A Novel Tree Shrew (Tupaia belangeri) Model of Glaucoma.

Purpose: Primates and rodents are used widely as animal models of glaucoma, but each has significant limitations. Researchers need additional animal models that closely resemble the relevant anatomy and pathologic features of the human disease to more quickly advance research. We validate a novel glaucoma animal model in tree shrews (Tupaia belangeri). Methods: Experimental glaucoma was induced in adult tree shrews (n = 8) by injecting 50 µL of a 25 mg/mL ferromagnetic bead solution into the anterior chamber. Beads were directed into the iridocorneal angle with a magnet to impede aqueous outflow. Animals were followed for 3 months with weekly IOP measurements and biweekly spectral domain optical coherence tomography (SD-OCT) images of the optic nerve head. Histopathology of the optic nerve and optic nerve axon counts were completed at the end of the study. Results: The 12-week average mean IOP was 22.7 ± 3.6 and 8.6 ± 2.9 mm Hg in the treated and control eyes, respectively. Longitudinal analysis showed significant retinal nerve fiber layer (RNFL) thinning throughout the study. Axon counts were significantly reduced (59.7%) in treated versus control eyes. SD-OCT imaging showed cupping and posterior displacement of the lamina cribrosa in glaucomatous eyes. RNFL thickness and optic nerve axon counts were reduced consistent with IOP elevation. Optic nerves demonstrated histopathology consistent with glaucomatous optic neuropathy. Conclusions: Tree shrews with experimental glaucoma show key pathologic characteristics of the human disease. The tree shrew model of glaucoma has the potential to help researchers accelerate our understanding of glaucoma pathophysiology.

Lamina cribrosa in glaucoma.

PURPOSE OF REVIEW: This article presents, summarizes, and interprets the most recent advances in the study and understanding of the lamina cribrosa in glaucoma, in the context of previous work. RECENT FINDINGS: The lamina is an active living structure that responds to strain by changing morphology at the micro-scale and macro-scale in glaucoma. Changes in lamina cribrosa morphology in glaucoma include posteriorization of the laminar insertion into the sclera, increased cupping or depth of the lamina cribrosa, and the development of focal lamina cribrosa defects. These lamina cribrosa changes are associated with disk hemorrhages and visual field damage, and are detectable with clinical imaging techniques such as optical coherence tomography. Glaucomatous changes in the lamina cribrosa are thought to be driven by cellular processes mediated by focal cyclical mechanical strain. Strain is eye specific and mediated by intraocular pressure, cerebrospinal fluid pressure, scleral and lamina cribrosa morphology, and structural stiffness; deleterious lamina cribrosa strains can occur at all levels of mean intraocular pressure. SUMMARY: Laminar morphology is ever changing in health and disease, and recent studies have identified several promising morphological changes that are indicative of glaucoma susceptibility, onset, and progression.

Posterior (outward) migration of the lamina cribrosa and early cupping in monkey experimental glaucoma.

PURPOSE: To quantify the lamina cribrosa insertion into the peripapillary sclera and optic nerve pia in normal (N) and early experimental glaucoma (EEG) monkey eyes. METHODS: Perfusion-fixed optic nerve heads (ONHs) from 21 animals were digitally reconstructed three dimensionally and delineated. Anterior Laminar Insertion Position (ALIP), Posterior Laminar Insertion Position (PLIP), Laminar Insertion Length (LIL; distance between the anterior and posterior laminar insertions), and Scleral Thickness (at the Anterior Sub-arachnoid space) were calculated for each ONH. Animals were pooled into four groups based on the kill condition (N vs. EEG) and perfusion IOP (10, 30, or 45 mm Hg) of each eye: N10-N10 (n = 6), N30/45-N10 (n = 6), EEG10-N10 (n = 3), and EEG30/45-N10 (n = 6). Glaucomatous EEG versus N eye differences in each group and each animal were required not only to achieve statistical significance (P < 0.05) but also to exceed physiologic intereye differences within the bilaterally normal groups. RESULTS: ALIP was significantly posterior (outward) in the EEG compared with N10 eyes of the EEG30/45-N10 group and 5 of 9 individual EEG eyes (difference range, 12-49 µm). PLIP was significantly posterior in the EEG eyes of both EEG groups and in 6 of 9 individual EEG eyes (range, 25-83 µm). LIL ranged from 90 to 190 µm in normal eyes and was significantly increased within the EEG eyes of both EEG groups and in 7 of 9 individual EEG eyes (difference range, 30-47 µm). CONCLUSIONS: Posterior migration of the lamina cribrosa is a component of early cupping in monkey EEG.

Physiologic intereye differences in monkey optic nerve head architecture and their relation to changes in early experimental glaucoma.

