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| September 2007 | Inside IOVS | Volume 48/9 |
Peripheral Vision and Refractive Development
Ocular growth and refractive development are regulated by visual feedback associated with the eye’s refractive status. Although it has generally been assumed that visual signals from the fovea dominate refractive development in primates, Smith et al. (p. 3914) showed that photoablation of the fovea in infant monkeys does not alter the normal course of emmetropization or prevent the axial myopia produced by form deprivation. These results suggest that peripheral vision can, in the absence of foveal signals, dominate central refractive development and that peripheral vision should be considered when assessing the effects of visual experience on refractive development.
Progression of Macular Edema: Blood-Retinal Barrier Permeability vs. Retinal Thickening
VEGF inhibition is now investigated in many studies of diabetic macular edema. However, the relation between increase in permeability and retinal thickening is not straightforward. As shown by Sander et al. (p. 3983), permeability increased at least 18 months before the authors observed a significant increase in retinal thickness. Thus, optical coherence tomography (OCT) is not sensitive for the early stages of progression. The time lag of at least 18 months also indicates that an increase in permeability is not the only factor responsible for edema formation and that a treatment based on VEGF alone may be insufficient in some stages of edema.
Chlamydia pneumoniae Infection and Early Age-Related Macular Degeneration
In the Australian Blue Mountains Eye Study, the development of early age-related macular degeneration (AMD) was not associated with exposure to Chlamydia pneumoniae infection. This contributes to the body of evidence that this infection does not play a causal role for early AMD. The study by Robman et al. (p. 4007) had insufficient power to test the hypothesis that Chlamydia pneumoniae infection can provoke and accelerate progression of AMD to advanced stages. Although it is possible that C. pneumoniae can merely colonize the already damaged macular tissues or be a chance finding, the previous positive findings on advanced AMD and biological plausibility of chronic infection to be involved in the inflammatory component of AMD warrant further research.
Allergic Conjunctivitis and Corneal Graft Rejection
Ocular surface inflammation increases the risk of immune-mediated rejection of corneal transplants. Many patients undergoing penetrating keratoplasty for keratoconus also suffer from allergic eye disease. Flynn et al. (p. 4044) report that allograft rejection is accelerated in the presence of perioperative allergic conjunctivitis. Graft-infiltrating eosinophils were an additional feature of rejection in the setting of allergic conjunctivitis suggesting a possible role for these cells as effectors of rejection.
Postnatal Development of Corneal Transparency
Mammalian corneas are translucent at birth and develop transparency during early postnatal life through unknown mechanisms. Jester et al. (p. 4061) quantitatively measured the postnatal development of corneal transparency using in vivo confocal microscopy and correlated corneal light scattering to expression of corneal crystallin proteins, stromal cell density, cell cycle entry, and stromal thickness. This paper provides evidence that postnatal development of transparency is significantly correlated with increased expression of corneal crystallins, decreased keratocyte density and keratocyte cell cycle exit or quiescence.
Infantile Nystagmus and Visual Acuity
Infantile nystagmus (IN) is the onset of involuntary ocular oscillations before 6 months of age. Weiss and Kelly (p. 4093) found that the rate of acuity development in children with IN, with or without associated visual-sensory disorders, parallels normal acuity development. Acuity reductions in isolated IN are less than one octave across ages. Therefore, acuity reductions in albinism, aniridia, and optic nerve hypoplasia are likely due to the underlying visual-sensory defect. These findings indicate that acuity development in IN is normal when adjusted for acuity differences due to associated visual sensory defects and that postnatal acuity development is largely independent of the gaze-holding instability.
Genome-Wide Scan of Exfoliation Syndrome
Exfoliation syndrome (XFS) is an age-related ocular syndrome characterized by the accumulation of extracellular material in ocular tissues and associated with increased risk for glaucoma. The purpose of the current study by Lemmelä et al. (p. 4136) was to identify the genetic basis of XFS, performing a genome-wide scan with 1000 microsatellite markers for a large Finnish family where XFS demonstrated autosomal dominant mode of inheritance. The most promising locus was assigned to 18q12.1-21.33. The study presented here is the first genome-wide scan in XFS and offers a starting point to unravel the molecular background of this syndrome.
VEGF and Basement Membrane Thickening
Basement membrane (BM) thickening is an early hallmark of preclinical diabetic retinopathy (PCDR). Kuiper et al. (p. 4267) show that vascular endothelial growth factor (VEGF) can induce BM thickening by induction of genes related with the extracellular matrix. The potential importance of this finding is that VEGF is also considered to cause increased vascular permeability and leukocyte adhesion in PCDR. These findings suggest that BM thickening is not just an epiphenomenon of the diabetic state, but may be instrumental in the progression to diabetic retinopathy. Modulation of BM thickening may therefore have a preventive effect on the development of diabetic retinopathy.
VEGF and Its Role in Retinal Regeneration
Nishiguchi et al. (p. 4315) show that vascular endothelial growth factor (VEGF), a known potent angiogenic factor, has the potential to enhance the proliferation of retinal progenitors at the peripheral retina in mice with retinal degeneration. This was mediated through one of its receptors, VEGFR2/Flk1. As expression of VEGF is altered in many retinal diseases, these findings point out that there may be under-recognized roles of VEGF on retinal progenitors or retinal regeneration by these cells other than those on vessels influencing their pathology. Caution should be taken with the possible unexpected influence of the widely explored anti-VEGF agents, particularly in young individuals who may still have retinal progenitors in their eyes.
Looking for JAMs in Cell Junctions of the Human Eye
Junctional adhesion molecules (JAMs) are a family of adhesion proteins expressed in epithelial and endothelial cell junctions and on the surface of leukocytes. Evidence suggests that JAMs are important for tight junction assembly, leukocyte transmigration, platelet activation, angiogenesis, and viral binding to mucosal tissues. Mandell et al. (p. 3928) demonstrate expression of a JAM protein in the corneal endothelium and retinal pigment epithelium. Their findings provide a basis for further study of the function of JAMs in the human eye and their potential role in eye disease.
Abnormal Nerve Activity in PRK Operated Corneas
Pain and discomfort sensations are frequent companions of photorefractive surgery. These sensations have been attributed to augmented activity in sensory nerves innervating the injured tissue. Gallar et al. (p. 4033) recorded the impulse activity in sensory fibers innervating the cornea 12-48 hours following photorefractive keratectomy (PRK) and show that nerve endings in the surroundings of the damaged area exhibit an enhanced spontaneous activity, whereas responsiveness of mechano- and polymodal nociceptors to noxious stimuli is absent or reduced. These disturbances in nerve responsiveness explain the presence of spontaneous pain combined with a low sensitivity to corneal stimulation observed in patients subjected to photorefractive surgical procedures.
A Model System for Corneal Fibril Orientation
In spite of the fact that the function of numerous connective tissues (including the cornea) depends on the highly-anisotropic organization of collagen fibrils, little is known about the mechanisms that control collagen synthesis and orientation. Guo et al. (p. 4050) developed a model in which primary human corneal fibroblasts were induced to generate organized arrays of collagen fibrils. The model should provide fertile ground for investigations of fibroblast control of matrix organization. Methods derived from this model have the potential to provide tissue-engineering scaffoldings suitable for the transport of cellular grafts to the cornea or for use as a stromal replacement.
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