Human photoreceptor topography

We have measured the spatial density of cones and rods in eight whole-mounted human retinas, obtained from seven individuals between 27 and 44 years of age, and constructed maps of photoreceptor density and between-individual variability. The average human retina contains 4.6 million cones (4.08-5.29 million). Peak foveal cone density averages 199,000 cones/mm2 and is highly variable between individuals (100,000-324,000 cones/mm2). The point of highest density may be found in an area as large as 0.032 deg2. Cone density falls steeply with increasing eccentricity and is an order of magnitude lower 1 mm away from the foveal center. Superimposed on this gradient is a streak of high cone density along the horizontal meridian. At equivalent eccentricities, cone density is 40-45% higher in nasal compared to temporal retina and slightly higher in midperipheral inferior compared to superior retina. Cone density also increases slightly in far nasal retina. The average human retina contains 92 million rods (77.9-107.3 million). In the fovea, the average horizontal diameter of the rod-free zone is 0.350 mm (1.25 degrees). Foveal rod density increases most rapidly superiorly and least rapidly nasally. The highest rod densities are located along an elliptical ring at the eccentricity of the optic disk and extending into nasal retina with the point of highest density typically in superior retina (5/6 eyes). Rod densities decrease by 15-25% where the ring crosses the horizontal meridian. Rod density declines slowly from the rod ring to the far periphery and is highest in nasal and superior retina. Individual variability in photoreceptor density differs with retinal region and is similar for both cones and rods. Variability is highest near the fovea, reaches a minimum in the midperiphery, and then increases with eccentricity to the ora serrata. The total number of foveal cones is similar for eyes with widely varying peak cone density, consistent with the idea that the variability reflects differences in the lateral migration of photoreceptors during development. Two fellow eyes had cone and rod numbers within 8% and similar but not identical photoreceptor topography.

Topography of ganglion cells in human retina

We quantified the spatial distribution of presumed ganglion cells and displaced amacrine cells in unstained whole mounts of six young normal human retinas whose photoreceptor distributions had previously been characterized. Cells with large somata compared to their nuclei were considered ganglion cells; cells with small somata relative to their nuclei were considered displaced amacrine cells. Within the central area, ganglion cell densities reach 32,000-38,000 cells/mm2 in a horizontally oriented elliptical ring 0.4-2.0 mm from the foveal center. In peripheral retina, densities in nasal retina exceed those at corresponding eccentricities in temporal retina by more than 300%; superior exceeds inferior by 60%. Displaced amacrine cells represented 3% of the total cells in central retina and nearly 80% in the far periphery. A twofold range in the total number of ganglion cells (0.7 to 1.5 million) was largely explained by a similar range in ganglion cell density in different eyes. Cone and ganglion cell number were not correlated, and the overall cone:ganglion cell ratio ranged from 2.9 to 7.5 in different eyes. Peripheral cones and ganglion cells have different topographies, thus suggesting meridianal differences in convergence onto individual ganglion cells. Low convergence of foveal cones onto individual ganglion cells is an important mechanism for preserving high resolution at later stages of neural processing. Our improved estimates for the density of central ganglion cells allowed us to ask whether there are enough ganglion cells for each cone at the foveal center to have a direct line to the brain. Our calculations indicate that 1) there are so many ganglion cells relative to cones that a ratio of only one ganglion cell per foveal cone would require fibers of Henle radiating toward rather than away from the foveal center; and 2) like the macaque, the human retina may have enough ganglion cells to transmit the information afforded by closely spaced foveal cones to both ON- and OFF-channels. Comparison of ganglion cell topography with the visual field representation in V1 reveals similarities consistent with the idea that cortical magnification is proportional to ganglion cell density throughout the visual field.

The renewal of photoreceptor cell outer segments

The utilization of methionine-(3)H by retinal photoreceptor cells has been studied by radioautographic technique in the rat, mouse, and frog. In all three species, the labeled amino acid is concentrated initially in the inner segment of the cell. Within 24 hr, the radioactive material is displaced to the base of the outer segment, where it accumulates as a distinct reaction band. The reaction band then gradually moves along the outer segment and ultimately disappears at the apex of the cell, which is in contact with the retinal pigment epithelium. These findings are interpreted to indicate that the photoreceptor cell outer segment is continually renewed, by the repeated lamellar apposition of material (membranous discs) at the base of the outer segment, in conjunction with a balanced removal of material at its apex. The outer segment renewal rate is accelerated in frogs when ambient temperature is raised, and is elevated in both frogs and rats when the intensity of retinal illumination is increased.

