Distribution of S- and M-cones in normal and experimentally detached cat retina

The lectin peanut agglutinin (PNA) and antibodies to short (S)- and medium to long wavelength (M/L)-sensitive cones were utilized in order to define the relative distributions of the two spectral types of cone across the domestic cat's retina. These values, in turn, were compared to those from retinas that had been experimentally detached from the retinal pigment epithelium. The pattern of cone distribution in the normal cat's retina is established by the preponderance of M-cones that constitute between 80% and 90% of all cones. Their peak density of over 26,000 cells/mm(2) resides at the area centralis. Though M-cone density decreases smoothly to the ora serrata where they have densities as low as 2,200 cells/mm(2), the density decrease along the nasotemporal axis is slower,creating a horizontal region of higher cone density. S-cones constitute between 10% and 20% of all cones, the number being quite variable even between individual animals of similar age. The highest S-cone densities are found in three distinct locations: at the superior far periphery near the ora serrata, immediately at the area centralis itself, and in a broad zone comprising the central and lower half of the inferior hemiretina. S-cones in the cat retina do not form a regular geometrical array at any eccentricity. As for the detached cat retina, the density of labeled S-cone outer segments (OS) decreases rapidly as early as 1 day postdetachment and continues decreasing to day 28 when the density of cones labeling with anti-S opsin has dropped to less than 10% of normal. This response points to a profound difference between rods and cones; essentially all rods, including those without OS, continue to express their opsin even in long-term detachments. The implications of these results for visual recovery after retinal reattachment are discussed.     

The development of parafoveal and mid-peripheral human retina.

The morphological development of parafoveal retina (1-1.5 mm from the foveal center) and the mid-peripheral (4 mm from the foveal center) human retina has been studied from fetal (F) 26 weeks to adulthood. At both retinal points, all layers and neuronal types are present at F26 weeks. In parafovea at F26 weeks photoreceptors have only a rudimentary inner segment and no outer segments. Short outer segments are present on both rods and cones at F36 weeks. By postnatal (P) 5-8 days the inner retina is relatively mature. Photoreceptors have elongated basal axons which cause the photoreceptor layer to become much thicker than in prenatal retina. At birth cone inner segments are untapered, but rod inner segments have already reached their adult width of 2 microns. Both rod and cone inner and outer segments are 30-50% of adult length. By 13 months both inner and outer retina are mature appearing, with the photoreceptors accounting for half the retinal thickness due to the elongation of the fibers of Henle. Cone outer segments elongate up to P5 years and rod outer segments to P13 years. At mid-peripheral or rod-ring retina outer segments are present on rods at F26 weeks and on cones at F36 weeks. At birth the inner retina is adultlike. The outer plexiform layer becomes thicker up to P45 months due to the elongation of fibers of Henle. At birth both rod and cone mid-peripheral inner segments are slightly longer and outer segments are 50% longer than in parafoveal retina. By P5 years mid-peripheral rod outer segments are slightly longer than in parafoveal retina, and this changes little thereafter. This anatomical study has found that the photoreceptors in peripheral rod-ring retina develop earlier than those in more central retina, and in turn parafoveal photoreceptors develop well in advance of foveal cones. This suggests that human neonates may utilize more peripheral retinal regions for some aspects of visual function before foveal cone vision becomes dominant.     

Topography of ganglion cells and photoreceptors in the sheep retina

Retinal topographies of some cell types and distribution of the tapetum lucidum in the sheep's eye were investigated in this study. The tapetum was observed macroscopically in the fundus. The topographical distributions of retinal ganglion cells (RGCs), cones, and rods were simultaneously analyzed in retinal whole mounts stained with cresyl violet. Short‐wavelength‐sensitive (S) cones were immunocytochemically identified in retinal whole mounts. The tapetum was located dorsal to the optic disc, with the nasal part elongated horizontally and the temporal part expanded dorsally. RGCs were distributed densely in the area centralis, horizontal visual streak, and anakatabatic area. The highest density in the area centralis was approximately 18,000 RGCs/mm². Cones showed high density in the horizontal area crossing the optic disc and dorsotemporal area, whereas rods showed high density in the horizontal area, which was greater in height than the horizontal area of high cone density. S cones showed high density in the dorsotemporal retina. The rod/cone ratios were high horizontally in the dorsal retina to the optic disc, with a mean value of 11:1. The cone/RGC and rod/RGC ratios were lower in the horizontal and dorsotemporal retina, and the rod/cone/RGC ratio was lowest in the area centralis (9:1:1). The retinal topographies and distribution of the tapetum were specialized in the horizontal and dorsotemporal fundus. This suggests that sheep have better visual acuity in horizontal and anteroinferior visual fields and that this specialization is related to the visual ecology of sheep. J. Comp. Neurol. 518:2305–2315, 2010. © 2010 Wiley‐Liss, Inc.     

