Aishwarya Jha, M.Optom

Adjunct Optometrist, Cornea and Refractive Services, Dr. Shroff’s Charity Eye Hospital

 

Cornea preservation and tissue banking

The cornea, though seemingly simple, plays a vital role in vision, serving as the eye’s primary refractive element due to its transparency and shape. Comprising mainly a collagenous stroma and a layer of endothelial cells, it maintains transparency by regulating hydration and structure. Additionally, it acts as a protective barrier against pathogens and maintains intraocular pressure (IOP).

Endothelial cells, crucial for corneal function, have limited regenerative capacity, leading to a gradual decline in density with age.(1) Disease or injury can accelerate this decline, leading to corneal oedema and loss of transparency. Endothelial dysfunction, whether primary or secondary, often necessitates corneal transplantation, along with conditions like keratoconus, trauma, and infections. (2)

Historically, corneal transplants relied on live donors until the 1930s when the use of corneas from deceased donors and preservation techniques advanced. The establishment of eye banks further facilitated transplantation, leading to thousands of successful procedures annually worldwide. (2,3)

The success of corneal transplants relies heavily on preserving the corneal endothelium, crucial for maintaining transparency. Cryopreservation, although successful in some cases, is limited due to its complexity and potential for endothelial damage. Hypothermia (2–8 °C) is the primary method, allowing storage for 7–14 days, while organ culture (28–37 °C) extends storage time to 4 weeks, with comparable graft outcomes. (3)

Cold Strategies in Eye Banking: Advancing Corneal Preservation

Hypothermic storage of cells, tissues, and organs relies on the principle that cold temperatures reduce the metabolic energy demand of cells, following the Arrhenius relation.(2)

Hypothermic conditions are commonly used for preserving biologics before use but understanding their effects on tissues is crucial. Corwin et al. studied the unfolded protein response (UPR) in human corneal endothelial cells (HCEC) during hypothermic storage. They found that UPR activation in response to cold exposure led to increased levels of certain proteins associated with cellular stress. This pathway appeared to play a role in mediating cell apoptosis. Targeting the UPR pathway could potentially improve the survival and function of HCECs, offering clinical benefits.(4) It is understood that additional exogenous FGF-2 (fibroblast growth factor) helped prevent irreversible damage to the corneal endothelium, highlighting its potential as a protective agent during corneal storage. (5)

Hypothermia protects corneal endothelial cells by reducing metabolic demand, which slows down cellular processes and decreases the production of harmful by-products associated with metabolism. Moreover, inhibiting cell death pathways, thereby preserving cellular integrity during storage or transplantation.

Figure 2: Endothelium (blue arrow) of donor’s cornea under slit-lamp Biomicroscopy.

Hypothermia: Preservation Challenges and Adverse Effects

Corneal endothelial damage imposes limitations on the storage duration of corneas under hypothermic conditions. When cultured porcine corneal endothelial cells are exposed to 4°C, they undergo morphological changes such as cellular retraction, rounding, and partial detachment. Additionally, rewarming induces apoptotic features like apoptotic bodies, nuclear shrinkage, and chromatin condensation. Mitochondrial permeability transition, a known intrinsic pathway of apoptosis, is observed during both hypothermia and rewarming, characterised by changes in mitochondrial morphology and membrane potential.

In summary, the presence of mitochondrial permeability transition during both hypothermia and rewarming underscores temperature’s critical impact on cellular physiology. Understanding these dynamics reveals the intricate interplay between temperature stress and mitochondrial function, paving the way for potential interventions to safeguard cellular health amid temperature fluctuations. Stay informed and proactive about the effects of temperature on your body’s health. (6)

 

References-

  1. Armitage, W. J., Dick, A. D., & Bourne, W. M. (2003). Predicting endothelial cell loss and long-term corneal graft survival. Investigative Ophthalmology & Visual Science, 44, 3326–3331.
  2. Armitage, W. J. (2011). Preservation of human cornea. Transfusion Medicine and Hemotherapy, 38(2), 143–147.
  3. Xi, L. (2020). Research progress of the application of hypothermia in the eye. Oxidative Medicine and Cellular Longevity, 2020, 1-13.
  4. Corwin, W. L., Baust, J. M., Baust, J. G., & Van Buskirk, R. G. (2011). The unfolded protein response in human corneal endothelial cells following hypothermic storage: Implications of a novel stress pathway. Cryobiology, 63(1), 46–55.
  5. Rieck, P. W., von Stockhausen, R. M., Metzner, S., Hartmann, C., & Courtois, Y. (2003). Fibroblast growth factor-2 protects endothelial cells from damage after corneal storage at 4°C. Graefe’s Archive for Clinical and Experimental Ophthalmology, 241(9), 757–764.
  6. Rauen, U., Kerkweg, U., Wusteman, M. C., & de Groot, H. (2006). Cold-induced injury to porcine corneal endothelial cells and its mediation by chelatable iron. Cornea, 25(1), 68–77.