Washington: Scientists at the Harvard Department of Ophthalmology’s Schepens Eye Research Institute and Massachusetts Eye and Ear Infirmary (MEEI) have claim to have learned why the cornea is free of any blood vessels.
Researchers said the VEGFR-3 halts angiogenesis (blood vessel growth) by acting as a “sink” to bind or neutralize the growth factors sent by the body to stimulate the growth of blood vessels.
For long it has been known that the cornea had the remarkable property of not having any blood vessels, but the exact reasons for this had remained unknown.
Scientists opine that their discovery has not only solved a scientific mystery, but also holds great promise for preventing and curing blinding eye disease and illnesses such as cancer. They have said that phenomenon present in the cornea can be used therapeutically in other tissues.
“This is a very significant discovery. A clear cornea is essential for vision. Without the ability to maintain a blood-vessel-free cornea, our vision would be significantly impaired. Clear, vessel-free corneas are vital to any animal that needs a high level of visual acuity to survive,” said Dr. Reza Dana, Senior Scientist at the Schepens Eye Research Institute, and senior author and principal investigator of the study.
The study is published in the July 25, 2006 issue of the Proceedings of the National Academy of Sciences and in the July 17 online edition.
Structure of Cornea: The cornea has unmyelinated nerve endings sensitive to touch, temperature and chemicals; a touch of the cornea causes an involuntary reflex to close the eyelid. Because transparency is of prime importance the cornea does not have blood vessels; it receives nutrients via diffusion from the tear fluid through the outside surface and the aqueous humour through the inside surface, and also from neurotrophins supplied by nerve fibres that innervate it. In humans, the cornea has a diameter of about 11.5 mm and a thickness of 0.5–0.6 mm in the center and 0.6–0.8 mm at the periphery. Transparency, avascularity, the presence of immature resident immune cells, and immunologic privilege makes the cornea a very special tissue. The cornea has no blood supply; it gets oxygen directly through the air. Oxygen first dissolves in the tears and then diffuses throughout the cornea to keep it healthy.
It borders with the sclera by the corneal limbus.
The most abundant soluble protein in mammalian cornea is albumin.
In lampreys, the cornea is solely an extension of the sclera, and is separate from the skin lying above it, but in more advanced vertebrates it is always fused with the skin to form a single structure, albeit one composed of multiple layers. In fish, and aquatic vertebrates in general, the cornea plays no role in focusing light, since it has virtually the same refractive index as water.
Research:
This unique characteristic makes vision possible and researchers believe that the unexpected presence of large amounts of the protein VEGFR-3 (vascular endothelial growth factor receptor-3) on the top epithelial layer of normal healthy corneas, might be something to do with this. Researchers said the VEGFR-3 halts angiogenesis (blood vessel growth) by acting as a “sink” to bind or neutralize the growth factors sent by the body to stimulate the growth of blood vessels.
For long it has been known that the cornea had the remarkable property of not having any blood vessels, but the exact reasons for this had remained unknown.
Scientists opine that their discovery has not only solved a scientific mystery, but also holds great promise for preventing and curing blinding eye disease and illnesses such as cancer. They have said that phenomenon present in the cornea can be used therapeutically in other tissues.
“This is a very significant discovery. A clear cornea is essential for vision. Without the ability to maintain a blood-vessel-free cornea, our vision would be significantly impaired. Clear, vessel-free corneas are vital to any animal that needs a high level of visual acuity to survive,” said Dr. Reza Dana, Senior Scientist at the Schepens Eye Research Institute, and senior author and principal investigator of the study.
The study is published in the July 25, 2006 issue of the Proceedings of the National Academy of Sciences and in the July 17 online edition.
General references
- Daxer, A; Misof, K; Grabner, B; Ettl, A; Fratzl, P (1998). "Collagen fibrils in the human corneal stroma: Structure and aging". Investigative Ophthalmology & Visual Science 39(3): 644–8. PMID 9501878.
- Daxer, A; Fratzl, P (1997). "Collagen fibril orientation in the human corneal stroma and its implication in keratoconus". Investigative Ophthalmology & Visual Science 38 (1): 121–9. PMID 9008637.
- Fratzl, P.; Daxer, A. (1993). "Structural transformation of collagen fibrils in corneal stroma during drying. An x-ray scattering study". Biophysical Journal 64 (4): 1210–4.doi:10.1016/S0006-3495(93)81487-5. PMC 1262438. PMID 8494978.
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