Unique topographic separation of two spectral classes of cones in the mouse retina.

We have found two immunologically distinguishable cone types in the retina of the mouse, each localized to two opposite halves of the eye. One cone type was labelled by the monoclonal antibody COS-1 specific to the middle-to-long wave sensitive visual pigment of the mammals, while the other type was stained by the shortwave-specific monoclonal antibody (OS-2). These results were confirmed with other antibodies directed against specific sequences of the visual pigments. As a result of the uneven distribution of the two cone types the mouse retina is divided into two fields separated by an oblique meridional line. The middlewave sensitive cones were present exclusively in the dorsal half of the mouse retina (M-field). The overwhelming majority of the shortwave sensitive cones occupied the ventral half (S-field), and only a small number was scattered among the middlewave sensitive cones in the dorsal retina. The ratio of the two cone types in the M-field corresponds to what has been found in the retina of other mammals, including rodents such as the gerbil and the rat. The S-field represents an entirely unique area with the unusually great number of shortwave sensitive cones and with the complete lack of the middlewave sensitive ones. The present study provides the structural basis for dichromacy in a rodent species considered for a long time to be monochromat. In addition, it shows that the ventral retina, containing exclusively S-cones in a relatively high density, is a unique retinal field not present in other mammalian species studied so far.     

Retinal photoreceptor arrangement,SWS1 and LWS opsin sequence,and electroretinography in the South American marsupial Thylamys elegans (Waterhouse, 1839)

We studied the retinal photoreceptors in the mouse opossum Thylamys elegans, a nocturnal South American marsupial. A variety of photoreceptor properties and color vision capabilities have been documented in Australian marsupials, and we were interested to establish what similarities and differences this American marsupial showed. Thylamys opsin gene sequencing revealed two cone opsins, a longwave‐sensitive (LWS) opsin and a shortwave‐sensitive (SWS1) opsin with deduced peak sensitivities at 560 nm and 360 nm (ultraviolet), respectively. Immunocytochemistry located these opsins to separate cone populations, a majority of LWS cones (density range 1,600–5,600/mm²) and a minority of SWS1 cones (density range 100–690/mm²). With rod densities of 440,000–590,000/mm², the cones constituted 0.4–1.2% of the photoreceptors. This is a suitable adaptation to nocturnal vision. Cone densities peaked in a horizontally elongated region ventral to the optic nerve head. In ventral—but not dorsal—retina, roughly 40% of the LWS opsin‐expressing cones occurred as close pairs (double cones), and one member of each double cone contained a colorless oil droplet. The corneal electroretinogram (ERG) showed a high scotopic sensitivity with a rod peak sensitivity at 505 nm. At mesopic light levels, the spectral ERG revealed the contributions of a UV‐sensitive SWS1 cone mechanism and an LWS cone mechanism with peak sensitivities at 365 nm and 555 nm, respectively, confirming the tuning predictions from the cone opsin sequences. The two spectral cone types provide the basis for dichromatic color vision, or trichromacy if the rods contribute to color processing at mesopic light levels. J. Comp. Neurol. 518:1589–1602, 2010. © 2009 Wiley‐Liss, Inc.     

The distribution of rods and cones in the retina of the cat (Felis domesticus)