Participation of the retinal pigment epithelium in the rod outer segment renewal process

The disposal phase of the retinal rod outer segment renewal process has been studied by radioautography in adult frogs injected with tritiated amino acids. Shortly after injection, newly formed radioactive protein is incorporated into disc membranes which are assembled at the base of the rod outer segment. During the following 2 months, these labeled discs are progressively displaced along the outer segment owing to the repeated formation of newer discs. When the labeled membranes reach the end of the outer segment, they are detached from it. They subsequently may be identified in inclusion bodies within the pigment epithelium by virtue of their content of radioactivity. These inclusions have been termed phagosomes. Disc membrane formation is a continuous process, but the detachment of groups of discs occurs intermittently. The detached outer segment fragments become deformed, compacted, undergo chemical changes, and are displaced within the pigment epithelium. Ultimately, the material contained in the phagosomes is eliminated from the cell. In this manner the pigment epithelium participates actively in the disposal phase of the rod outer segment renewal process.

Central projections of melanopsin-expressing retinal ganglion cells in the mouse

A rare type of ganglion cell in mammalian retina is directly photosensitive. These novel retinal photoreceptors express the photopigment melanopsin. They send axons directly to the suprachiasmatic nucleus (SCN), intergeniculate leaflet (IGL), and olivary pretectal nucleus (OPN), thereby contributing to photic synchronization of circadian rhythms and the pupillary light reflex. Here, we sought to characterize more fully the projections of these cells to the brain. By targeting tau-lacZ to the melanopsin gene locus in mice, ganglion cells that would normally express melanopsin were induced to express, instead, the marker enzyme beta-galactosidase. Their axons were visualized by X-gal histochemistry or anti-beta-galactosidase immunofluorescence. Established targets were confirmed, including the SCN, IGL, OPN, ventral division of the lateral geniculate nucleus (LGv), and preoptic area, but the overall projections were more widespread than previously recognized. Targets included the lateral nucleus, peri-supraoptic nucleus, and subparaventricular zone of the hypothalamus, medial amygdala, margin of the lateral habenula, posterior limitans nucleus, superior colliculus, and periaqueductal gray. There were also weak projections to the margins of the dorsal lateral geniculate nucleus. Co-staining with the cholera toxin B subunit to label all retinal afferents showed that melanopsin ganglion cells provide most of the retinal input to the SCN, IGL, and lateral habenula and much of that to the OPN, but that other ganglion cells do contribute at least some retinal input to these targets. Staining patterns after monocular enucleation revealed that the projections of these cells are overwhelmingly crossed except for the projection to the SCN, which is bilaterally symmetrical.

Crx, a novel otx-like homeobox gene, shows photoreceptor-specific expression and regulates photoreceptor differentiation

We have isolated a novel otx-like homeobox gene, Crx, from the mouse retina. Crx expression is restricted to developing and mature photoreceptor cells. CRX bound and transactivated the sequence TAATCC/A, which is found upstream of several photoreceptor-specific genes, including the opsin genes from many species. Overexpression of Crx using a retroviral vector increased the frequency of clones containing exclusively rod photoreceptors and reduced the frequency of clones containing amacrine interneurons and Müller glial cells. In addition, presumptive photoreceptor cells expressing a dominant-negative form of CRX failed to form proper photoreceptor outer segments and terminals. Crx is a novel photoreceptor-specific transcription factor and plays a crucial role in the differentiation of photoreceptor cells.

The primary cilium as a complex signaling center

Respect for the primary cilium has undergone a remarkable renaissance over the past decade, and it is now thought to be an essential regulator of numerous signaling pathways. The primary cilium's functions range from the movement of cells and fluid, to sensory inputs involved with olfaction and photoreception. Disruption of cilia function is involved in multiple human syndromes collectively called 'ciliopathies'. The cilium's activities are mediated by targeting of receptors, channels, and their downstream effector proteins to the ciliary or basal body compartment. These combined properties of the cilium make it a critical organelle facilitating the interactions between the cell and its environment. Here, we review many of the recent advances contributing to the ascendancy of the primary cilium and how the extraordinary complexity of this organelle inevitably assures many more exciting future discoveries.

