Wound Healing in the Cornea Epithelium Injuries to the corneal epithelium heal by regeneration. Shortly after (6-12 hours or so) an injury, intact epithelial cells around the injured area loosen their connections, flatten out and then migrate. The cells migrate as sheet in a centripetal fashion until the whole of the injured area has been covered by a single layer of cells. In the case of large injuries, cells in the periphery of the cornea and even the stem cells in the limbus will proliferate to provide a supply of cells to cover the wound. Once the injured area has been covered by a single layer, the cells then proliferate and differentiate to re-form the normal multilayered structure of the epithelium.
The events of corneal epithelial wound healing are initiated and regulated by a variety of growth factors and cytokines (including EGF, IL-1, IL-6, HGF) coming from intact epithelial cells, the tear film, corneal keratocytes and infiltrating neutrophils. There appears to be significant redundancy in the system in terms of the factors having overlapping activities. Rapid corneal epithelial healing is essential to restore the refractive and protective functions of the cornea and if it all depended on just one factor, loss of activity of that factor (e.g. as a result of genetic mutation) would cause severe vision problems. So having multiple factors that can do the same job makes sense. Stroma Interesting things happen when the corneal stroma is injured such as in laser refractive surgery. In a typical scenario, the keratocytes (the main cells in the stroma) in the injured area die off (some due to direct injury, others by apoptosis likely induced by local release of cytokines such as IL-1). Soon (24-48 hours) these cells are replaced by “activated keratocytes”. The latter are uninjured keratocytes that have undergone a process of transformation and are now very active and can migrate from their original location to the injured area. Often the activated keratocytes are referred to as corneal fibroblasts. Once they get to their new location the corneal fibroblasts secrete new extracellular matrix to replace that which was lost. Also some of the corneal fibroblasts have the ability to contract, this is because they express a protein called smooth muscle actin. In this case, the cells are referred to as corneal myofibroblasts. The myofibroblasts typically appear in large wounds and, by contracting, they help bring the edges of the wound together.
While we need the fibroblasts and myofibroblasts to heal a corneal wound, they can also cause problems for example by secreting too much new extracellular matrix. It’s important that we find out more about the corneal fibroblasts and myofibroblasts so ultimately we will be able to regulate their appearance/behaviour after injury and so maximize their beneficial effects but reduce their detrimental effects.
One major problem people often experience after refractive surgery is an increase in the amount of light reflected (scattered) by the cornea which impairs vision. A number of researchers have suggested that normal keratocytes contain proteins called “corneal crystallins” which help make the cell itself transparent. Then after injury, the normal keratocytes are replaced by the corneal fibroblasts, but these express less corneal crystallins so are not as transparent and so contribute to impaired vision. We have been doing a little research on corneal crystallins and find that the expression of the putative crystallin aldehyde dehydrogenase 3 is markedly reduced in human corneal fibroblasts and myofibroblasts compared to normal keratocytes. This confirms similar findings made by other researchers on corneal fibroblasts from other species. Also, when keratocytes transform in to corneal fibroblasts, they start to express a variety of different proteins. We have recently found that one of the proteins the fibroblasts start to express is Thy-1 (CD90), a cell surface protein anchored to the plasma membrane by a GPI tail. Because Thy-1 is only present on corneal fibroblasts and myofibroblasts, and these cell types are only found after injury, we think that Thy-1 may have a role during wound healing. The question is what role? We are investigating several possibilities such as mediating interactions with inflammatory cells after injury.
References if you want to learn more about corneal wound healing:
Pei Y, Reins RY,McDermott AM. (2006) Aldehyde dehydrogenase (ALDH)3A1 Expression by the Human Keratocyte and Its Repair Phenotypes. In Press Exp Eye Res. West-Mays JA,Dwivedi DJ. The keratocyte: Corneal stromal cell with variable repair phenotypes. Int J Biochem Cell Biol. 2006 Apr 3;[Epub ahead of print]
Netto MV, MohanRR, Ambrosio R Jr, Hutcheon AE, Zieske JD, Wilson SE. Wound healing in the cornea: a review of refractive surgery complications and new prospects for therapy. Cornea. 2005, 24, 509-22. Fini ME, StramerBM. How the cornea heals: cornea-specific repair mechanisms affecting surgical outcomes. Cornea. 2005, 24, S2-S11.
Jester JV, BudgeA, Fisher S, Huang J. Corneal keratocytes: phenotypic and species differences in abundant protein expression and in vitro light-scattering. InvestOphthalmol Vis Sci. 2005, 46, 2369-78.
Pei Y, Sherry D and McDermott AM. Thy-1 distinguishes human corneal fibroblasts and myofibroblasts from keratocytes. Exp Eye Res. 2004, 79, 705-712. Wilson SE, NettoM, Ambrosio R Jr. Corneal cells: chatty in development, homeostasis, wound healing, and disease. Am J Ophthalmol. 2003, 136, 530-6.
Piatigorsky J.Review: A case for corneal crystallins. J Ocul Pharmacol Ther 2000, 16,173-180. Fini ME. Keratocyte and fibroblast phenotypes in the repairing cornea. Prog Retin Eye Res 1999, 18, 529-551.
Jester JV, PetrollWM, Cavanagh HD. Corneal stromal wound healing in refractive surgery: the role of myofibroblasts. Prog Retin Eye Res 1999, 18, 311-356.