The distribution of rods and cones in the cat retina was studied by light microscopy. The rods and cones were counted from the area centralis to the temporal periphery in photomicrographs of transverse sections through the inner segments. A 16% correction for the effect of tissue shrinkage was applied to the densities obtained in fixed-dehydrated tissue, by comparing these counts with those obtained from similar retinal areas in fresh tissue. The cone distribution was characterized by a steep increase in cone density centrally, which was elongated in the nasotemporal axis, and coincided approximately with the central increase in ganglion cell density (Stone, '65). Cone density in the area centralis peaked at 26,000–27,000/mm², and fell to a plateau of 4000/mm² in the periphery and to less than 3000/mm² near the ora serrata. The density of rods was greater than the density of cones in all regions. The rods reached a maximum density of 460,000/mm² at an eccentricity of 10–15°, in a region which completely surrounded the central cone elevation. Centrally, the rod distribution was characterized by a sharp fall in density, reaching a low of 275,000/mm² at the point of peak cone density. Peripherally, there was a plateau of high rod density at above 400,000/mm², out to about 30° temporal eccentricity. Beyond this plateau, rod density steadily fell, and reached a low of 250,000/mm² near the ora serrata. The rod/cone ratio reached a low value of 10.5–11.0 in the central region of maximum cone density. It then rose steeply to reach a plateau of about 65 in the periphery, while near the ora serrata there were 100 rods for each cone. The rods were arranged in well defined rows, and each rod was surrounded by six other rods. The cones were interspersed throughout the rod mosaic and were less regularly arranged than the rods. In fresh tissue, rod outer segment diameters ranged from 1.0 μ to 1.6 μ. rod diameters were smallest in the region of maximum rod density and increased, along with the decrease in rod density, both in the periphery and in the area centralis. Two thirds of the central decrease in rod density from the pericentral peak could not be accounted for by the increase in the percentage of space occupied by cones, and was consistent with a central increase in rod diameter. Receptor-ganglion cell convergence was estimated by comparing receptor densities with the ganglion cell densities of Stone ('65). Cone-ganglion cell convergence reached a minimum in the area centralis at the cone density peak, probably indicating the retinal region in which photopic acuity is at an optimum. Minimum rod-ganglion cell convergence, as estimated from the rod/large ganglion cell ratio, reached a minimum at about 7.5° central to the peak of rod density, and at about the same eccentricity as the human scotopic acuity optimum.     

Photoreceptor topography and spectral sensitivity in the common brushtail possum (Trichosurus vulpecula)

Marsupials are believed to be the only non‐primate mammals with both trichromatic and dichromatic color vision. The diversity of color vision systems present in marsupials remains mostly unexplored. Marsupials occupy a diverse range of habitats, which may have led to considerable variation in the presence, density, distribution, and spectral sensitivity of retinal photoreceptors. In this study we analyzed the distribution of photoreceptors in the common brushtail possum (Trichosurus vulpecula). Immunohistochemistry in wholemounts revealed three cone subpopulations recognized within two spectrally distinct cone classes. Long‐wavelength sensitive (LWS) single cones were the largest cone subgroup (67–86%), and formed a weak horizontal visual streak (peak density 2,106 ± 435/mm²) across the central retina. LWS double cones were strongly concentrated ventrally (569 ± 66/mm²), and created a “negative” visual streak (134 ± 45/mm²) in the central retina. The strong regionalization between LWS cone topographies suggests differing visual functions. Short‐wavelength sensitive (SWS) cones were present in much lower densities (3–10%), mostly located ventrally (179 ± 101/mm²). A minority population of cones (0–2.4%) remained unlabeled by both SWS‐ and LWS‐specific antibodies, and may represent another cone population. Microspectrophotometry of LWS cone and rod visual pigments shows peak spectral sensitivities at 544 nm and 500 nm, respectively. Cone to ganglion cell convergences remain low and constant across the retina, thereby maintaining good visual acuity, but poor contrast sensitivity during photopic vision. Given that brushtail possums are so strongly nocturnal, we hypothesize that their acuity is set by the scotopic visual system, and have minimized the number of cones necessary to serve the ganglion cells for photopic vision. J. Comp. Neurol. 522:3423–3436, 2014. © 2014 Wiley Periodicals, Inc.     

Absence of short-wavelength sensitive cones in the retinae of seals (Carnivora) and African giant rats (Rodentia)

Most non-primate mammals have two types of cone: short-wavelength sensitive (S) and middle-to-long-wavelength sensitive (M/L) cones. In two species of African giant rats, Cricetomys gambianus and C. emini, and in two species of earless seals, Phoca hispida and P. vitulina, the retinal cone types and cone distributions were assessed with antibodies specific for the M/L-cone opsin and the S-cone opsin, respectively. All four species were found to completely lack S-cones, while M/L-cones were present in low densities. M/L-cone densities, rod densities and cone/rod ratios were determined across the retina. Cone proportions are about 0.3–0.5% in C. gambianus, 0.5–0.8% in C. emini, and 1.5–1.8% in P. hispida. An absence of S-cones has previously been reported in a few nocturnal mammals. As earless seals are visually active during night and day, we conclude that an absence of S-cones is not exclusively associated with nocturnality. The functional and comparative aspects are discussed.     