Structural basis for ON-and OFF-center responses in retinal ganglion cells

The inner plexiform layer of the mammalian retina has a bisublaminar organization determined by restricted branching of the terminals of cone bipolar cells and dendrites of class I (large) and class II (small) ganglion cells. Comparison of dendritic field diameters and receptive fiedl center sizes of large ganglion cells suggests that neural circuitry in sublamina a conveys "OFF"-center properties and connections in sublamina b "ON"-center properties to retinal ganglion cells.

Otx2 homeobox gene controls retinal photoreceptor cell fate and pineal gland development

Understanding the molecular mechanisms by which distinct cell fate is determined during organogenesis is a central issue in development and disease. Here, using conditional gene ablation in mice, we show that the transcription factor Otx2 is essential for retinal photoreceptor cell fate determination and development of the pineal gland. Otx2-deficiency converted differentiating photoreceptor cells to amacrine-like neurons and led to a total lack of pinealocytes in the pineal gland. We also found that Otx2 transactivates the cone-rod homeobox gene Crx, which is required for terminal differentiation and maintenance of photoreceptor cells. Furthermore, retroviral gene transfer of Otx2 steers retinal progenitor cells toward becoming photoreceptors. Thus, Otx2 is a key regulatory gene for the cell fate determination of retinal photoreceptor cells. Our results reveal the key molecular steps required for photoreceptor cell-fate determination and pinealocyte development.

A qualitative and quantitative analysis of the human fovea during development

The anatomical development of the human fovea has been sampled from 22 weeks gestation to adulthood, using both qualitative and quantitative methods. The foveal depression continues to deepen after birth until 15 months, due to the migration of the cells of the inner retina toward the periphery. Before birth the rod-free zone or foveola is over 1000 microns in diameter, but it becomes progressively narrower after birth because of a centralward migration of cones. It reaches the adult diameter of 650-700 microns by 45 months of age. Postnatally, foveolar cone development is characterized by maturation, elongation, and an increase in packing density. Foveolar cone diameter changes markedly after birth, going from 7.5 microns at 5 days postnatal to 2 microns by 45 months. During this time the foveolar cone develops both its outer segment and basal axon process (fiber of Henle). This combination of elongation and centralward migration results in an increase of foveolar cone density from 18 cones/100 microns at 1 week postnatal to 42 cones/100 microns in the adult. Measures of foveola width and cone diameter reach the adult stage of development at 45 months of age, but the two important visual factors of outer segment length and cone packing density still are only half the adult values at 45 months of age.

The role of the pigment epithelium in the etiology of inherited retinal dystrophy in the rat

Visual cell outer segment renewal was studied in eyes of mutant Royal College of Surgeons (RCS) and Sprague-Dawley (control) rats by a combination of microscopy and radioautography with the light and electron microscopes. RCS and control rats were injected with amino acids-(3)H at 11 days of age. Radioactive rod outer segment discs were assembled at the outer segment base from radioactive proteins synthesized in the rod inner segments. In controls, all radioactive discs assembled at 11 days of age were displaced the length of the outer segments, removed from outer segment tips, and phagocytized by the pigment epithelium by 8 days after injection. In the RCS rats, disc assembly and displacement resembled controls for the first 3 days after injection. However, as disc assembly continued for some time thereafter, a layer of labeled, disorganized, lamellar debris accumulated between the outer segment tips and the pigment epithelium. The buildup of debris was accompanied by visual cell death. At no time during the study was there evidence for phagocytic activity by the pigment epithelium. 61 days after injection, the layer of debris was the only heavily radioactive component in the retina. In the retina of RCS rats, the outer segment renewal mechanism malfunctions because the pigment epithelium does not fulfill its normal phagocytic role. The end result is visual cell death and blindness.

