Glaucomas are a group of progressive optic neuropathies characterized by degeneration of retinal ganglion cells (RGCs) and resulting changes in the optic nerve head. RGCs are central nervous system neurons whose cell bodies lie in the inner retina, while their axons form the optic nerve, so injury to either compartment can translate into characteristic optic disc cupping and visual loss. Across the clinical spectrum, loss of ganglion cells is related to the level of intraocular pressure (IOP), yet multiple additional factors may contribute, and the biological basis of glaucoma remains poorly understood.

A consistent clinicopathological theme is that glaucoma is not a single mechanism disease. Reviews describing current concepts emphasize that glaucoma is heterogeneous and that its pathophysiology is believed to be multifactorial, with multiple factors acting either on RGC cell bodies or on their axons and converging on RGC death. This framing matters because it accommodates several well described observations: glaucomatous changes can be observed in individuals whose IOP is considered normal, and therapeutic control of IOP in many patients is not sufficient to arrest disease progression. At the same time, IOP remains central to clinical management, because reduction of IOP is the only proven method to treat the disease, and treatment is based mainly on lowering IOP, which is considered a main risk factor. These points can be held together without contradiction if glaucoma is viewed as a neurodegenerative process with layered risk and injury pathways, in which IOP lowering reduces one major driver but does not eliminate all contributors to RGC vulnerability.

Within the IOP related component of glaucoma pathology, the optic nerve head region is a key anatomic and biomechanical interface. IOP can cause mechanical stress and strain on posterior structures of the eye, notably the lamina cribrosa and adjacent tissues. The lamina cribrosa is the site where the sclera is perforated and optic nerve fibers exit the eye, and it is described as the weakest point in the wall of the pressurized eye. In this context, IOP induced stress and strain may lead to compression, deformation, and remodeling of the lamina cribrosa, with consequent mechanical axonal damage and disruption of axonal transport. This transport disruption is pathophysiologically important because it interrupts retrograde delivery of essential trophic factors to retinal ganglion cells. Experimental systems have demonstrated that disrupted axonal transport occurs early in the pathogenesis of glaucoma, and similar ultrastructural changes in optic nerve fibers have been reported in postmortem human eyes with glaucoma. In parallel, there may be mitochondrial dysfunction in retinal ganglion cells and astrocytes, raising the possibility that energy supply and metabolic stress intersect with mechanical and transport mediated injury.

Beyond IOP linked biomechanics, glaucoma pathology is often described as a cascade in which early insults are amplified by secondary mechanisms. In a multifactorial model, elevated IOP and vascular dysregulation are proposed contributors to an initial insult that includes obstruction to axoplasmic flow within RGC axons at the lamina cribrosa, altered optic nerve microcirculation at the level of the lamina, and changes in laminar glial and connective tissue. Secondary insult factors described in the same review framework include excitotoxic damage linked to neurotransmitters released from injured neurons and oxidative damage driven by reactive species. While the relative weight and timing of these components can vary, the convergent endpoint described is dysfunction and death of RGCs leading to irreversible visual loss, reflecting complex interplay of multiple factors rather than a single isolated pathway. Consistent with this, other reviews characterize glaucoma as a disease with complex physiopathogenic mechanisms that are not entirely known, and they highlight the continuing aim of research to elucidate mechanisms involved in the survival, adaptation, and death of retinal ganglion cells, both to clarify how damage occurs and to identify factors that may protect these cells.

A major practical challenge in glaucoma research is that RGCs are highly polarized neurons and glaucoma injury is anatomically focal, yet many experimental systems blur cellular compartments. This becomes especially salient when considering axonal pathology. A recent full text open access study in Proceedings of the National Academy of Sciencesaddressed this gap by developing a pluripotent stem cell based microfluidic platform designed to model human RGC compartmentalization. In the study’s framing, RGCs are highly compartmentalized neurons whose long axons serve as the sole connection between the eye and the brain, and in both injury and disease, RGC degeneration is described as occurring in a similarly compartmentalized manner with distinct molecular and cellular responses in axonal and somatodendritic regions. The authors therefore set the goal of establishing a microfluidic based platform to investigate RGC compartmentalization in both health and disease states.

Methodologically, the platform seeded human pluripotent stem cell derived RGCs into microfluidic devices that allow physical separation of axons from the somatodendritic compartment, enabling precise study of each region. In the study’s significance statement, the platform is described as isolating RGC axons apart from the somatodendritic compartment and including a unique orientation of astrocytes along the proximal axonal compartment. This design choice aligns with the broader pathophysiology literature that locates early glaucoma injury within axonal pathways and highlights the optic nerve head microenvironment as biologically active. The same PNAS article explicitly states that in glaucoma it is well established that the primary site of injury occurs along the initial part of the axon and that degeneration of RGC compartments occurs by different mechanisms in axonal and somatodendritic regions, implying that compartmentalization is a relevant requirement when developing in vitro models for glaucoma.

