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Inside IOVS 2001
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December 2001 |
Ocular Structural Biology: Macro and Micro
Proprioceptors in EOM
Proprioception from mammalian extraocular muscles (EOMs) is thought to be important for the development of a normal binocular vision. In the present study by Blumer et al. (p. 3085), the entire complement of encapsulated proprioceptors was investigated in pig EOMs. The precise locations of muscle spindles and Golgi tendon organs within the EOMs were determined. Structural particularities were observed in several Golgi tendon organs. The present investigation might contribute to solve the major enigma why mammals exhibit considerable species differences in their proprioceptive complement.
Keratocan and Cornea Plana
Keratocan is a member of a family of molecules that mediate the normal assembly of connective tissue. The motifs responsible for this activity are highly conserved, and Lehmann et al. (p. 3118) have identified a novel mutation in a similar region of Keratocan. The mutation is predicted to alter the regular arrangement of the leucine rich repeat motifs and results in a distinctive corneal phenotype, cornea plana. As cornea plana co-segregates with microphthalmia, it raises the intriguing possibility that Keratocan has a wider role in ocular development than one confined to the cornea.
Contact Lenses and Corneal Swelling
Contact lens-induced hypoxia is a factor in several types of ocular complications. As the corneal swelling response provides a direct indication of hypoxic exposure and altered corneal metabolism, it is assumed that contact lenses which induce comparatively low levels of swelling are safer, particularly for overnight wear. However, Graham et al. (p. 3150) show that a lower swelling response does not imply a reduced probability of ocular complications over 12 months of overnight RGP wear. These findings will contribute to understanding factors that put patients at risk for contact lens-associated keratopathy and to identifying measures of lens safety.
Scleral Thickness and IOP
Biomechanical modeling of the effect of IOP on the optic nerve head requires characterization of the geometry and physical properties of the involved tissues. As a first step toward this goal, Downs et al. (p. 3202) report measurement of posterior scleral shell thickness in normal and early glaucomatous perfusion-fixed monkey eyes at normal and elevated IOPs. The results show regional differences in scleral thickness in normal eyes and verify that increased pressure correlates with scleral thinning, and hence, increased stress, in some eyes. Eventually, these data, along with peripapillary scleral thickness and elastic and viscoelastic parameters, will provide the boundary conditions for finite element models of the normal and early glaucomatous monkey optic nerve head.
Neuron Shrinkage in Glaucoma
Glaucomatous damage extends beyond the eye to involve central visual pathways. The question of whether magno- and/or parvocellular pathways are selectively or diffusely affected has been of considerable interest. Yücel et al. (p. 3216) show that neurons in the lateral geniculate nucleus projecting to the visual cortex undergo significant shrinkage in experimental glaucoma. Furthermore, this damage involves both magno- and parvocellular layers. These insights on the diffuse nature of glaucomatous damage are relevant to understanding disease progression, visual dysfunction and its detection. In addition, shrinkage of neurons prior to cell death may be a window of opportunity for treatment strategies in glaucoma.
GPX-1 KOs and Lens Fibers
Oxidative damage from H2O2 or free radicals is a major factor in the etiology of age onset cataracts. The critical role of glutathione peroxidase-1 (GPX-1) in antioxidant defense of the lens was investigated in mice, which were null to this enzyme. Reddy et al. (p. 3247) report that the development of late onset cataracts in gene knockouts, compared to age-matched controls, occurs between 6-10 months with complete opacification in animals older than 15 months. This is the first phenotype noted in GPX-1 knockouts. The increased light scatter in these lenses was associated with damage to nuclear fiber membranes. The authors speculate that the membrane changes in the nuclear fibers of knockouts is due to lipid peroxide formation and that these animals may serve as an important model for age-related cataracts.
Retinal Precursor Cell Integration
Transplantation of neural precursor cells is a promising approach to treat a variety of neurodegenerative diseases. Pressmar et al. (p. 3311) studied the fate of heterotopically grafted neural precursors in the dystrophic retina of adult mice. Neural precursors were isolated from the embryonic mouse spinal cord, expanded in vitro and transplanted into the retina of mutant mice displaying apoptotic degeneration of photoreceptor cells. The authors observed wide-spread integration and long-term survival of grafted cells in the diseased adult retina. Donor cells differentiated into glia cells but not into nerve cells. A major challenge for future work is thus to achieve differentiation of grafted neural precursor cells into retina-specific cell types.
TTT and CNV
Transpupillary thermotherapy (TTT) has been used to treat occult subfoveal choroidal neovascular membranes (CNVM) in age-related macular degeneration (AMD), but its mechanism of action is currently unclear. Ciulla et al. (p. 3337) used color Doppler imaging to reveal transiently decreased volumetric blood flow in the retinal circulation at 24 hours post treatment. In the posterior ciliary arteries, which supply the choroid, there were no changes observed at 24 hours, but at one month, there was a decrease in the mean end diastolic velocity and increase in the resistive index in the nasal and temporal posterior ciliary arteries. TTT may lead to alterations in choroidal blood flow, consistent with complete or partial occlusion of occult subfoveal CNVM.
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