A Million Fibers That Won't Regrow

Blindness caused by optic nerve damage (trauma, glaucoma, tumors) is irreversible because mammalian central nervous system neurons — including the ~1.2 million retinal ganglion cell axons that form the optic nerve — do not regenerate after injury. While cornea transplants have been routine for decades and retinal prostheses can provide rudimentary vision, no approach can restore vision lost to optic nerve destruction. A whole-eye transplant was performed for the first time in 2023 (at NYU Langone, as part of a face transplant), and the transplanted eye maintained structural integrity and blood supply, but vision was not restored because the severed optic nerve did not reconnect to the brain. The fundamental barrier is axon regeneration: severed mammalian CNS axons form growth-inhibiting scar tissue and encounter molecular signals that actively prevent regrowth.

An estimated 43 million people worldwide are blind, and approximately 1.1 million Americans have bilateral blindness. Causes include glaucoma, optic nerve trauma, tumors compressing the optic nerve, and inherited optic neuropathies. Unlike corneal blindness (treatable by transplant) or photoreceptor loss (addressable by gene therapy for some conditions), optic nerve damage has no treatment path. If whole-eye transplantation could restore vision, it would represent the first successful transplant of a central nervous system organ and could open pathways to nerve repair in spinal cord injury and other neurological conditions.

The 2023 NYU whole-eye transplant demonstrated that surgical vascular anastomosis can maintain a transplanted eye's structural viability and intraocular pressure for months — a significant milestone. However, the optic nerve was completely severed, and no functional visual signal reached the brain. In animal models, retinal ganglion cell axon regeneration has been achieved over short distances (a few millimeters) using combinations of growth factor overexpression (CNTF, IGF1), mTOR pathway activation (PTEN deletion), and removal of inhibitory signals (Nogo receptor knockout). But regenerating the full length of the human optic nerve (~50 mm from eye to chiasm) and correctly mapping ~1.2 million axons to their retinotopic targets in the lateral geniculate nucleus has never been attempted in any species. The axon guidance cues that directed original development may not be present in the adult brain.

Three convergent advances are needed: (1) donor eye preservation techniques that maintain retinal ganglion cell viability for hours to days between retrieval and transplantation (currently, RGCs begin dying within minutes of ischemia); (2) gene and cell therapies that stimulate robust, long-distance axon regeneration from transplanted RGCs through the host optic nerve sheath, overcoming both intrinsic growth arrest and extrinsic inhibitory signals; (3) methods to guide regenerating axons to their correct retinotopic targets in the brain — or evidence that sufficient plasticity exists for the visual cortex to interpret misrouted inputs. Success would also require long-term immunosuppression management for the transplanted organ.

A student team could develop an ex vivo perfusion system for maintaining donor eye viability after enucleation, measuring retinal ganglion cell survival time under different perfusion conditions and temperatures. A computationally oriented team could model axon guidance in the optic nerve, simulating how different combinations of molecular cues affect the probability of correct retinotopic targeting across the ~50 mm regeneration distance. Relevant disciplines: neuroscience, biomedical engineering, ophthalmology, tissue preservation science.