Cornea Implants Topics: Structure of the cornea Implant materials for the cornea History, current and future research

The focus of the cornea is fixed; the lens provides fine focus tuning The cornea is the transparent front part of the eye that covers the iris, pupil, and anterior chamber. Together with the lens, it refracts light and provides 80% of the eye’s optical power The focus of the cornea is fixed; the lens provides fine focus tuning

Cornea Structure The cornea is transparent, thus it has no blood vessels. It is the only part of the body that has no blood supply It receives its oxygen directly through the air, and its nutrients via diffusion from the tear fluid on the outside and aqueous humour on the inside, and slo from neurotrophins supplied by the nerve fibers that innervate it. It is ~0.6 mm thick at the center, and 11.5 mm in diameter

Layers 1. Corneal Epithelium: a thin multicellular, stratified tissue layer of fast-growing and easily-regenerated cells 2. Bowman’s layer: a tough layer, 8-14 mm thick, that protects the corneal stroma, consisting of irregularly-arranged collage fibers. 3. Corneal stroma: a thick, transparent middle layer made up of ~200 regularly-arranged layers of collagen fibers along with sparsely populated keratocytes. 90% of the cornea's thickness is made up of stroma. 4. Descemet’s membrane: a thin acellular layer that serves as the modified basement membrane of the corneal endothelium. 5. Corneal endothelium: a simple squamous or low cuboidal monolayer of mitochondria-rich cells responsible for regulating fluid an solute transport between the aqueous and corneal stromal compartments For the meaning of the letters a- consult Gray’s anatomy, subject #225 1006

Innervation The cornea is one of the most sensitive tissues of the body, and is densely populated with sensory nerve fibers Nerve enter the cornea through the sclera, episcleral and conjunctival levels. Receptive fields at each nerve ending are very large and may overlap Corneal nerves of the subepithelial layer converge and terminate near the apex of the cornea in a spiral pattern

Maintaining Transparency Upon death, or removal from the eye, the cornea absorbs the aqueous humor, thickens and becomes hazy. Transparency can be restored by placing it in a warm, well-ventilated chamber at 31 oC, allowing fluid to leave the cornea When placed in reservoirs of glycogen and sugar, the cornea can remain transparent for at least a day The endothelium controls the ejection of fluid from the cornea When the cornea is damaged, e.g. a viral infection, the collagen used to repair the process is not regularly arranged, leading to an opaque patch

Diseases and disorders Laser eye surgery uses photoablation to reshape the cornea If the corneal stroma develops opacity, irregularity or edema, a cornea of a deceased donor can be transplanted. Because there are no blood vessels in the cornea, there are few problems with tissue rejection Synthetic corneas are being developed that encourage tissue ingrowth and promote biointegration

Corneal disease is a leading cause of blindness Corneal disease is a leading cause of blindness. Examples of corneal disease include infection such as ocular herpes and corneal dystrophy. It is estimated that 10 million people worldwide have lost their sight to corneal disease Stanford scientists have developed a cornea made of two interwoven polymer gels, similar to the materials used in soft contact lenses One layer is exceptionally strong, and the other can absorb large amounts of water This results in a transparent, highly permeable substance with a water content similar to that of the natural cornea High water content is needed in order to feed nutrients to the cells within the eye – otherwise, they die The artificial cornea is currently being tested in animals Only one artificial cornea is approved in the U.S. Called AlphaCor, the failure rate is ~20% per year