PURPOSE: To characterize physiologic intereye differences (PIDs) in optic nerve head (ONH) architecture in six normal rhesus monkeys and compare them to intereye differences in three previously reported cynomolgus monkeys with early experimental glaucoma (EEG). METHODS: Trephinated ONH and peripapillary sclera from both eyes of six normal monkeys were serial sectioned, 3-D reconstructed, 3-D delineated, and parameterized. For each normal animal and each parameter, PID was calculated (both overall and regionally) by converting all left eye data to the right eye configuration and subtracting the right eye value from that of the left eye. Physiologic intereye percent difference (PIPD) was calculated as the PID divided by the measurement mean of the two eyes. For each EEG monkey, intereye (EEG minus normal) differences and percent differences for each parameter overall and regionally were compared to the PID and PIPD maximums. RESULTS: For all parameters the PID maximums were relatively small overall. Compared to overall PID maximums, overall intereye differences in EEG monkeys were greatest for laminar deformation and thickening, posterior scleral canal enlargement, cupping, and prelaminar neural tissue thickening. Compared with the regional PID maximums, the lamina cribrosa was posteriorly deformed centrally, inferiorly, inferonasally, and superiorly and was thickened centrally. The prelaminar neural tissues were thickened inferiorly, inferonasally, and superiorly. CONCLUSIONS: These data provide the first characterization of PID and PIPD maximums for ONH neural and connective tissue parameters in normal monkeys and serve to further clarify the location and character of early ONH change in experimental glaucoma. However, because of the species differences, the findings in EEG should be confirmed in EEG rhesus monkey eyes.

Premise and prediction-how optic nerve head biomechanics underlies the susceptibility and clinical behavior of the aged optic nerve head.

We propose that age-related alterations in optic nerve head (ONH) biomechanics underlie the clinical behavior and increased susceptibility of the aged ONH to glaucomatous damage. The literature which suggests that the aged ONH is more susceptible to glaucomatous damage at all levels of intraocular pressure is reviewed. The relevant biomechanics of the aged ONH are discussed and a biomechanical explanation for why, on average, the stiffened peripapillary scleral and lamina cribrosa connective tissues of the aged eye should lead to a shallow (senile sclerotic) form of cupping is proposed. A logic for why age-related axon loss and the optic neuropathy of glaucoma in the aged eye may overlap is discussed. Finally, we argue for a need to characterize all forms of clinical cupping into prelaminar and laminar components so as to add precision to the discussion of clinical cupping which does not currently exist. Such characterization may lead to the early detection of ONH axonal and connective tissue pathology in ocular hypertension and eventually aid in the assessment of etiology in all forms of optic neuropathy including those that may be purely age-related.

3-D histomorphometry of the normal and early glaucomatous monkey optic nerve head: prelaminar neural tissues and cupping.

PURPOSE: To introduce a three-dimensional (3-D) histomorphometric strategy for characterizing the connective tissue (laminar) and prelaminar neural tissue (prelaminar) components of optic nerve head (ONH) cupping in one bilaterally normal monkey and three monkeys with early experimental glaucoma (EG) in one eye. METHODS: Trephined ONH and peripapillary sclera from both eyes of four monkeys were serially sectioned at either 3-mum thickness (three EG monkeys) or 1.5-microm thickness (the bilaterally normal monkey) with the embedded tissue block face stained and imaged after each cut. Digital section images were aligned and stacked to create a 3-D reconstruction of each ONH. Within 40 digital radial sagittal sections of each reconstruction, Bruch's membrane opening (BMO), the neural canal wall, and the anterior laminar surface were delineated by two delineators. The 80 BMO points were used to establish a BMO-zero reference plane. The parameters prelaminar tissue volume, post-BMO cup (the estimate of the clinical cup), and post-BMO total prelaminar volume (a global measure of ONH connective tissue deformation) were calculated overall and within 15 degrees radial regions. The parameter prelaminar tissue thickness was calculated at each delineated anterior laminar surface point. For each monkey, an intra-animal difference map was generated for each parameter. Overall volume and thickness data were compared between normal and EG eyes by analysis of variance (ANOVA). RESULTS: Regionally variable expansion of post-BMO cup volume and post-BMO total prelaminar volume were present in all three EG eyes and far exceeded the intra-animal, physiologic differences for these parameters in the bilaterally normal monkey. Prelaminar tissue thickness was increased in all three EG monkeys, with the greatest effects present within the peripheral regions of the canal. CONCLUSIONS: These data suggest that in young adult monkeys with more compliant connective tissues, clinical cupping in early glaucoma is primarily due to fixed deformation of the ONH connective tissues and occurs in the setting of prelaminar tissues that are thickened rather than thinned.