Structure and composition of the rodent lamina cribrosa

Factors contributing to neuronal degeneration in retinas of experimental glaucomatous rats

After our studies on ganglion cell degeneration in the glaucomatous retina, the current work further confirmed the reduction of amacrine cells in the retina after the onset of glaucoma. Present study also tried to understand the possible mechanisms underlying neuronal degeneration in the glaucomatous retina. Changes of expressions in immediate early genes (IEGs), glutamate receptors (GluRs), calcium-binding proteins (CaBPs), 8-hydroxy-deoxyguanosine (8-OH-dG) and nitric oxide synthase (NOS), as well as apoptotic-related factors including caspase 3, bax, and bcl-2 were examined. IEGs such as c-fos and c-jun were induced in the retina of the glaucomatous rat as early as 2 hr after the onset of glaucoma and lasted up to 2 weeks. Expressions of GluRs and CaBPs (i.e., parvalbumin and calbindin D-28k) were observed to be increased in the retinal ganglion cell layer (GCL) and inner nuclear layer (INL) at 3 days and 1 week after the onset of glaucoma. The increase occurred well before and during the phase where significant neuronal death was observed in the GCL and INL of the glaucomatous retinae. Induction of 8-OH-dG was present in both the GCL and INL of the glaucomatous retina at 3 days after the onset of glaucoma before significant neuronal death was observed. Furthermore, confocal microscopy study showed the complete colocalization of immunohistochemical expression of caspase 3 with glial fibrillary acidic protein (GFAP), but not with neuronal nuclei (NeuN). It indicates that astrocytes and Müller cells are involved in the pathological processes of neuronal death. The relationship between the linked factors and neuronal degeneration is also discussed.

Elevated intraocular pressure and optic nerve injury models in the rat

Models of experimentally elevated intraocular pressure in rats provide valuable opportunities to discover and study mechanisms of pressure-induced optic nerve damage. The structure and vasculature of the rat optic nerve head have several anatomic similarities and differences from the primate that allow useful comparisons and insights into human glaucoma. Specifically, the ultrastructural relationship between astrocytes, retinal ganglion cell axons and the connective tissues of the optic nerve head appear quite similar to the primate, and have a high potential for revealing cellular mechanisms of axonal injury. Three widely used models of creating elevated IOP in rats exist. However, they are not all equivalent and appear to differ in the relationship they exhibit between the level of pressure and extent of optic nerve damage. This indicates that these models may differ in the mechanisms by which they produce elevated eye pressure. All of these models are amenable to a variety of methods for evaluating damage. These include objective and subjective histologic assessment of the optic nerve, counting cells in the retinal ganglion cell layer of the retina and the use of retinal whole mounts to count retinal ganglion cells that have been back-labeled with dye applied to the superior colliculus. In the decade since their introduction, these versatile models have provided important insights into mechanisms of pressure-induced optic nerve damage using sensitive molecular biology techniques. They have also allowed the evaluation of several potential strategies for neuroprotection in glaucoma, ranging from currently available drugs to gene transfer studies.

Understanding mechanisms of pressure-induced optic nerve damage

Patients with glaucoma can suffer progressive vision loss, even in the face of what appears to be excellent intraocular pressure (IOP) control. Some of this may be secondary to non-pressure-related (pressure-independent) factors. However, it is likely that chronically elevated IOP produces progressive changes in the optic nerve head, the retina, or both that alter susceptibility of remaining optic nerve fibers to IOP. In order to understand the nature of these progressive changes, relevant, cost-effective animal models are necessary. Several rat models are now used to produce chronic, elevated IOP, and methods exist for measuring the resulting IOP and determining the extent of the damage this causes to the retina and optic nerve. A comparison of damage, pressure and duration shows that these models are not necessarily equivalent. These tools are beginning to uncover clear evidence that elevated IOP produces progressive changes in the optic nerve head and retina. In the optic nerve head, these include axonal and non-axonal effects, the latter pointing to involvement of extracellular matrix and astrocyte responses. In the retina, retinal ganglion cells appear to undergo changes in neurotrophin response as well as morphologic changes prior to actual cell death. These, and other, as yet uncovered, abnormalities in the optic nerve head and retina may influence relative susceptibility to IOP and explain progressive optic nerve damage and visual field loss, in spite of apparent, clinically adequate IOP control. Ultimately, this knowledge may lead to the development of new treatments designed to preserve vision in these difficult patients.

Developmental changes of aggrecan, versican and neurocan in the retina and optic nerve

We have used a monoclonal antibody to neurocan and specific polyclonal antibodies to the non-homologous glycosaminoglycan attachment regions of aggrecan and mRNA splice variants of versican to compare the localization and developmental changes of these structurally related hyaluronan-binding chondroitin sulfate proteoglycans in the rat retina and optic nerve. Staining for aggrecan and versican was first seen at embryonic day 16 in the optic nerve and retina, whereas neurocan was not detected in the embryonic eye. At postnatal day 0 (P0), beta-versican staining is largely confined to the inner plexiform layer whereas alpha-versican is also apparent in the neuroblastic layer. Both aggrecan and, much more weakly, neurocan immunoreactivity is present throughout the neonatal retina. At P9, aggrecan and versican immunoreactivity is most intense in the inner and outer plexiform and ganglion cell layers, accompanied by diffuse staining in the inner and outer nuclear layers. Aggrecan and alpha-versican are also present throughout the optic nerve and disk, whereas beta-versican and neurocan are confined to the laminar beams of the optic nerve. Between P0 and P9 there is a marked increase in beta-versican expression in the inner and outer nuclear layers and in the outer plexiform layer, whereas there is only weak staining of neurocan in the inner plexiform and ganglion cell layers of P9 retina. By 1 month postnatal the staining pattern of the fully differentiated retinal layers is essentially identical to that seen in the adult, where there is strong aggrecan and alpha-versican immunoreactivity in the retina and optic nerve, whereas beta-versican has essentially disappeared from the adult retina and, similarly to neurocan, is present only in the laminar beams of the optic nerve. The marked decrease of beta-versican in the retina is consistent with >90% decrease in its concentration in brain during postnatal development, suggesting that the developmental time-course for these proteoglycans in retina parallels that seen in other areas of the central nervous system.

