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Regional topography of rod and immunocytochemically characterized “blue” and “green” cone photoreceptors in rabbit retina
- E. V. Famiglietti, S. J. Sharpe
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- Journal:
- Visual Neuroscience / Volume 12 / Issue 6 / November 1995
- Published online by Cambridge University Press:
- 02 June 2009, pp. 1151-1175
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Evidence from several sources indicates that the photoreceptors of rabbit retina include rods, green cones and blue cones, and that blue-green color opponency occurs in select retinal ganglion cells. One of us (Famiglietti) has identified wide-field cone bipolar cells as probable blue-cone-selective bipolars, and type C horizontal cells as possibly connected to blue cones. We wished to extend the analysis of blue cone pathways in rabbit retina and to characterize the topographic distribution of blue and green cones. Two monoclonal antibodies raised against chicken visual pigments are reported to label medium- and long-wavelength cones (COS-1) and short-wavelength cones (OS-2) in all mammalian retinas studied thus far (Szél and colleagues). Using selective labeling with these two antibodies and a nonselective method in nasal and temporal halves of the same retinas, we have found that densities of photoreceptors vary systematically, depending upon the size of the eye and age of the animal. In ‘standard’ New Zealand rabbits of 2–3 kg (2–3 months old), rods reached a peak density of about 300,000/mm2 just dorsal to the visual streak, while cones exhibit peak density at mid-visual streak of about 18,000/mm2. Published measurements of visual acuity in rabbit are less than predicted by this calculation. The ratio of cones to rods is significantly higher in ventral retina, where the density of cones declines to a plateau of 10,000–12,000/mm2, when compared to dorsal retina, where cones are uniformly distributed at a density of about 7000/mm2. The density of OS-2 labeled (presumably “blue”) cones is uniformly low, 1000–1500/mm2, in a wide expanse that includes dorsal retina, the visual streak, and much of ventral retina, except for a region of higher density along the vertical midline. We confirm that there is a far ventral horizontal region near the perimeter that is populated exclusively by a high density (about 13,000/mm2) of OS-2-positive cones (Juliusson and colleagues). This region does not extend to the ventral retinal margin, however, where cone density drops precipitously. Transitional zones between COS-1 and OS-2 labeling, in a region of relatively high and uniform cone density, where sums of COS-1 and OS-2 labeling are higher than expected and in which weakly and strongly labeled cones are intermixed, raise questions about the identities of the visual pigment epitopes, the possibility of double labeling, and therefore the possibility of dual expression of pigments in single cones. The “inverted- T -shaped” topography of higher density OS-2 labeling raises doubts about the significance of a ventral concentration of blue cones for visual function in rabbit retina.
Distribution of photoreceptor types in the retina of a marsupial, the tammar wallaby (Macropus eugenii)
- JAN M. HEMMI, ULRIKE GRÜNERT
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- Journal:
- Visual Neuroscience / Volume 16 / Issue 2 / March 1999
- Published online by Cambridge University Press:
- 01 March 1999, pp. 291-302
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Mammalian retinae generally contain low numbers of short-wavelength-sensitive cones (S-cones) and higher numbers of middle- to long-wavelength-sensitive cones (M-cones). Some recent studies found topographic differences between the different photoreceptor types and in some instances between photoreceptors and ganglion cells. To investigate this question further, we constructed topographical maps of the different photoreceptors found in an Australian marsupial, the tammar wallaby. We used two polyclonal antibodies that have been shown to label S-cones (JH455) or M-cones (JH492) in a range of mammals. In the tammar wallaby, the antisera clearly distinguish two cone types. JH455 recognizes a small subset of cones (S-cones) with a density of less than 500 cells/mm2 in the ventral retina. Their density increases towards the dorsal retina to about 1600–2000 cells/mm2. JH492 recognizes all remaining cones (M-cones), but also faintly labels most cone cells recognized by JH455. The distribution of M-cones, unlike that of the S-cones, shows a clear horizontal streak of high cell density through the central retina, just like the ganglion cells. Unlike the ganglion cells, however, the M-cones do not peak in the temporal retina but show a very broad peak (12,000–18,000 cells/mm2) in the central or even slightly nasal retina. Based on our findings, the retina of the tammar can be divided into three distinct regions: firstly, the dorsal retina, which has a low ganglion and low cone cell density but a high percentage of S-cones (30%), is thought to provide good spectral sensitivity; secondly, the central horizontal band of retina, which has a high ganglion and high cone cell density and therefore provides good spatial resolution; and thirdly, the ventral retina, which has a low ganglion cell but high cone cell density with few S-cones (5%) and is therefore thought to have a high contrast sensitivity but low acuity.