S-cones do not contribute to the OFF-midget pathway in the retina of the marmoset, Callithrix jacchus

It is well established that in primate retina both medium- and long-wavelength-sensitive cone types provide input to the midget-parvocellular pathway. The question, however, whether short-wavelength-sensitive (S or 'blue') cones provide input to the OFF-division of the midget-parvocellular pathway is still controversial. In the present study, we investigated the connections of nearly 400 S-cones with OFF-midget bipolar cells in central and peripheral retina of a New World monkey, the marmoset. Horizontal sections or pieces of whole retinae were double-labelled with an antiserum to S-cone opsin to identify S-cones and antibodies to the cell adhesion molecule CD15 to identify OFF-midget bipolar cells. Peanut agglutinin coupled to a fluorescent tag was used to label the cone pedicles of all cone types. Peanut agglutinin was also used to distinguish S-cones from the other cone types. The sections were analysed with deconvolution microscopy. We found that nearly all pedicles of medium- and long-wavelength-sensitive cones are located opposite distinct dendritic clusters formed by OFF-midget bipolar cells. By contrast, the S-cone pedicles are not located opposite dendritic clusters. Instead, S-cones make sparse contacts with CD15-labelled processes. Some of these processes protruded from OFF-midget bipolar clusters, whereas others could be traced to a diffuse bipolar cell type. Thus, in the marmoset retina the midget-parvocellular system does not carry a blue-OFF signal.     

Effect of Rds abundance on cone outer segment morphogenesis, photoreceptor gene expression, and outer limiting membrane integrity

We examined the molecular, structural, and functional consequences on cone photoreceptors of the neural retinal leucine zipper knockout (Nrl(-/-)) mice when only one allele of retinal degeneration slow (Rds) is present (Rds(+/-)/Nrl(-/-)). Quantitative RT-PCR and immunoblot analysis were used to assess the expression levels of several phototransduction genes; electroretinography was used to assess quantitatively the retinal responsiveness to light; and immunohistochemistry and ultrastructural analysis were used to examine retinal protein distribution and morphology, respectively. In Rds/Nrl double-null mice, S-cones form dysmorphic outer segments that lack lamellae and fail to associate properly with the cone matrix sheath and the outer limiting membrane. In Rds(+/-)/Nrl(-/-) mice, cones form oversized and disorganized outer segment lamellae; although outer limiting membrane associations are maintained, normal interactions with cone matrix sheaths are not, and photoreceptor rosette formation is observed. These retinas produce significantly higher photopic a-wave and b-wave amplitudes than do those of Rds(-/-)/Nrl(-/-) mice, and the levels of several cone phototransduction genes are significantly increased coincidently with the presence of Rds and partial lamellae formation. Thus, as in rod photoreceptors, expression of only one Rds allele is unable to support normal outer segment morphogenesis in cones. However, cone lamellae assembly, albeit disorganized, concomitantly permits outer limiting membrane association, and this appears to be linked to photoreceptor rosette formation in the rodless (cone-only) Nrl(-/-) retina. In addition, photoreceptor gene expression alterations occur in parallel with changes in Rds levels.     

The distributions of photoreceptors and ganglion cells in the California ground squirrel,Spermophilus beecheyi

The topographical distributions of photoreceptors and ganglion cells of the California ground squirrel (Spermophilus beecheyi) were quantified in a light microscopic study. The central retina contains broad, horizontal streaks of high photoreceptor density (40–44,000/mm²) and high ganglion cell density (20–24,000/mm²). The isodensity contours of both cell types are elliptical and oriented along the nasal-temporal axis. There are roughtly fivefold decreases in both photoreceptor and ganglion cell densities with increasing eccentricity, the lowest densities being found in the superior retina. Large transitions in cell density and retinal thickness occur across the linear optic nerve head. Rod frequency increases with increasing eccentricity, from 5 to 7% in the central retina to 15 to 20% in the periphery. Roughly 10% of the cones possess wide, dark-staining ellipsoids. These cones are uniformly distributed across the retina which suggests that they may belong to a separate cone class, possibly blue-sensitive cones. The ganglion cell soma size distribution is unimodal, with the majority of somata being 25–50 μm². Large ganglion cells (somata > 100 μm²) are rare in the central retina, but their frequency increases with increasing eccentricity. No evidence for separate size classes of ganglion cells was found. The gradual decrement of photoreceptor density across the ground squirrel retina suggests that there are only relatively small changes in acuity across much of the animal's visual space compared with species possessing either a narrow visual streak or fovea or area centralis.     

