Proliferative vitreoretinopathy (PVR) is still a major cause of failure of retinal detachment surgery. Despite a dramatic increase in our pathobiologic knowledge of PVR during the last 10 years, little of this information has been used to modify the surgical management of the disease, and, thus, the anatomic and functional results are still unsatisfactory. Collaborative research involving clinicians and basic researchers must be encouraged. PVR must be considered a multifactorial disease caused by interaction of several cells and intra- and extraocular factors. Therefore, therapeutic options based on the inhibition of one factor or phenomenon may be regarded with scepticism. To prevent PVR, it is necessary to determine the factors involved in its development, and because of its relatively small prevalence, large, prospective, multicenter studies seem necessary. In addition, clinical research must not be underestimated. PVR affects both sides of the retina and the retina itself, a point to which little attention has been paid and that is critical for surgical results. Therefore, a new classification that provides information about clinical relevance, such as the evolutionary stages of the disease (biologic activity) and the degree of surgical difficulty (location of the fibrotic process), seems necessary.
Three different aspects of the morphological organisation of deep-sea fish retinae are reviewed: First, questions of general cell biological relevance are addressed with respect to the development and proliferation patterns of photoreceptors, and problems associated with the growth of multibank retinae, and with outer segment renewal are discussed in situations where there is no direct contact between the retinal pigment epithelium and the tips of rod outer segments. The second part deals with the neural portion of the deep-sea fish retina. Cell densities are greatly reduced, yet neurohistochemistry demonstrates that all major neurotransmitters and neuropeptides found in other vertebrate retinae are also present in deep-sea fish. Quantitatively, convergence rates in unspecialised parts of the retina are similar to those in nocturnal mammals. The differentiation of horizontal cells makes it unlikely that species with more than a single visual pigment are capable of colour vision. In the third part, the diversity of deep-sea fish retinae is highlighted. Based on the topography of ganglion cells, species are identified with areae or foveae located in various parts of the retina, giving them a greatly improved spatial resolving power in specific parts of their visual fields. The highest degree of specialisation is found in tubular eyes. This is demonstrated in a case study of the scopelarchid retina, where as many as seven regions with different degrees of differentiation can be distinguished, ranging from an area giganto cellularis, regions with grouped rods to retinal diverticulum.
The human visual system can discriminate increment and decrement light stimuli over a wide range of ambient illumination; from moonlight to bright sunlight. Several mechanisms contribute to this property but the major ones reside in the retina and more specifically within the photoreceptors themselves. Numerous studies in retinae from cold- and warm-blooded vertebrates have demonstrated the ability of the photoreceptors to respond in a graded manner to light increments and decrements even if these are applied during a background illumination that is expected to saturate the cells. In all photoreceptors regardless of type and species, three cellular mechanisms have been identified that contribute to background desensitization and light adaptation. These gain controlling mechanisms include; response-compression due to the non-linearity of the intensity-response function, biochemical modulation of the phototransduction process and pigment bleaching. The overall ability of a photoreceptor to adapt to background lights reflects the relative contribution of each of these mechanisms and the light intensity range over which they operate. In rods of most species, response-compression tends to dominate these mechanisms at light levels too weak to cause significant pigment bleaching and therefore, rods exhibit saturation. In contrast, cones are characterized by powerful background-induced modulation of the phototransduction process at moderate to bright background intensities where pigment bleaching becomes significant.Therefore, cones do not exhibit saturation even when the level of ambient illumination is raised by 6-7 log units.
The link between morphology and physiology for some of the cell types of the macaque monkey retina is reviewed with emphasis on understanding the neural mechanism for spectral opponency in the light response of ganglion cells. An in vitro preparation of the retina is used in which morphologically identified cell types are selectively targeted for intracellular recording and staining under microscopic control. The goal is to trace the physiological signals from the long (L), middle (M) and short-wavelength sensitive (S) cones to identified cell types that participate in opponent and non-opponent signal pathways. Heterochromatic modulation photometry and silent substitution are used to characterize L-, M- or S-cone inputs to the receptive fields of distinct horizontal cell, bipolar cell, ganglion cell and amacrine cell types. The majority of the retinal cell types await detailed analysis, and knowledge of the mechanisms of opponency remains incomplete. However results thus far have established: (1) Horizontal cell interneurons make preferential connections with the three cone types, but cannot provide a basis for spectral opponency in the circuitry of the outer retina. (2) A morphologically distinctive bistratified ganglion cell type transmits a blue-ON yellow-OFF spectral opponent signal to the parvocellular division of lateral geniculate nucleus. The morphology of this ganglion cell type suggests a simple synaptic mechanism for blue yellow opponency via converging input from an S-cone connecting ON-bipolar cell and an L - M cone connecting OFF-bipolar cell. (3) Midget ganglion cells, whose axons project to the parvocellular layers of the lateral geniculate nucleus and are assumed to be the origin of red/green opponent signals, show a non-opponent, achromatic physiology when recorded in the retinal periphery the underlying circuitry for red green opponency thus remains controversial, and (4) recent recordings from identified bipolar and amacrine cells in macaque suggest that a more complete accounting of opponent circuitry is a realistic goal.
