Fenestrations and preferential flow routes in the prelaminar optic nerve through wet scanning electron microscope and perfusion of tracers

On the protective role of the blood vessels in glaucomatous damage: A transversal study

PURPOSE

To corroborate whether vessels on the surface of the optic nerve head can provide protection against the loss of underlying axons in subjects with manifest glaucoma.

METHODS

In this pilot study, thirty-six glaucomatous eyes with a perimetric defect in the Bjerrum area were included. The retinal nerve fiber layer (RNFL) thickness was measured in each of the sectors of the clock-hour map obtained by Cirrus HD-OCT considering the presence or absence of blood vessels. These sectors were related with their corresponding areas of the retina examined in the visual field using a mathematical model of the retina introduced by Jansonius, in order to determine the values of threshold sensitivity in those areas in the presence or absence of vessels.

RESULTS

We corroborated the protective role of the blood vessel for peripapillary RNFL thickness of clock-hour 12 despite obtaining a p-value (p = 0.023; w = 228.5) close to the acceptance zone (p ≥ 0.05). The mean ± standard deviation with vessel and without vessel were 70.95 ± 24.35 and 88.46 ± 23.96, respectively. No differences were found between the mean values of threshold sensitivity to the presence or absence of blood vessels in each of the sectors considered.

CONCLUSIONS

Our findings do not allow us to affirm that there is an association between the presence of a vessel and protection against glaucomatous damage in subjects with an advanced manifestation of the disease. In the future, more extensive studies are needed to study this relationship in subjects with early glaucoma.

A neuroglia-based interpretation of glaucomatous neuroretinal rim thinning in the optic nerve head

Neuroretinal rim thinning (NRR) is a characteristic glaucomatous optic disc change. However, the precise mechanism of the rim thinning has not been completely elucidated. This review focuses on the structural role of the glioarchitecture in the formation of the glaucomatous NRR thinning. The NRR is a glia-framed structure, with honeycomb geometry and mechanically reinforced astrocyte processes along the transverse plane. When neural damage selectively involves the neuron and spares the glia, the gross structure of the tissue is preserved. The disorganization and loss of the glioarchitecture are the two hallmarks of optic nerve head (ONH) remodeling in glaucoma that leads to the thinning of NRR tissue upon axonal loss. This is in contrast to most non-glaucomatous optic neuropathies with optic disc pallor where hypertrophy of the glioarchitecture is associated with the seemingly absent optic disc cupping. Arteritic anterior ischemic optic neuropathy is an exception where pan-necrosis of ONH tissue leads to NRR thinning. Milder ischemia indicates selective neuronal loss that spares glia in non-arteritic anterior ischemic optic neuropathy. The biological reason is the heterogeneous glial response determined by the site, type, and severity of the injury. The neuroglial interpretation explains how the cellular changes underlie the clinical findings. Updated understandings on glial responses illustrate the mechanical, microenvironmental, and microglial modulation of activated astrocytes in glaucoma. Findings relevant to the possible mechanism of the astrocyte death in advanced glaucoma are also emerging. Ultimately, a better understanding of glaucomatous glial response may lead to glia-targeting neuroprotection in the future.

Lessons from glaucoma: rethinking the fluid-brain barriers in common neurodegenerative disorders

Glaucoma has been recently characterized as a member of the group of anoikis-related diseases. Anoikis, a form of apoptosis, can be triggered by the unfastening of adherent junctions present in astrocytes. In those areas of the central nervous system in which the soma of the neurons or their axons and dendrites are metabolically dependent on the activity of astrocytes, a derangement of the lactate shuttle caused by a separation between the plasma membranes of neurons and astrocytes would result in metabolic impairment of the neurons themselves. In glaucoma, the triggering event has been attributed to the posterior deviation of aqueous humor towards the astrocyte-rich prelaminar tissue of the optic nerve head. The mean calcium content in the aqueous is able to interfere with calcium-dependent adherent junctions and induce anoikis of the astrocytes. As the cerebrospinal fluid has a similar base calcium concentration, a shunt of cerebrospinal fluid through the cerebral parenchyma would be able to interfere in the astrocytic architecture with dire consequences to the metabolically dependent neurons. Here the similitude between glaucoma, amyotrophic lateral sclerosis and Alzheimer's disease are discussed and the concept of the break in the fluid-brain barrier, as an event separated from the blood-brain barrier, is stressed.

Expression of glucose transporters in the prelaminar region of the optic-nerve head of the pig as determined by immunolabeling and tissue culture

Background

To develop the use of cultured tissue of the prelaminar optic nerve of the pig to explore possible alterations of the astrocyte-axon metabolic pathways in glaucoma, we map the distribution of the glucose transporters GLUT1 and GLUT3 in fresh and cultured tissue.

Methods

We monitor cell survival in cultures of the prelaminar optic-nerve tissue, measuring necrosis and apoptosis markers biochemically as well as morphologically, and establish the presence of the glucose transporters GLUT1 and GLUT3. We map the distribution of these transporters with immunolabeling in histological sections of the optic nerve using confocal and electronic transmission microscopy.

Results

We find that the main death type in prelaminar culture is apoptosis. Caspase 7 staining reveals an increment in apoptosis from day 1 to day 4 and a reduction from day 4 to day 8. Western blotting for GLUT1 shows stability with increased culture time. CLSM micrographs locate GLUT1 in the columnar astrocytes and in the area of axonal bundles. Anti-GLUT3 predominantly labels axonal bundles. TEM immunolabeling with colloidal gold displays a very specific distribution of GLUT-1 in the membranes of vascular endothelial cells and in periaxonal astrocyte expansions. The GLUT-3 isoform is observed with TEM only in axons in the axonal bundles.

Conclusions

Tissue culture is suitable for apoptosis-induction experiments. The results suggest that glucose is transported to the axonal cleft intracytoplasmically and delivered to the cleft by GLUT1 transporters. As monocarboxylate transporters have been reported in the prelaminar region of the optic-nerve head, this area is likely to use both lactate and glucose as energy sources.

Automatic identification and quantitative morphometry of unstained spinal nerve using molecular hyperspectral imaging technology

Quantitative observation of nerve fiber sections is often complemented by morphological analysis in both research and clinical condition. However, existing manual or semi-automated methods are tedious and labour intensive, fully automated morphometry methods are complicated as the information of color or gray images captured by traditional microscopy is limited. Moreover, most of the methods are time-consuming as the nerve sections need to be stained with some reagents before observation. To overcome these shortcomings, a molecular hyperspectral imaging system is developed and used to observe the spinal nerve sections. The molecular hyperspectral images contain both the structural and biochemical information of spinal nerve sections which is very useful for automatic identification and quantitative morphological analysis of nerve fibers. This characteristic makes it possible for researchers to observe the unstained spinal nerve and live cells in their native environment. To evaluate the performance of the new method, the molecular hyperspectral images were captured and the improved spectral angle mapper algorithm was proposed and used to segment the myelin contours. Then the morphological parameters such as myelin thickness and myelin area were calculated and evaluated. With these morphological parameters, the three dimension surface view images were drawn to help the investigators observe spinal nerve at different angles. The experiment results show that the hyperspectral based method has the potential to identify the spinal nerve more accurate than the traditional method as the new method contains both the spectral and spatial information of nerve sections.