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

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 (oairs) a 2.0. LS (total) z 1.4-2.0, MSS z 0.4-0.8. and MLS < 0.1. The form dimensions. and deniities’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 q (1000-2500 SSimm’). Double cones form the sides of rhombi whose centers are occupied by SS cones. MSS cones lie at the comers 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 (j.,., E 625 nm). The SD and other LS cones contain the green-sensitive pigment (i.,,, 3 530 nm). The probable ratio of LS (red): LS (green) z 2: 1. The SS cones contain the blue-sensitive pigment (&,,,, 2 455 nm). The pigment contained in the MSS and MLS cones has not been determined. Many vertebrate retinas are capable of discriminating between lights of different wave lengths. For example, members of the teleostean family Cyprinidae, including carp and goldfish, are known to have trichromatic color vision (Northmore and Muntz, 1974). By means of microspectrophotometry (Marks, 1963, 1965&b; Liebman and Entine, 1964; Svaetichin, Negishi and Fatehchand, 1965; Harosi and MacNichol, 1974a) and microelectrode recording (Tomita, Murakami and Pautler, 1967), it has been shown that each carp and goldfish cone contains one of three visual pigments, which absorb light maximally at about 455, 530 and 625 nm. For convenience, we can refer to these receptors as “blue” (B), “green” (G), and “red” (R), respectively. By recording the activity of single units with microelectrodes, it has been shown that the responses of some neurons of all major classes in the cyprinid retina are coded for chromaticity (see, e.g., Kaneko, 1970: Spekreijse, Wagner and Wol’ Supported in part by Research Grant No. EY 01190 and Vision Research Center Grant No. 00331 from the National Institutes of Health. ’ Laboratory of Neurophysiology, National Institute of Neurological and Communicative Disorders and Stroke, National Institutes of Health, Bethesda, MD 20014, U.S.A. barsht, 1972). It has not been possible to establish the pathways along which color information passes through the retina by electrophysiological methods. The cones of many lower vertebrates (unlike those of mammals) are not all identical in structure, but may differ markedly from their immediate neighbors. Hannover (1840. 1844). in one of the earliest microscopic studies of vertebrate retinas, observed both single (individual) and twin (double or closely paired) cones in the pike, Esox. Subsequently, many investigators were concerned with the form and arrangements of cones in teleostean retinas (see Engstrom, 1963b, for review). Marks (1965b) reported that in goldfish the single cones which have short, rounded ellipsoids are blue-sensitive, while the double cones are pairs of redand green-sensitive elements. At about the same time, Svaetichin er al. (1965) made similar observations and noted that the members of double cones in goldfish are not identical, but they did not define the structural distinction between the redand green-sensitive members. H&rosi and MacNichol(1974a) reconfirmed these localixations of Cone pigments and described structural differences between the outer segments of the redand green-sensitive members of double cones. If the structure of a cone cell is a reliable indicator of the visual pigment it contains, then it should be possible to trace pathways for color by morphological MS W. K. STELL and F. I. H&XI methods. This principle has been applied in an elegant analysis of conecone and cone-bipolar cell connections in the rudd (Scholes and Morris, 1973; Scholes. 1975). A similar analysis of cone-horizontal cell connections in goldfish has been undertaken by Stell and Lightfoot (1975). In this paper we describe the cones and their distribution in goldfish retina, and present evidence for the correlations of structure and pi_gment content to which we have referred briefly elsewhere (Stell, Harosi and Lightfoot, 1974). Our purpose here is to provide not an encylopedic analysis of goidfish cones, but a sufficiently accurate description to allow important conclusions with regard to synaptic connectivity and function of retinal cells. SlATERIALS AND METHODS