Unlike in birds and cold-blooded vertebrates' retinas, the photoreceptors of mammalian retinas were long supposed to be morphologically uniform and difficult to distinguish into subtypes. A number of new techniques have now begun to overcome the previous limitations. A hitherto unexpected variability of spectral and morphological subtypes and topographic patterns of distribution in the various retinas are being revealed. We begin to understand the design of the photoreceptor mosaics, the constraints of evolutionary history and the ecological specialization of these mosaics in all the mammalian subgroups. The review discusses current cytological identification of mammalian photoreceptor types and speculates on the likely "bottleneck-scenario" for the origin of the basic design of the mammalian retina. It then provides a brief synopsis of current data on the photoreceptors in the various mammalian orders and derives some trends for phenomena such as rod/cone dualism, spectral range, preservation or loss of double cones and oil droplets, photopigment co-expression and mono- and tri-chromacy. Finally, we attempt to demonstrate that, building on the limits of an ancient rod dominant (probably dichromatic) model, mammalian retinas have developed considerable radiation. Comparing the nonprimate models with the intensively studied primate model should provide us with a deeper understanding of the basic design of the mammalian retina.
Successful restoration of vision in human patients with gene therapy affirmed its promise to cure ocular diseases and disorders. The efficacy of gene therapy is contingent upon vector and mode of therapeutic DNA introduction into targeted cells/tissues. The cornea is an ideal tissue for gene therapy due to its ease of access and relative immune-privilege. Considerable progress has been made in the field of corneal gene therapy in last 5 years. Several new gene transfer vectors, techniques and approaches have evolved. Although corneal gene therapy is still in its early stages of development, the potential of gene-based interventions to treat corneal abnormalities has begun to surface. Identification of next generation viral and nanoparticle vectors, characterization of delivered gene levels, localization, and duration in the cornea, and significant success in controlling corneal disorders, particularly fibrosis and angiogenesis, in experimental animal disease models, with no major side effects have propelled gene therapy a step closer toward establishing gene-based therapies for corneal blindness. Recently, researchers have assessed the delivery of therapeutic genes for corneal diseases and disorders due to trauma, infections, chemical, mechanical, and surgical injury, and/or abnormal wound healing. This review provides an update on the developments in gene therapy for corneal diseases and discusses the barriers that hinder its utilization for delivering genes in the cornea.
Mutations in the genes necessary for the metabolism of vitamin A (all-trans retinol) and cycling of retinoids between the photoreceptors and retinal pigment epithelium (RPE) (the visual cycle) have recently emerged as an important class of genetic defects responsible for retinal dystrophies and dysfunctions. Research into the causes and treatment of diseases resulting from defects in retinal vitamin A metabolism is currently the subject of intense interest, since disorders affecting the RPE are, in principle, more accessible to therapeutic intervention than those affecting the proteins of photoreceptor cells. This chapter presents an overview of the visual cycle, as well as the function of the RPE genes involved in the conversion of vitamin A to 11-cis retinal, the chromophore of the visual pigments. The identification of disease-causing mutations in this group of genes is described as well as the associated phenotypes that range from stationary night blindness to childhood-onset severe visual handicap. Consideration is also given to alternative genetic paradigms potentially relevant to defects in vitamin A metabolism, including a discussion of the relationship of this pathway to age-related macular degeneration, a non-Mendelian disease of late onset. Finally, progress and prospects for targeted therapeutic intervention in vitamin A metabolism are presented, including retinoid and gene replacement therapy. On the basis of early successes in animal models, and plans underway for Phase I/II clinical trials, it is hoped that the near future will bring effective therapies for many retinal dystrophy patients with defects in vitamin A metabolism.
