The cornea is the clear, dome-shaped front surface of your eye. It covers the iris (the colored part) and the pupil, acting as both a protective shield and the eye’s primary lens. Despite being only about half a millimeter thick at its center, the cornea is responsible for roughly 70% of the eye’s focusing power, bending light at about 43 diopters out of the eye’s total 60.
How the Cornea Focuses Light
When light hits your eye, the cornea is the first structure it passes through. The curved front surface of the cornea, combined with the difference in density between air and corneal tissue, bends incoming light sharply inward. This single surface alone generates about 48 diopters of focusing power. The back surface of the cornea slightly reduces that by about 5 diopters because the fluid behind it has a similar density to the cornea itself, producing less bending. The net result is around 43 diopters, making the cornea far more important for focusing than the lens sitting deeper inside the eye.
This is why procedures like LASIK reshape the cornea rather than the internal lens. Even tiny changes to corneal curvature produce large shifts in focus.
Five Layers, Each With a Job
The cornea is remarkably thin, averaging 515 micrometers at its center (roughly half a millimeter), with a normal range of about 420 to 625 micrometers across different people. Within that sliver of tissue, five distinct layers are stacked together.
The outermost layer is the epithelium, a sheet of five to seven cell layers that acts as the cornea’s first line of defense. These cells form tight junctions with their neighbors, creating a barrier that prevents tears and debris from seeping into deeper tissue. Because this layer is exposed to the outside world, it regenerates constantly. Stem cells at the edge of the cornea (in a region called the limbus) divide and push new cells toward the center. Complete turnover of the epithelial surface takes roughly two weeks.
Beneath the epithelium sits Bowman’s layer, a thin sheet of collagen about 12 micrometers thick. It helps the cornea hold its shape and provides a smooth foundation for the epithelium above it. Unlike the epithelium, Bowman’s layer does not regenerate if damaged, so injuries that reach this depth can leave permanent scars.
The stroma makes up 80 to 85% of the cornea’s total thickness. It consists of roughly 300 collagen sheets, stacked in a precise arrangement. This regularity is what makes the cornea transparent. If those fibers become disorganized, through swelling or scarring, the cornea turns hazy and vision suffers.
Descemet’s membrane is a thin elastic layer, about 7 micrometers thick, that sits just in front of the innermost cell layer. It is continuously produced throughout life, so it gradually thickens with age.
The innermost layer is the endothelium, a single sheet of hexagonal cells only 5 micrometers thick. These cells run a critical pump that controls how much water sits in the stroma. By actively moving ions from the stroma into the fluid chamber behind the cornea, the endothelium creates an osmotic gradient that draws water out of the stromal tissue. This keeps the stroma at just the right hydration level to remain transparent. If endothelial cells are lost (they do not regenerate in adults), the pump weakens, the stroma swells with excess water, and the cornea clouds over.
No Blood Vessels, by Design
The cornea is one of the few tissues in the body that contains no blood vessels. This avascularity is essential for transparency. Blood vessels would scatter light and blur vision. Instead, the cornea gets its oxygen directly from the air when your eyes are open. When your eyes are closed during sleep, oxygen diffuses from the blood vessels lining the inside of the eyelid.
Nutrients arrive through two routes: tears washing over the front surface and the aqueous humor, the clear fluid that fills the chamber behind the cornea. This system works well under normal conditions, but it makes the cornea vulnerable to oxygen deprivation. When oxygen levels drop (for example, from wearing contact lenses too long), the epithelium shifts to a less efficient form of metabolism that produces excess lactate. That lactate raises the concentration of dissolved particles in the stroma, which draws in water and causes the cornea to swell and become cloudy. This is one reason eye care professionals emphasize not sleeping in contact lenses that aren’t designed for overnight wear.
The Most Sensitive Tissue in the Body
The cornea is densely packed with nerve endings, far more than most other tissues. Researchers mapping the full nerve architecture of the human cornea found roughly 525 nerve terminals per square millimeter in the central cornea, dropping to about 230 per square millimeter in the periphery. This density is what makes even a tiny speck of dust in your eye so intensely painful.
That sensitivity serves a protective purpose. It drives the blink reflex, which shields the cornea from injury, and it stimulates tear production to keep the surface moist. As people age, corneal nerve density gradually declines, which can contribute to dry eye and slower wound healing because the reflex circuits that trigger blinking and tear flow become less responsive.
How the Cornea Heals
Minor scratches on the corneal surface heal remarkably fast. In experimental models, small epithelial wounds close within about 24 hours. After an injury, the stem cells at the corneal edge ramp up dramatically. Under normal conditions, only about 4.5% of these stem cells enter a division cycle in a given day. Within 24 hours of a wound, that jumps sevenfold to about 33%, and by 24 to 48 hours after injury, more than 85% of stem cells are actively dividing.
This rapid healing applies mainly to the epithelium. Deeper injuries that penetrate into the stroma heal more slowly and often produce scar tissue that can permanently reduce transparency. The endothelium, as noted, does not regenerate its cells. When endothelial cells are lost, the remaining cells stretch to cover the gap rather than dividing, which means there is a finite limit to how much damage that layer can absorb.
Common Corneal Conditions
Keratitis is inflammation of the cornea, most commonly caused by infections related to contact lens use. Symptoms typically include redness, pain, blurred vision, and sensitivity to light. Bacterial keratitis from contaminated lenses or lens cases is among the most frequent forms, though viral infections (particularly herpes simplex) can also affect the cornea.
Keratoconus is a condition in which the cornea progressively thins and bulges outward into a cone shape, distorting vision. It usually begins in the teenage years or early twenties. In mild cases, specially fitted contact lenses can compensate for the irregular curvature. In advanced cases, a corneal transplant may be necessary.
Fuchs’ endothelial dystrophy is a gradual loss of the endothelial pump cells described earlier. As the endothelium deteriorates, the cornea swells and vision becomes increasingly blurry, often worse in the morning (because the closed eyelid during sleep further limits oxygen, compounding the swelling). It typically affects both eyes and progresses over years or decades.
Corneal Transplant Surgery
When the cornea is too damaged or diseased to function, transplantation is an option. The most established procedure is penetrating keratoplasty, in which the full thickness of the damaged cornea is replaced with donor tissue. The most common reasons for this surgery include repeat transplants (about 41% of cases), keratoconus (15%), Fuchs’ dystrophy (9%), and corneal swelling after previous eye surgeries (about 8%).
Success rates vary significantly depending on the underlying condition. At five years after surgery, keratoconus transplants have a 98% survival rate, making it one of the most successful indications. Fuchs’ dystrophy and viral keratitis transplants survive at around 85 to 86%. Repeat transplants fare less well: first regrafts survive at about 80% at five years, second regrafts at 60%, and third regrafts at 44%.
Newer techniques allow surgeons to replace only the specific damaged layer rather than the entire cornea. For conditions affecting just the endothelium, such as Fuchs’ dystrophy, only the thin back layer is transplanted. This approach generally means faster recovery and fewer complications because the front surface of the cornea remains untouched.