Correlation between retinal ganglion cell death and chronically developing inherited glaucoma in a new rat mutant

Glaucoma is a progressive optic neuropathy with characteristic optic disc changes, retinal ganglion cell loss and progressive visual field defects. Elevated intraocular pressure is considered to be a major risk factor in glaucomatous neuropathy. This study aimed to characterize and document a new chronic glaucoma model in the rat with respect to the effect of elevated intraocular pressure on overall retinal dysfunction and retinal ganglion cell loss, and to elucidate the possible mechanisms underlying this cell loss. Intraocular pressure (IOP) was measured in rats using a Tonopen. RGCs were retrogradely labeled with the fluorescent dye, 4-[didecylaminostyryl]-N-methyl-pyridinium-iodide (4-Di-10 ASP) and quantified on retinal flat mounts using fluorescence microscopy. The optic nerve head was examined fundoscopically. Changes in the histological appearance of the whole eyes was studied in paraffin sections, and immunohistochemistry was carried out on cryostat sections. The levels of mRNA for several genes were compared between control and glaucomatous retinae using semi-quantitative RT-PCR. Mutant animals are affected with either a unilateral or bilateral enlargement of the globes having an IOP that ranged from 25 to 45 mmHg, as compared to control values of 12-16 mmHg. The IOP of glaucomatous eyes increased significantly with age to attain a value of 35+/-7.3 at 1.5 years. Concomitant with the rise in IOP, the number of labeled RGCs continued to decrease in number with age. A total of 1887+/-117RGC mm(-2) could be labeled in wild-type control and juvenile mutant pre-glaucomatous retinas, whereas this number dropped to 92+/-26RGC mm(-2) at 1.5 years. Ophthalmoscopy revealed atrophied optic nerve heads in the affected eyes. The pars plicata and the pars plana of the ciliary body of glaucomatous eyes were hypertrophied and elongated, respectively. The anterior chamber was narrow and the irido-corneal angle open in glaucoma eyes. The mRNA of glial-fibrillary-acidic protein, endothelin-1, STAT-3 and STAT-6 increased in the retinas correlating with the severity and duration of the disease. Changes in the expression of GFAP and endothelin-1 could be confirmed using immunohistochemistry. This model may help to address several fundamental issues in the pathogenesis of glaucoma and aid in the development of neuroprotective strategies.

Comparative study of the lamina cribrosa and the pial septa in the vertebrate optic nerve and their relationship to the myelinated axons

The optic nerve contains the connective tissues, i.e. the lamina cribrosa and pial septa. This report presents a histological comparison of the lamina cribrosa and pial septa in the five classes (mammals, birds, reptiles, amphibians and teleosts) of vertebrates. Furthermore, the distribution of myelinated fibers was observed from the optic nerve through the retina in the same animals. The lamina cribrosa is found in mammals except for mice, and in birds. Structural complexity of the lamina was different in animals but generally dependent of the optic nerve thickness. The pial septa were present in the optic nerve proper of the mammals except for the mice, in birds and in a part of teleosts. Fasciculation of the optic nerve by the pial septa tended to be more prominent as the optic nerve become thicker. The optic nerve consisted of largely myelinated fibers in vertebrates. The retina contained some myelinated fibers in submammals but was thoroughly devoid of myelinated fibers in mammals. The borderline between myelinated and unmyelinated portions in the optic nerve of different species did not related to the lamina cribrosa. Amphibians had exceptionally only a few myelinated fibers in the optic nerve and no myelinated fibers in the retina.

Evidence that a locus for familial high myopia maps to chromosome 18p

Myopia, or nearsightedness, is the most common human eye disorder. A genomewide screen was conducted to map the gene(s) associated with high, early-onset, autosomal dominant myopia. Eight families that each included two or more individuals with >=-6.00 diopters (D) myopia, in two or more successive generations, were identified. Myopic individuals had no clinical evidence of connective-tissue abnormalities, and the average age at diagnosis of myopia was 6.8 years. The average spherical component refractive error for the affected individuals was -9.48 D. The families contained 82 individuals; of these, DNA was available for 71 (37 affected). Markers flanking or intragenic to the genes for Stickler syndrome types 1 and 2 (chromosomes 12q13.1-q13.3 and 6p21.3, respectively), Marfan syndrome (chromosome 15q21.1), and juvenile glaucoma (chromosome 1q21-q31) were also analyzed. No evidence of linkage was found for markers for the Stickler syndrome types 1 and 2, the Marfan syndrome, or the juvenile glaucoma loci. After a genomewide search, evidence of significant linkage was found on chromosome 18p. The maximum LOD score was 9.59, with marker D18S481, at a recombination fraction of .0010. Haplotype analysis further refined this myopia locus to a 7.6-cM interval between markers D18S59 and D18S1138 on 18p11.31.