Retinal angiogenesis and choroidal angiogenesis are major causes of vision loss, and the pathogenesis of this angiogenesis process is still uncertain. However, several key steps of the angiogenic cascade have been elucidated. In retinal angiogenesis, hypoxia is the initial stimulus that causes up regulation of growth factors, integrins and proteinases, which result in endothelial cell proliferation and migration that are critical steps in this process. Once the endothelial tube is formed from the existing blood vessels, maturation starts with recruitment of mural cell precursors and formation of the basement membrane. Normally, there is a tight balance between angiogenic factors and endogenous angiogenesis inhibitors that help to keep the angiogenic process under control. Although the steps of choroidal angiogenesis seem to be similar to those of retinal angiogenesis, there are some major differences between these two processes. Several anti-angiogenic approaches are being developed in animal models to prevent ocular angiogenesis by blocking the key steps of the angiogenic cascade. Based on these pre-clinical studies, several anti-angiogenic clinical trials are ongoing in patients with diabetic retinopathy and age-related macular degeneration. This review discusses the pathogenesis of retinal and choroidal angiogenesis, and alternative pharmacological approaches to inhibit angiogenesis in ocular diseases.
Human retinal dystrophies and degenerations and light-induced retinal degenerations in animal models are sharing an important feature: visual cell death by apoptosis. Studying apoptosis may thus provide an important handle to understand mechanisms of cell death and to develop potential rescue strategies for blinding retinal diseases. Apoptosis is the regulated elimination of individual cells and constitutes an almost universal principle in developmental histogenesis and organogenesis and in the maintenance of tissue homeostasis in mature organs.
Recent studies demonstrated a number of links between chromatin structure, gene expression, extracellular signaling and cellular differentiation during lens development. Lens progenitor cells originate from a pool of common progenitor cells, the pre-placodal region (PPR) which is formed from a combination of extracellular signaling between the neural plate, naïve ectoderm and mesendoderm. A specific commitment to the lens program over alternate choices such as the formation of olfactory epithelium or the anterior pituitary is manifested by the formation of a thickened surface ectoderm, the lens placode. Mouse lens progenitor cells are characterized by the expression of a complement of lens lineage-specific transcription factors including Pax6, Six3 and Sox2, controlled by FGF and BMP signaling, followed later by c-Maf, Mab21like1, Prox1 and FoxE3. Proliferation of lens progenitors together with their morphogenetic movements results in the formation of the lens vesicle. This transient structure, comprised of lens precursor cells, is polarized with its anterior cells retaining their epithelial morphology and proliferative capacity, whereas the posterior lens precursor cells initiate terminal differentiation forming the primary lens fibers. Lens differentiation is marked by expression and accumulation of crystallins and other structural proteins. The transcriptional control of crystallin genes is characterized by the reiterative use of transcription factors required for the establishment of lens precursors in combination with more ubiquitously expressed factors (e.g. AP-1, AP-2alpha, CREB and USF) and recruitment of histone acetyltransferases (HATs) CBP and p300, and chromatin remodeling complexes SWI/SNF and ISWI. These studies have poised the study of lens development at the forefront of efforts to understand the connections between development, cell signaling, gene transcription and chromatin remodeling.
Vision scientists long ago pointed to black and white as separate sensations and saw confirmation in the fact that in the absence of light, one perceives the visual field as gray against which the negative after-image of a bright light appeared blacker. The first recordings from optic nerve fibers in vertebrates revealed ON and OFF signals, later associated with separate streams, arising already at the synapse between receptors and bipolar cells. These can be identified anatomically and physiologically and remain distinct all the way to the lateral geniculate nucleus, whose fibers form the input to the primary visual cortex. The dichotomy has been probed by electroretinography and analyzed by means of pharmacological agents and dysfunction due to genetic causes. The bi- rather than a unidirectional nature of the retinal output has advantages in allowing small signals to remain prominent over a greater dynamic range. The two streams innervate cortical neurons in a push-pull manner, generating receptive fields with spatial sensitivity profiles featuring ON and OFF subregions. Manifestations of the dichotomy appear in a variety of simple visual discriminations where there are often profound threshold differences in patterns with same polarity as compared with mixed contrast-polarity components. But even at levels in which the spatial, contrast and color attributes have already been securely established and black and white elements participate equally, a categorical difference between blackness and whiteness of a percept persists. It is an opponency, akin to the ones in the color domain, derived from the original ON and OFF signals and subsequently bound with the other attributes to yield a feature's unitary percept.
