Elsevier

Experimental Eye Research

Volume 141, December 2015, Pages 9-14
Experimental Eye Research

Microbead models in glaucoma

https://doi.org/10.1016/j.exer.2015.06.020Get rights and content

Highlights

  • Particulate occlusion of the trabecular meshwork provides an effective method for the induction of experimental glaucoma.

  • Anterior chamber injection techniques can be refined to minimise transient increases in IOP following injection.

  • Use of ferromagnetic beads can ensure a clear visual axis and enhance occlusion of the iridocorneal angle.

Abstract

The sustained and moderate elevation of intraocular pressure, which can be initiated at precise time points, remains the cornerstone of research into the mechanisms of glaucomatous retinal damage. We focus on the use of microbeads to block the outflow of aqueous following anterior chamber injection in a range of animals (mouse, rat and primate). We describe some of the most commonly used parameters and present guidance on injection technique and bead manipulation to facilitate the successful generation of experimental glaucoma.

Introduction

The study of glaucomatous pathophysiology has relied on the development of technologies to generate sustained and moderate increases in intraocular pressure (IOP) in a range of species. The chronic elevation of IOP presents significant challenges in small eyes where ocular tissues are thin and vulnerable to the effects of inflammation and remodelling.

The choice of animal is guided by cost, handling characteristics and the availability of mutant strains. Not surprisingly, rodents have superseded primates as the most common experimental model for the investigation of mechanism underlying retinal ganglion cell loss. For the rat, the injection of hypertonic saline for the sclerosis of episcleral vessels has been one of the most successful models (Morrison et al., 2008) but its use has been constrained by need for a high level of surgical skill to cannulate small episcleral veins. With the mouse, genetic models of glaucoma such as the DBA2J strain which rely on the deposition of pigment cells within the trabecular meshwork (TM) (John et al., 1998) have the advantage that they do not require surgical intervention for the development of ocular hypertension. However, the model incurs significant financial and time costs since animals have to be aged before glaucoma develops and control of IOP at the single animal level can be problematic (John, 2005). Other models in which the episcleral vessels are cauterized externally raise the possibility of the confounding effects of increases in episcleral venous pressure (Vecino and Sharma, 2011).

A cost effective model would be one in which the induction of IOP elevation is technically undemanding, rapid in onset and works in as many animals as possible. In the last decade the availability of high quality microbeads with diameters that match the pores in the TM has facilitated the generation of sustained ocular hypertension in primates and rodents. With these methods, the onset of ocular hypertension can be timed and the level of the IOP increase controlled. In this review we cover the salient features of currently used models with technical advice on methods of bead delivery.

Section snippets

Background

Obstruction of the TM by the injection of microparticles has a long track record as a method for the generation of experimental glaucoma. Some of the earliest attempts were based on primate models in which glutaraldehyde treated autologous red blood cells were injected into the anterior chamber. These cells mimicked the ghost red blood cells (GBCs) in their occlusion of the TM by virtue of their membrane rigidity. Sustained elevation in IOP could be achieved in both rabbit and primates

Mouse

The small anterior chamber of the rodent eye is ideal for microbead occlusion. The mouse has particular appeal because of the availability of numerous genetic constructs and the ability to measure IOP in awake (manually or by telemetry (Ruixia, 2008)) and anaesthetized animals (Wang et al., 2005, Cone et al., 2012).

The use of microbeads in mouse glaucoma models has generated a wide range of injection parameters and techniques which are summarized in Table 1. The table is not intended as an

Rat

There are advantages to working on rat, rather than mouse glaucoma models. Rat eyes are larger and therefore easier to inject and rats can be easier to handle in the context of obtaining awake IOP readings. The rat is increasingly being used for mechanistic studies of glaucoma pathophysiolgy, supported by the increasing availability of genetically altered rats (Ma et al., 2014). Gene array analyses in wild type strains have been valuable in pinpointing potential mechanism for early axonal

Cannulation technique

Rodents have thin corneas which present a particular challenge for the retention of beads. These incisions are more challenging with the rodent eyes where the corneal thickness is in the range 160 μm for the rat eye to 90 μm for the mouse eye (Bawa et al., 2013). In the mouse, the use of a glass injection micropipette is optimal since these can be pulled to very fine outside diameters and bevelled to a sharp tip. For 10 μm beads, a cannula with an external diameter of 100um should facilitate

Conclusion

Microbead injection models are in routine use for the development of experimental glaucoma in rodents and primates. They allow the generation of persistent ocular hypertension and some element of control for the level of IOP elevation. The use of fluorescent markers can greatly facilitate the identification of beads within the anterior chamber while the use of ferromagnetic particles allows the injection of beads and carrier solution to be separated and for a clear visual axis to be maintained.

Commercial interest

None.

References (32)

  • S. Asrani et al.

    Large diurnal fluctuations in intraocular pressure are an independent risk factor in patients with glaucoma

    J. Glaucoma

    (2000 Apr 1)
  • S. Baltan et al.

    Metabolic vulnerability disposes retinal ganglion cell axons to dysfunction in a model of glaucomatous degeneration

    J. Neurosci.

    (2010 Apr 21)
  • G. Bawa et al.

    Variational analysis of the mouse and rat eye optical parameters

    Biomed. Opt. Express

    (2013)
  • S. Bunker et al.

    Experimental glaucoma induced by ocular injection of magnetic microspheres

    J. Vis. Exp.

    (2015)
  • C. Dai et al.

    Structural basis of glaucoma: the fortified astrocytes of the optic nerve head are the target of raised intraocular pressure

    Glia

    (2012)
  • B.J. Frankfort et al.

    Elevated intraocular pressure causes inner retinal dysfunction before cell loss in a mouse model of experimental glaucoma

    Invest Ophthalmol Vis Sci.

    (2013)
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