The receptor mosaic of Aotes trivirgatus: Distribution of rods and cones

The distribution and density of rods and cones have been determined in the temporal retina of the owl monkey Aotes trivirgatus. Rod density varied from 175,000/mm² in the far periphery to 387,000/mm² in the area centralis. Cone density was 4000/mm² in the periphery and 7300/mm² within the central retinal area. Variation of densities across the retina was continuous but the ratio of rods to cones remained about 50:1 at all locations. The cones are not randomly scattered, but are separated by distances of about 10 μ in central retina and 13 μ in peripheral retina.     

Development of the neural retina and its vasculature in the marmoset Callithrix jacchus

The morphological sequence of retinal development in the New World marmoset monkey Callithrix jacchus is similar to previous reports in Macaca and humans. The incipient fovea is present at fetal day (Fd) 100 as the only part of the retina that contains five distinct layers, including a single layer of cone photoreceptors. A foveal pit begins to form at Fd 135 in the center of the foveal avascular zone which is surrounded by a ring of blood vessels (BV) and astrocytes. At birth (Fd 144) the fovea has a single layer of cones over the pit center where the inner retinal layers are thinned but still separated. After birth the fovea rapidly matures so that foveal cone and pit morphology are similar to adult by 4 months. Five distinct layers and the BV plexus in the nerve fiber layer are present to the retinal edge in neonatal marmosets. Near the optic disc BV are sprouting into outer retinal layers at birth and vascularization of the outer retina is completed by 2 to 3 months. Retinal length increases sharply up to Fd 135, but undergoes a quiescent period around birth during which pit formation begins. Length then increases again up to 4mo, followed by a slow increase into adulthood. The postnatal increase is accompanied by a marked thinning of the peripheral retina. The pars plana appears after birth and its length increases at least until 2 years of age. The major difference between marmoset and Macaca is the relative immaturity of the marmoset fovea at birth, and its rapid development after birth. This makes the marmoset a good candidate for neonatal experimental manipulation of retinal and eye development.     

Topography of cones and rods in the tree shrew retina

The topographical distribution of cones and rods in the tree shrew retina was analysed quantitatively in whole-mounted retinae and horizontal semithin sections stained with cresyl violet or toluidine blue. The outer nuclear layer consists of a single layer of photoreceptor nuclei with the rod nuclei slightly displaced towards the outer plexiform layer. This facilitated quantification of the photoreceptor populations. The density of cones ranges from 12,000/mm2 in the peripheral retina to a maximum of 36,000/mm2 in the inferior retina. Unlike ganglion cell density, the density of cones does not peak in the temporal retina. Rod density, between 500/mm2 and 3,500/mm2, also peaks in the inferior retina, but not in the same region as cone density. Rods constitute from 1 to 14% of the photoreceptor population, depending on retinal location, and have a local minimum at the central area. Amongst the cones a regularly arrayed subpopulation of presumed blue-sensitive cones is distinguished by its special staining properties. These cones constitute between 4 and 10% of the cone population depending on retinal location. A second, irregularly spaced, subpopulation of possibly pathological cones is also described.     

Retinal photoreceptor and ganglion cell types and topographies in the red fox (Vulpes vulpes) and Arctic fox (Vulpes lagopus)