The appropriate guidance and patterning of vessels during vascular development is critical for proper tissue function. The loss of these guidance mechanisms can lead to abnormal vascularization and a number of pathological conditions. The molecular basis of endothelial cell guidance and subsequent tissue specific vascular patterning remains largely unknown in spite of its clinical relevance and biological importance. In this regard, retinal vascular development offers many advantages for studying endothelial cell guidance and the mechanisms by which characteristic vascular patterns are formed. In this review, we will provide an overview of the known mechanisms that mediate vascular patterning during mouse retinal development, synthesizing these data to formulate a model of how growth factors, cellular adhesion molecules, and vascular-associated cells mediate directed endothelial cell migration and appropriate vascular remodeling. Finally, we will discuss the many aspects of retinal vascular development that remain unknown and cite evidence that many of these gaps may be addressed by further studying the guidance cues shared by vascular and neuronal elements in the retina and other parts of the central nervous system.
Retinal diseases such as age-related macular degeneration (ARMD) and retinitis pigmentosa (RP) affect millions of people. Replacing lost cells with new cells that connect with the still functional part of the host retina might repair a degenerating retina and restore eyesight to an unknown extent. A unique model, subretinal transplantation of freshly dissected sheets of fetal-derived retinal progenitor cells, combined with its retinal pigment epithelium (RPE), has demonstrated successful results in both animals and humans. Most other approaches are restricted to rescue endogenous retinal cells of the recipient in earlier disease stages by a 'nursing' role of the implanted cells and are not aimed at neural retinal cell replacement. Sheet transplants restore lost visual responses in several retinal degeneration models in the superior colliculus (SC) corresponding to the location of the transplant in the retina. They do not simply preserve visual performance - they increase visual responsiveness to light. Restoration of visual responses in the SC can be directly traced to neural cells in the transplant, demonstrating that synaptic connections between transplant and host contribute to the visual improvement. Transplant processes invade the inner plexiform layer of the host retina and form synapses with presumable host cells. In a Phase II trial of RP and ARMD patients, transplants of retina together with its RPE improved visual acuity. In summary, retinal progenitor sheet transplantation provides an excellent model to answer questions about how to repair and restore function of a degenerating retina. Supply of fetal donor tissue will always be limited but the model can set a standard and provide an informative base for optimal cell replacement therapies such as embryonic stem cell (ESC)-derived therapy.
Glaucomatous optic neuropathy is classified by morphologic changes in the intrapapillary and parapapillary region of the optic nerve head and the retinal nerve fibre layer. These changes can be evaluated using descriptive optic nerve head variables which are the size and shape of the optic disc; size, shape and pallor of the neuroretinal rim; size of the optic cup in relation to the area of the disc; configuration and depth of the optic cup; cup-to-disc diameter ratio and cup-to-disc area ratio; position of the exit of the central retinal vessel trunk on the lamina cribrosa surface; presence and location of splinter-shaped haemorrhages; occurrence, size, configuration and location of parapapillary chorioretinal atrophy; diffuse and/or focal decrease of the diameter of the retinal arterioles; and visibility of the retinal nerve fibre layer. Assessment of these variables is useful for the early detection of glaucomatous optic nerve damage, to follow-up patients with glaucoma, to differentiate various types of the chronic open-angle glaucomas, and to get hints for the pathogenesis of glaucomatous optic nerve fibre loss.
Electroretinography (ERG) is a non-invasive method that can contribute to a description of the functional organization of the human retina under normal and pathological circumstances. The physiological and pathophysiological processes leading to an ERG signal can be better understood when the cellular origins of the ERG are identified. The ERG signal recorded at the cornea is initiated by light absorption in the photoreceptors which leads to activity in the photoreceptors and in their post-receptoral pathways. Light absorption in distinct photoreceptor types may lead to different ERG responses caused either by differences between the photoreceptors or between their post-receptoral pathways. The description of contributions of the different photoreceptor types to the ERG may therefore give more detailed insight in the origins of the ERG. Such a description can be obtained by isolating the responses of a single photoreceptor type. Nowadays, careful control of differently colored light sources together with the relatively well-known cone and rod fundamentals enables a precise description and control of photoreceptor excitation. Theoretically, any desired combination of photoreceptor excitation modulation can be achieved, including conditions in which the activity in only one photoreceptor type is modulated (silent substitution). In this manner the response of one photoreceptor type is isolated without changing the state of adaptation. This stimulus technique has been used to study the contribution of signals originating in the different photoreceptor types to the human ERG. Furthermore, by stimulating two or more photoreceptor types simultaneously, the interaction between the different signals can be studied.