Stable and efficient gene transfer into the retina using an HIV-based lentiviral vector

The development of methods for efficient gene transfer to terminally differentiated retinal cells is important to study the function of the retina as well as for gene therapy of retinal diseases. We have developed a lentiviral vector system based on the HIV that can transduce terminally differentiated neurons of the brain in vivo. In this study, we have evaluated the ability of HIV vectors to transfer genes into retinal cells. An HIV vector containing a gene encoding the green fluorescent protein (GFP) was injected into the subretinal space of rat eyes. The GFP gene under the control of the cytomegalovirus promoter was efficiently expressed in both photoreceptor cells and retinal pigment epithelium. However, the use of the rhodopsin promoter resulted in expression predominantly in photoreceptor cells. Most successfully transduced eyes showed that photoreceptor cells in >80% of the area of whole retina expressed the GFP. The GFP expression persisted for at least 12 weeks with no apparent decrease. The efficient gene transfer into photoreceptor cells by HIV vectors will be useful for gene therapy of retinal diseases such as retinitis pigmentosa.

Notch is required for successive cell decisions in the developing Drosophila retina

Mutations in the Notch locus affect a variety of developmental decisions in Drosophila. In this paper, we examine the role of Notch in the developing retina. We reduced Notch activity at successive intervals during development of the retina, and then examined the effect on individual cells. When Notch activity was reduced, cells responded by selecting inappropriate developmental pathways. We found that all cell types appear to require Notch when establishing their fate. To examine further Notch's role in eye development, we examined two alleles of Notch--split and facet-glossy. split flies show defects in the initial clustering of photoreceptors, whereas the defects in facet-glossy flies are due to the misrouting of presumptive primary pigment cells into the secondary pigment cell pathway. Our results suggest that Notch plays a permissive role in the cell-cell interactions used to assemble the eye.

An SH3-SH2-SH3 protein is required for p21Ras1 activation and binds to sevenless and Sos proteins in vitro

Activation of the sevenless protein-tyrosine kinase is required for the proper specification of R7 photoreceptors in the Drosophila eye. The activation of a Ras protein, p21Ras1, is a crucial early event in the signaling pathway, and constitutive activation of p21Ras1 is sufficient to induce all of the effects of sevenless action. Here we report that another gene, E(sev)2B, required for proper signaling by sevenless encodes a protein of the structure SH3-SH2-SH3. We further provide evidence that the E(sev)2B protein is required for activation of p21Ras1 but not for any subsequent events, and that this protein can bind in vitro to sevenless and to Son of sevenless (Sos), a putative guanine nucleotide exchange factor for p21Ras1. These results suggest that the E(sev)2B protein may act to stimulate the ability of Sos to catalyze p21Ras1 activation by linking sevenless and Sos in a signaling complex. We have renamed the E(sev)2B locus downstream of receptor kinases (drk).

Rod and cone pathways in the inner plexiform layer of cat retina

In cat retina, rod bipolar terminials do not synapse on ganglion cells but on two types of amacrine cell (types I and II). Cone bipolars synapse directly on ganglion cells and on type I amacrines. The type II amacrine appears to play a special internuncial role between bipolars and ganglion cells in the rod system.

Rod bipolar cells in the mammalian retina show protein kinase C-like immunoreactivity

An antibody directed against protein kinase C (PKC) was applied to various mammalian retinae. In the cat, rat, rabbit, and macaque monkey we found PKC-like immunoreactivity in bipolar cells which had the morphology of rod bipolar cells; in the rat some amacrine cells were also immunoreactive. In the outer plexiform layer, labeled dendrites were always the central elements of the rod spherule invagination, and in the inner plexiform layer only rod bipolar axons and their axon terminals were immunoreactive. The antibody against PKC thus can be used to distinguish rod bipolar cells from cone bipolar cells. The antibody against PKC was used to determine the densities of rods and rod bipolar cells in the cat retina. In the central retina we found a rod to rod bipolar ratio of 16 to 1, in the periphery the ratio increases to 25 to 1. In freshly dissociated retina, cells with rod bipolar morphology could be identified; these cells were also labeled with the anti-PKC antibody. Hence, PKC-like immunoreactivity can be used to recognize rod bipolar cells in vitro.