The red fox (Vulpes vulpes) is the carnivore with the widest distribution in the world. Not much is known about the visual system of these predominantly forest‐dwelling animals. The closely related Arctic fox (Vulpes lagopus) lives in more open tundra habitats. In search for corresponding adaptations, we examined the photoreceptors and retinal ganglion cells (RGCs), using opsin immunohistochemistry, lucifer yellow injections and Nissl staining. Both species possess a majority of middle‐to‐longwave‐sensitive (M/L) and a minority of shortwave‐sensitive (S) cones, indicating dichromatic color vision. Area centralis peak cone densities are 22,600/mm² in the red fox and 44,800/mm² in the Arctic fox. Both have a centro‐peripheral density decrease of M/L cones, and a dorsoventrally increasing density of S cones. Rod densities and rod/cone ratios are higher in the red fox than the Arctic fox. Both species possess the carnivore‐typical alpha and beta RGCs. The RGC topography shows a centro‐peripheral density gradient with a distinct area centralis (mean peak density 7,900 RGCs/mm² in the red fox and 10,000 RGCs/mm² in the Arctic fox), a prominent visual streak of higher RGC densities in the Arctic fox, and a moderate visual streak in the red fox. Visual acuity and estimated sound localization ability were nearly identical between both species. In summary, the red fox retina shows adaptations to nocturnal activity in a forest habitat, while the Arctic fox retina is better adapted to higher light levels in the open tundra.     

Migration and synaptogenesis of cone photoreceptors in the developing mouse retina

Mouse retinal photoreceptor cell generation and morphogenesis take place in a well-characterized temporal sequence. Both rod and cone photoreceptor differentiation and synaptogenesis occur postnatally, but the relative timing of these events has been difficult to document due to the paucity of cell-specific markers. We have found that antibodies to neuron-specific enolase (NSE) preferentially label a subpopulation of photoreceptors in the outer nuclear layer (ONL) of the mouse retina in addition to labeling ganglion, amacrine, bipolar, and horizontal cells within the inner layers of the retina. The appearance of NSE immunoreactivity in the different classes of retinal neurons during development showed a close temporal relationship to the onset of expression of the synaptic vesicle-associated protein SV2 and clearly preceded the sequential development of synaptic connections in both inner and outer synaptic layers. The NSE-immunoreactive photoreceptors were identified as cones by dual labeling of their inner segments with the lectin peanut agglutinin or by colabeling with antisera to cone photopigments. Axonal extensions of NSE-labeled cone cells were shown to interact with those of differentiating horizontal cells as early as postnatal day 3 (P3). Colocalization of NSE with SV2 indicated that cone cells began to make synaptic contacts with horizontal cell processes several days prior to the development of rod synaptic terminals. Between P4 and P11, cone photoreceptor cell nuclei were observed to be scattered at various levels throughout the ONL and thus appeared to have become displaced from their previous position directly beneath the outer limiting membrane (OLM). By P12, the cone nuclei had migrated sclerad once again and were now observed to be neatly aligned adjacent to the OLM. In the rd mouse mutant, this migratory process was delayed, so that, at P12, positioning of the cone cell nuclei within the ONL was still quite irregular. Thus, we have identified a late migratory phase for cone photoreceptors during the second week after birth that correlates with the timing of maturation of the rod synaptic terminals just prior to eye opening. The types of cues used by maturing cone cells for their eventual sclerad location remain to be elucidated. J. Comp. Neurol. 388:47–63, 1997. © 1997 Wiley-Liss, Inc.     

Rods and cones in the mouse retina. I. Structural analysis using light and electron microscopy.

Rods and cones of the C57BL/6J mouse retina have been examined by light and electron microscopy to distinguish the structural features of the two photoreceptor types. By light microscopy, cone nuclei are conspicuously different from rod nuclei in 1-2 micrometer plastic sections. Cone nuclei have an irregularly shaped clump of heterochromatin that appears in single sections to be one to three clumps, whereas rod nuclei are more densely stained and have one large, central clump of heterochromatin. Cone nuclei make up approximately 3% of the photoreceptor nuclei in both the central and peripheral retina at all ages examined up to 267 days. Cone nuclei are confined to the outer half of the outer nuclear layer, and more than 50% of the cone nuclei lie adjacent to the outer limiting membrane. By electron microscopy, cones in the mouse retina meet virtually every morphological criterion of mammalian cones. The outer segments are conically shaped. Many, if not all of the outer segment discs are continuous with the outer plasma membrane, whereas almost all of the rod discs are not. Cone outer segments are only about half the length of the rod outer segments, and they are contacted by long, villous pigment epithelial cell processes. The cone inner segment diameter is greater than the outer segment diameter, and the accumulation of mitochondria present at the apical end of the inner segment forms a more conspicuous ellipsoid than in rods. The internal fiber or axon of the cone is larger in diameter than that of the rod, and it terminates in a large synaptic pedicle with multiple ribbon synapses, whereas the rod terminal is a smaller spherule with only a single ribbon synaptic complex.     