Glaucoma can be defined as an optic nerve disease with typical morphological and functional changes. There are many risk factors associated with this neuropathy. The best known factor is an increased intraocular pressure. There are, however, many other risk factors. Among them, vascular factors play a major role. Although such vascular factors have been postulated more than hundred years ago, it is only recently that the physiology and pathophysiology of the optic nerve head circulation is, to some extent, understood. New instruments have been developed to measure ocular blood flow including blood flow in the optic nerve head. Although most of the studies indicate that circulation is changed in glaucoma patients, there is little association between glaucoma and arteriosclerosis. The main cause for the circulation disturbance in glaucoma seems rather to be a vascular dysregulation leading to local vasospasm and to systemic hypotension.
Connectomics is a strategy for mapping complex neural networks based on high-speed automated electron optical imaging, computational assembly of neural data volumes, web-based navigational tools to explore 10(12)-10(15) byte (terabyte to petabyte) image volumes, and annotation and markup tools to convert images into rich networks with cellular metadata. These collections of network data and associated metadata, analyzed using tools from graph theory and classification theory, can be merged with classical systems theory, giving a more completely parameterized view of how biologic information processing systems are implemented in retina and brain. Networks have two separable features: topology and connection attributes. The first findings from connectomics strongly validate the idea that the topologies complete retinal networks are far more complex than the simple schematics that emerged from classical anatomy. In particular, connectomics has permitted an aggressive refactoring of the retinal inner plexiform layer, demonstrating that network function cannot be simply inferred from stratification; exposing the complex geometric rules for inserting different cells into a shared network; revealing unexpected bidirectional signaling pathways between mammalian rod and cone systems; documenting selective feedforward systems, novel candidate signaling architectures, new coupling motifs, and the highly complex architecture of the mammalian AII amacrine cell. This is but the beginning, as the underlying principles of connectomics are readily transferrable to non-neural cell complexes and provide new contexts for assessing intercellular communication.
Within the last three years, triamcinolone acetonide has increasingly been applied intravitreally as treatment option for various intraocular neovascular edematous and proliferative disorders. The best response in terms of gain in visual acuity after the intravitreal injection of triamcinolone acetonide was found in eyes with intraretinal edematous diseases such as diffuse diabetic macular edema, branch retinal vein occlusion, central retinal vein occlusion, and pseudophakic cystoid macular edema. Visual acuity increased and degree of intraocular inflammation decreased in eyes with various types of non-infectious uveitis including acute or chronic sympathetic ophthalmia and Adamantiadis-Behcet's disease. Intravitreal triamcinolone may be useful as angiostatic therapy in eyes with iris neovascularization and proliferative ischemic retinopathies. Possibly, intravitreal triamcinolone may be helpful as adjunct therapy for exudative age-related macular degeneration, possibly in combination with photodynamic therapy. In eyes with chronic, therapy resistant, ocular hypotony, intravitreal triamcinolone can induce an increase in intraocular pressure and may stabilize the eye. The complications of intravitreal triamcinolone therapy include secondary ocular hypertension in about 40% of the eyes injected, cataractogenesis, postoperative infectious and non-infectious endophthalmitis, and pseudo-endophthalmitis. Intravitreal triamcinolone injection can be combined with other intraocular surgeries including cataract surgery. Cataract surgery performed some months after the injection does not show a markedly elevated rate of complications. If vision increases and eventually decreases again after an intravitreal triamcinolone acetonide injection, the injection can be repeated. The duration of the effect of a single intravitreal injection of triamcinolone depended on the dosage given. Given in a dosage of about 20mg to non-vitrectomized eyes, the duration of the effect and of the side-effects was 6-9 months. Intravitreal triamcinolone acetonide may offer a possibility for adjunctive treatment of intraocular edematous and neovascular disorders. One has to take into account the side-effects and the lack of long-term follow-up observations.
Circadian clocks are self-sustaining genetically based molecular machines that impose approximately 24h rhythmicity on physiology and behavior that synchronize these functions with the solar day-night cycle. Circadian clocks in the vertebrate retina optimize retinal function by driving rhythms in gene expression, photoreceptor outer segment membrane turnover, and visual sensitivity. This review focuses on recent progress in understanding how clocks and light control arylalkylamine N-acetyltransferase (AANAT), which is thought to drive the daily rhythm in melatonin production in those retinas that synthesize the neurohormone; AANAT is also thought to detoxify arylalkylamines through N-acetylation. The review will cover evidence that cAMP is a major output of the circadian clock in photoreceptor cells; and recent advances indicating that clocks and clock networks occur in multiple cell types of the retina.