The mosaic of nerve cells in the mammalian retina

Cells from three different layers of the retina were tested for regular horizontal arrangement or random distribution. Monkey cones, cat cones, cat A-type horizontal cells and cat alpha-ganglion cells are all arranged in a regular mosaic. This was assessed by measuring the distance to the nearest neighbour of each cell. The nearest neighbour distributions differed significantly from those of random dot patterns. The precision of the mosaic decreased in the order monkey cones, cat cones, cat A-type horizontal cells, cat alpha-ganglion cells. All four cell types were - independent of density - more regularly arranged in the centre of the retina than in the periphery. The advantages of a regular arrangement compared with a random distribution and mechanisms which could generate a regular mosaic are discussed.

The emergence of order in the Drosophila pupal retina

During pupation, long-range order is imposed on the autonomously developing ommatidia which compose the Drosophila eye. To accomplish this, eight additional cell types arise: the primary, secondary, and tertiary pigment cells, and the four cells that form the bristle. These cells form an interweaving lattice between ommatidia. The lattice is refined when excess cells are removed to bring neighboring ommatidia into register. Recent evidence suggests that in larval development, local contacts direct cell fate. The same appears to be true during pupal development: the contacts a cell makes predict the cell type it will become. Cells which contact the anterior or posterior cone cells in an ommatidium invariably become primary pigment cells. Cells which contact primary pigment cells from different ommatidia become secondary and tertiary pigment cells. Bristle development is in several ways distinct from ommatidial development. The four cells of each bristle group appear to be immediate descendents of a single founder cell. During their early differentiation, they do not make stereotyped contacts with surrounding ommatidial cells, but do make particular contacts within the bristle group. And unlike the surrounding ommatidia, differentiation of the bristles radiates from the center of the eye to the edges. As cells are removed during two stages of programmed cell death, the bristles are brought into their final position. When all cells in the lattice have achieved their final position, a second stage of retinal development begins as structures specific to each cell type are produced. This paper follows these various stages of pupal development, and suggests how local cell-cell contacts may produce the cells needed for a functional retina.

Rods and cones in the mouse retina. II. Autoradiographic analysis of cell generation using tritiated thymidine

The period of cell genesis of rod and cone photoreceptor cells has been determined in the retinas of C57BL/6J mice. Embryonic mice were exposed to a single dose of 3H-thymidine at embryonic day (E) 10--18 by injecting pregnant mice intraperitoneally. Animals at postnatal ages were injected subcutaneously once between postnatal day (P) 0--10. The eyes were removed at one to three months of age. After fixation, they were embedded in glycol methacrylate, sectioned at 1.5 micrometers and prepared for autoradiographic analysis. All of the cone cells are generated over a relatively short time interval during the fetal period. In the posterior retina, the peak of cone cell genesis occurs at E13-E14, and no cones are generated after E16. The rods, by contrast, are generated later and over a longer time period. They first begin to be generated in the posterior retina on E13, but the peak of cell genesis is not reached until the day of birth, and some rods are generated as late as P5. For both rods and cones the peaks of cell genesis in the peripheral retina occur two to three days later than in the posterior retina. The findings demonstrate that rods and cones are developmentally distinct cell types in the mouse retina.

Live astrocytes visualized by green fluorescent protein in transgenic mice

Green fluorescent protein (hGFP-S65T) was expressed in transgenic mice under the control of the astrocyte-specific glial fibrillary acidic protein (GFAP) promoter. Tissues from two independent transgenic lines were characterized by Northern blot analysis and by confocal microscopy. The expression pattern in these two lines was identical in all tissues examined, and similar to that found previously with a lacZ transgene driven by the same promoter. Bright fluorescence was observed in the cell bodies and processes of unfixed or fixed astrocytes, using both whole mount and brain slice preparations, from multiple areas of the central nervous system. However, in contrast to GFAP-lacZ transgenics, retinal Müller cells expressed the GFP transgene in response to degeneration of neighboring photoreceptors. These data indicate that the 2.2-kb hGFAP promoter contains sufficient regulatory elements to direct expression in Müller cells, and that GFP is a suitable reporter gene for use in living preparations of the mammalian nervous system. Such mice should prove useful for studies of dynamic changes in astrocyte morphology during development, and in response to physiological and pathological conditions.