unequal double cones (DC) comprise long (LD) and short (SD) members which differ in form as well as length. Long single (LS) cones are similar in length and form to SD cones. Although evidence indicates that there are two subpopulations of LS cones, normally they are structurally indistinguishable. Short single (SS) cones have short, broad outer segments and ellipsoids and virtually no myoids. Miniature long single (MLS) cones have the same shape as LS cones, but they are shorter and more slender and their nuclei project farther into the outer nuclear layer. Miniature short single (MSS) cones have the same shape as SS cones, but they are shorter and more slender and their nuclei are found with those of MLS cones. The average cone densities, relative to SS = 1.0, are: DC (pairs) ≅ 2.0, LS (total) ≅ 1.4–2.0, MSS ≅ 0.4–0.8, and MLS < 0.1. The form, dimensions, and densities of cones do not vary systematically with location in the retina. An irregular mosaic is discernible. In the fundus, this is fundamentally a rhombic array of SS cones separated by 20–28 μm ( 1000–2500 SS/mm 2 ). Double cones form the sides of rhombi whose centers are occupied by SS cones. MSS cones lie at the corners of the rhombi, while LS cones are scattered. The location of MLS cones in the mosaic has not been determined. In the far periphery the double cones form rows instead of rhombi, but otherwise the mosaic is the same. Microspectrophotometric measurement of visual pigments, accompanied by microscopic identification of the cones containing them, reveals a strong correlation between structure (cone type) and function (visual pigment). The LD and some LS cones contain the red-sensitive pigment ( λ max ≅ 625nm ). The SD and other LS cones contain the green-sensitive pigment ( λ max ≅ 530nm ). The probable ratio of LS (red):LS (green) ≅ 2:1. The SS cones contain the blue-sensitive pigment ( λ max ≅ 455nm ). The pigment contained in the MSS and MLS cones has not been determined.