The diversity of cones in the retina of vertebrates: A review

Abstract

Introduction The most miraculous structure of the eye is the retina, a receptive layer containing a lining of closely packed visual cells/photoreceptors. It is well known that the retina of the vertebrates contains two types of visual cells: the rods and cones. The rods are specialised for dim vision while the cones are for bright and colour vision. The cones can be further subdivided into at least three subcategories: the single cone, twin cone and the double cone. The ordinary single cone usually exists alone, while the twin cone and the double cone represent two cones adhering to each other. The two components of the twin cone are identical in morphology. By contrast, the double cone consists of a tall and slender component, which is termed as the chief cone, and a rounder and shorter component, which is known as the accessory cone. The single cone is the most common type that can be found in most species of vertebrates, while others are less frequently seen, but still appear in many species up the vertebrate scale from fish to mammals. The aim is to introduce the variety of cone cells in the retinae of some selected species of vertebrates and discuss their structures with regard to the possible functions. Conclusion This critical review illustrates the diversity of cones in selected retinae of animals. The cones and their internal components are phylogenetically old. Yet, the coordination of structure and function are unknown.

Vision adaptation in the laughing dove (Streptopelia senegalensis, Linnaeus, 1766) inferred from structural, ultrastructural, and genetic characterization

“…Second, Cheung et al (2013) clarified that the paraboloid represents a high-energy storage and is equipped for the high-energy demand for the visual acuity. As proposed by Amemiya (1975), the paraboloid and hyperboloid are valuable stations for storing and metabolism of glycogen and thus for energy.…”

“…A high intricate visual processing is suggested in S . senegalensis because of the high density of the retinal neural cells and their various synapses, represented in having a very thick IPL (Cheung et al, 2013). According to Sosula and Glow (1970), a functional significance of having a high incidence of cone and rod bipolar cells is attributed to the high number of dyads and synaptic ribbons.…”

“…Ellipsosomes also protect the outer segment and propose a tetra‐chromatic vision induced by the alternation of spectral sensitivity (Karl et al, 2018). Second, Cheung et al (2013) clarified that the paraboloid represents a high‐energy storage and is equipped for the high‐energy demand for the visual acuity. As proposed by Amemiya (1975), the paraboloid and hyperboloid are valuable stations for storing and metabolism of glycogen and thus for energy.…”

Systematic analysis of ocular features and responses of cultured spotted wolffish (Anarhichas minor)

“…Doublet cones are commonly observed in most fish, reptiles and birds but are not observed in catfish, elasmobranchs or placental mammals (Pignatelli et al., 2010). Doublet cones have also historically been further categorized as “twin” cones, which are identical in morphology, or “doublet” cones, which are composed of a larger chief cone and a smaller accessory cone (Cheung et al., 2013). Herein, we will refer to double cones as “doublet” cones since we did not observe any instances of larger chief cone and smaller accessory cone in lumpfish or spotted wolffish.…”

Rods and cones in an enantiornithine bird eye from the Early Cretaceous Jehol Biota

“…These are found in some fish (e.g. sturgeon and lungfish), amphibians, reptiles, and birds ( Bowmaker, 2008 ; Cheung et al, 2013 ). The colour of the oil droplets represents the wavelengths of light that are transmitted; e. g., red oil droplets transmit long wavelength (“red”) light, whereas colourless (to the human eye) oil droplets absorb the shortest (UV-A) wavelength light for vision in many animals ( Kolb and Jones, 1982 ).…”