Glaucoma, the most common optic neuropathy (GON) is characterised by the loss of retinal ganglion cells and their axons, as well as tissue remodelling of both the retina and the optic nerve head with corresponding visual field defects. Elevated intraocular pressure (IOP) is generally regarded as the major risk factor for glaucoma and its reduction is the most common target for therapy of GON. There are indications that the greater the IOP reduction, the better is the visual field prognosis. This article investigates, on the basis of two beta-blockers, betaxolol and timolol, whether the amount of IOP reduction is truly a good surrogate for successful glaucoma therapy with respect to visual field outcome. Contrary to what is generally expected, our analysis of the literature exemplifies that despite a smaller IOP reduction, patients treated with betaxolol had a smaller rate of visual field deterioration than patients treated with timolol. Based on the dissociation of IOP reduction and visual field prognosis, we postulate that for successful treatment in glaucoma not only the amount of IOP reduction is relevant but also the drug by which the reduction is achieved. This seeming paradox phenomenon highlights that ocular hypotensive drugs have relevant effects on GON other than IOP-related. Some of these effects on retinal ganglion cells (neuroprotection) or on ocular blood flow are mediated by calcium- and sodium channels. Future studies on glaucoma treatment should focus on their effect on visual field function, and not just on IOP. This should particularly be considered when comparing drugs from different classes.
In this work we advance the hypothesis that omega-3 (ω-3) long-chain polyunsaturated fatty acids (LCPUFAs) exhibit cytoprotective and cytotherapeutic actions contributing to a number of anti-angiogenic and neuroprotective mechanisms within the retina. ω-3 LCPUFAs may modulate metabolic processes and attenuate effects of environmental exposures that activate molecules implicated in pathogenesis of vasoproliferative and neurodegenerative retinal diseases. These processes and exposures include ischemia, chronic light exposure, oxidative stress, inflammation, cellular signaling mechanisms, and aging. A number of bioactive molecules within the retina affect, and are effected by such conditions. These molecules operate within complex systems and include compounds classified as eicosanoids, angiogenic factors, matrix metalloproteinases, reactive oxygen species, cyclic nucleotides, neurotransmitters and neuromodulators, pro-inflammatory and immunoregulatory cytokines, and inflammatory phospholipids. We discuss the relationship of LCPUFAs with these bioactivators and bioactive compounds in the context of three blinding retinal diseases of public health significance that exhibit both vascular and neural pathology.
The patterns of eye movement that accompany static activities such as reading have been studied since the early 1900s, but it is only since head-mounted eye trackers became available in the 1980s that it has been possible to study active tasks such as walking, driving, playing ball games and ordinary everyday activities like food preparation. This review examines the ways that vision contributes to the organization of such activities, and in particular how eye movements are used to locate the information needed by the motor system in the execution of each act. Major conclusions are that the eyes are proactive, typically seeking out the information required in the second before each act commences, although occasional 'look ahead' fixations are made to establish the locations of objects for use further into the future. Gaze often moves on before the last act is complete, indicating the presence of an information buffer. Each task has a characteristic but flexible pattern of eye movements that accompanies it, and this pattern is similar between individuals. The eyes rarely visit objects that are irrelevant to the action, and the conspicuity of objects (in terms of low-level image statistics) is much less important than their role in the task. Gaze control may involve movements of eyes, head and trunk, and these are coordinated in a way that allows for both flexibility of movement and stability of gaze. During the learning of a new activity, the eyes first provide feedback on the motor performance, but as this is perfected they provide feed-forward direction, seeking out the next object to be acted upon.
Pigment epithelium-derived factor (PEDF) is a 50-kDa protein encoded by a single gene that shows strong conservation across phyla from fish to mammals. It is secreted by the retinal pigment epithelium (RPE) and a select number of other cell types in the eye, as well as by other tissues in the body. PEDF was originally defined by its ability to induce differentiation in retinoblastoma cells. It also promotes a non-proliferative, differentiated state in a number of other cell types. PEDF protects retinal neurons from light damage, oxidative stress and glutamate excitotoxicity. PEDF is also antiangiogenic and can inhibit the growth of blood vessels in the eye induced in a variety of ways. A balance in the levels of PEDF and the proangiogenic factor vascular endothelial growth factor-A is perturbed in a range of retinal neovascular diseases. Some of the pathways by which PEDF exerts its actions on cells have now been defined. Peptide fragments of PEDF carry biological activity and may be valuable therapeutic agents that readily penetrate the eye.