Selective lectin binding of the developing mouse retina

A battery of eight lectins with different carbohydrate specificities was used to study changes in glycoconjugate expression during cell differentiation in the mouse retina. The lectins tested included concanavalin A (Con A), wheat germ agglutinin (WGA), soybean agglutinin (SBA), peanut agglutinin (PNA), Ulex europaeus agglutinin (UEA), Ricinus communis agglutinin I (RCA), Dolichos biflorus agglutinin (DBA), and Limulus polyphemus agglutinin (LPA). Unfixed frozen sections of adult and early postnatal mouse retina were treated with fluorescein isothiocyanate-conjugated lectins and examined by fluorescence microscopy. The results showed selective lectin binding in both cellular and synaptic retinal layers of the adult mouse and throughout postnatal development. In general, an increase in intensity of fluorescent lectin staining during retinal development was observed for Con A, WGA, DBA, LPA, RCA, and PNA. This suggests an increase in the expression or accessibility of carbohydrate moieties during development. SBA and UEA showed little to no binding to adult or neonatal retina. Retinal vasculature was intensely stained by RCA, both during development and in the adult. All lectins binding to adult or neonatal retinal layers showed some degree of reactivity with the inner segment region of photoreceptor cells. However, only Con A, PNA and WGA bound to photoreceptor outer segments, suggesting significant differences in the glycosylated components of inner and outer segment membranes. PNA bound specifically to a subpopulation of photoreceptor cells and to discrete regions within the outer synaptic layer. The pattern of PNA binding suggests that this lectin binds preferentially to cone photoreceptor inner and outer segments and cone synaptic pedicles rather than to rod photoreceptor cells. This marked specificity of PNA binding suggests that it may provide a basis for the physical separation of cone and rod photoreceptor cells.     

Horizontal Cells in the Monkey Retina: Immunocytochemical staining with antibodies against calcium binding proteins

Immunocytochemistry with antibodies against the calcium binding proteins parvalbumin (PV) and calbindin (CaBP-28kD) was used to study horizontal cells in the macaque monkey retina. Both morphological types (H1 and H2) are stained by PV immunocytochemistry. Horizontal cells in the centre of the fovea are described for the first time. From a minimum of 250/mm2 at the foveal centre, total horizontal cell density rapidly increases to a peak of 23,000/mm2 in an annulus of 0.6 mm radius around the fovea. Horizontal cell density then drops continuously to approximately 800 - 1000 cells/mm2 in peripheral retina. The density ratio of cones to horizontal cells is 1.5 in central and 4 in peripheral retina. The cones were stained with antibodies against CaBP-28kD. Thus the spatial correlation between the cone inner segments and the cone pedicles could be measured to show that they are in perfect register. There is no reorganization of the spatial array of cone outer segments to produce chromatically specific connections between cone pedicles and horizontal cells.     

S-cone connections of the diffuse bipolar cell type DB6 in macaque monkey retina

Previous studies of primate retinae have shown that diffuse bipolar (DB) cells contact all the cones in their dendritic field, suggesting there is no spectral selectivity in the functional input to DB cells. However, since short-wavelength sensitive (S) cones make up less than 10% of the total cone population, specialized connectivity with S-cones is difficult to detect. In the present study, the S-cone connectivity of a subtype of DB cells, the DB6 cell, was studied in macaque monkey retina. Pieces of macaque retina were processed with antibodies to CD15 to stain DB6 cells and antibodies to the S-cone opsin to identify S-cones. Immunoreactivity was visualized using immunoperoxidase or immunofluorescence. Some preparations were additionally processed with peanut agglutinin coupled to fluorescein to reveal medium- and long-wavelength sensitive (M/L) cones. The preparations were analyzed using conventional and deconvolution light microscopy. The majority of DB6 cells had one or two S-cones in their dendritic field and the majority of S-cones were located in the dendritic field of DB6 cells. On average, 80% of the S-cones and 81% of the M/L cones contacted DB6 cells. The average number of dendritic terminals at cone pedicles did not differ between the cone types. However, the total number of DB6 dendritic terminals receiving input from M/L-cone pedicles was about eight times higher than the total number of dendritic terminals at S-cone pedicles. In conclusion, DB6 cells make indiscriminate contact with all cone types, but receive their major input from M/L-cones and thus carry a "Yellow-ON" spectral signal.