Within this framework, the authors used the platform to interrogate glaucoma associated neurodegeneration in a compartment specific way. They examined compartment specific phenotypes in RGCs carrying the OPTN(E50K) glaucoma mutation compared to isogenic controls, including differences in axonal growth and axonal transport efficiency. In their abstract level summary, OPTN mutant RGCs showed reduced axon length and slower transport, described as hallmarks of neurodegeneration. To further probe molecular correlates, the authors performed axonal RNA sequencing and reported that axonal RNA seq analyses revealed transcriptomic alterations related to disease states, including transcriptomic changes along OPTN axons. While such transcriptomic data do not in themselves resolve causality, they add a compartment specific molecular layer to the concept that glaucomatous injury is not uniform across the neuron.

The same study also provides an experimental entry point into axon glia interactions, a theme that is prominent in contemporary glaucoma pathology discussions. In the PNAS work, to assess glial influences on axonal health, the authors developed models with astrocytes localized specifically to the proximal axonal compartment and modulated their disease states to simulate pathological conditions. Importantly, they reported that induction of diseased astrocytes solely along proximal axons triggered compartment specific neurodegenerative changes in RGCs. This aligns conceptually with the statement, presented in the same article, that across many underlying causes of glaucoma, glia in the optic nerve head play prominent roles associated with phenotypes observed in glaucoma patient eyes, and that the focal nature of glial activation and localized injury to RGC axons requires greater attention to the highly compartmentalized nature of glaucomatous injury. In other words, the platform is used not only to separate neuron compartments, but also to spatially localize glial states along the proximal axonal region, mirroring an optic nerve head adjacent zone where pathology is often discussed.

From a translational perspective, the PNAS authors characterize their platform as a successful recapitulation of spatially distinct features of human pluripotent stem cell derived RGCs under both healthy and disease conditions, offering a physiologically relevant, human specific in vitro system to study neuronal development, axon glia interactions, and mechanisms underlying neurodegeneration. They further state that the results underscore the importance of specifically investigating RGC axons in disease states and provide a foundation and justification for future studies targeting RGC axons for disease modeling. These claims are positioned as enabling statements rather than clinical promises, but they speak directly to a recognized need in glaucoma research: tools that can represent compartment specific injury biology while remaining experimentally tractable.

At the same time, careful interpretation requires attention to limitations that the authors themselves acknowledge, as well as the broader uncertainty in glaucoma mechanisms emphasized by review literature. The JAMA review states that the pathogenesis of glaucoma is not fully understood, even while providing a coherent mechanical and transport centered model of optic nerve head injury and reinforcing that IOP reduction is the only proven treatment method. Similarly, the evidence based review characterizes glaucoma mechanisms as complex and not entirely known. Within the microfluidic platform study, the authors explicitly recognize that other possible contributing factors exist in their experimental comparisons, including differences in the surface material used for microfluidic chips versus traditional cultures. Such acknowledgments matter because they underscore that compartmentalization and axonal isolation do not remove all sources of variability, and that platform effects can arise from multiple design features beyond compartment geometry.

Taken together, the pathology oriented reviews and the microfluidic modeling study converge on a coherent narrative without requiring oversimplification. Glaucoma can be described as a progressive optic neuropathy in which RGC degeneration and optic nerve head changes define the disease phenotype, while a multifactorial pathophysiology incorporates IOP related biomechanics, axonal transport disruption, vascular dysregulation, and secondary injury processes such as excitotoxic and oxidative damage. Within this landscape, the compartmentalized nature of RGCs becomes more than an anatomic detail: it becomes a constraint on how experimental systems should be built when the primary site of injury is described as occurring along the initial part of the axon and when glia at the optic nerve head are implicated in patient phenotypes. The 2025 PNAS platform offers one human cell based approach to meet this constraint by physically separating axons from somatodendritic compartments, modeling a glaucoma associated OPTN mutation with compartment specific phenotypes, and enabling localized astrocyte driven perturbations along proximal axons. As research continues to elucidate mechanisms of RGC survival and death, such compartment aware systems may provide complementary evidence alongside in vivo and ex vivo studies, helping to map where within the neuron, and in response to which microenvironmental cues, glaucomatous degeneration is initiated and propagated.

References

Weinreb, R. N., Aung, T. & Medeiros, F. A. The Pathophysiology and Treatment of Glaucoma: A Review. JAMA311, 1901–1911 (2014). PMCID: PMC4523637.
Agarwal, R., Gupta, S. K., Agarwal, P., Saxena, R. & Agrawal, S. S. Current concepts in the pathophysiology of glaucoma. Indian J Ophthalmol 57, 257–266 (2009). PMCID: PMC2712693.
Alexandrescu, C. et al. Evidence-based pathophysiology of glaucoma. Maedica (Bucur) 5, 207–213 (2010). PMCID: PMC3177542.
Gomes, C. et al. Modeling human retinal ganglion cell axonal outgrowth, development, and pathology using pluripotent stem cell–based microfluidic platforms. Proc Natl Acad Sci U S A 122, e2423682122 (2025). PMCID: PMC12452894.