The cornea: structure and protection of the eye

The cornea is the transparent anterior portion of the fibrous membrane of the eye, located in front of the anterior chamber, iris, and pupil, and transitioning into the sclera at the limbus. Its key property is the combination of transparency and mechanical strength, which allows it to simultaneously protect internal structures and transmit light. [1]

In the eye's optical system, the cornea accounts for the majority of refraction: approximately 65%-75% of the total refraction, and the average optical power of the anterior surface is approximately 43 diopters. Therefore, surface microroughness and even moderate swelling can significantly degrade image quality. [2]

The cornea is avascular, not a "deficiency" but a prerequisite for transparency. Nutrients are supplied by diffusion from the tear film and aqueous humor, as well as from the limbal vessels, while the central optic tract itself remains avascular. [3]

The epithelium also plays a significant role in protecting the eye: it acts as a barrier to microbes and chemicals, and its smooth surface, together with the tear film, forms the air-tear "optical interface." This explains why dry eye or erosion often causes stinging, photophobia, and a "film" even in the absence of deep damage. [4]

Geometry and reference parameters: dimensions, thickness, optical power

The shape of the cornea resembles a convex-concave lens: the anterior surface is usually steeper, while the posterior surface is flatter. Normally, it is wider horizontally than vertically, and it is this geometry that contributes to the formation of astigmatism and individual differences in refraction. [5]

Reference dimensions in adults: horizontal diameter 12-12.5 mm, vertical about 11 mm. These numbers are important not only for anatomy, but also for interpreting congenital and acquired conditions where the cornea may be enlarged or reduced. [6]

The cornea's thickness varies: approximately 540 µm in the center, but usually greater near the limbus. Central thickness is used as a practical biomarker: it influences the assessment of intraocular pressure and is involved in safety calculations for refractive surgery. [7]

The cornea's optical power is determined by the contributions of the anterior and posterior surfaces. On average, the air-tear interface contributes approximately 43 diopters, with the posterior surface having a smaller contribution of the opposite sign, which is important in modern calculations for artificial lenses and in topography analysis. [8]

Table 1. Reference parameters of the cornea in adults

Parameter	Typical value
Horizontal diameter	12-12.5 mm
Vertical diameter	about 11 mm
Central thickness	about 540 microns
Thickness near the limb	700 µm - 1.0 mm
Epithelial thickness	about 50 microns
Bowman's membrane thickness	about 10 microns
Air-tear optical power	about 43 dioptres

[9]

Anterior surface: tear film, epithelium and limbus

The most "optical" part of the cornea is not only the tissue but also the tear film on its surface. With each blink, tears are distributed across the epithelium, maintaining smoothness, hydration, particle removal, and oxygen delivery, especially in the open area of the palpebral fissure. [10]

The corneal epithelium is a stratified, nonkeratinizing squamous epithelium that renews rapidly and typically heals without scarring unless damage extends deeper. The barrier properties of the epithelium rely on tight junctions and a basement membrane. [11]

The key "service center" of the epithelium is the limbus, the transition zone between the cornea and the sclera. Limbal epithelial stem cells are located here, maintaining constant surface renewal; their deficiency leads to chronic epithelial defects, conjunctival ingrowth onto the cornea, and pathological vascular reactions.

The limbal border is also important as an anatomical barrier for blood vessels: normally, the vessels terminate at the limbus, leaving the central cornea transparent and avascular. When this barrier is "broken" by inflammation or chemical burns, the vessels and scarring dramatically impair optics. [13]

Table 2. Anterior surface of the cornea: elements and functions

Element	Main function	What happens when a crash occurs?
Tear film	Optical surface smoothness, hydration, oxygen	"Veil", burning, unstable vision
Epithelium	Barrier, rapid regeneration, protection from microbes	Erosions, severe pain, photophobia
Basement membrane of the epithelium	"Base" for cell attachment	Recurrent erosions in violation
Limbal stem cells	Constant renewal of the epithelium	Chronic defects, vascularization
Limbo	Transition to the sclera, vascular "border"	Growth of blood vessels in the cornea during inflammation

[14]

Stroma and posterior surface: strength, membranes and endothelium

The stroma makes up the bulk of the cornea and is its main "framework." It is composed of collagen fibrils organized into lamellae and an intercellular matrix containing glycosaminoglycans; this ordering helps maintain both shape and transparency. [15]

Above the stroma lies Bowman's membrane, which is conveniently understood as the dense, acellular anterior layer of the stroma. It rarely regenerates as a "layer," so trauma with damage to Bowman's membrane more often leaves a scar and optical opacity than superficial epithelial erosion. [16]

Descemet's membrane and the endothelium are located on the anterior chamber side. Descemet's membrane is the basement membrane of the endothelium and thickens over time; the endothelium is represented by a monolayer of hexagonal cells and is critical for controlling stromal hydration. [17]

Endothelial cells in adults regenerate in a limited manner: when cells are lost, the remaining cells enlarge and "stretch," closing the defect, but overall density decreases with age. When density becomes too low, pumping function fails, leading to stromal edema and persistent loss of transparency. [18]

The pre-Descemet layer, known as the Dua layer, is discussed in modern literature: it is described as a robust, acellular zone at the junction of the posterior stroma and Descemet's membrane and is considered a possible factor in the biomechanics and surgical techniques of posterior layer-by-layer transplantation. However, some experts interpret it as a specialized part of the posterior stroma, so it is more accurate to consider it as a "clinical-surgical" concept rather than a completely separate, essential layer in all people. [19]

Table 3. Layers of the cornea from outside to inside: composition and regeneration

Layer	Approximate thickness	Key role	Recovery potential
Epithelium	about 50 microns	Barrier, smoothness, protection	High
Bowman's membrane	about 10 microns	Front surface strength	Low, prone to scarring
Stroma	about 90% of the thickness	Optics and mechanics	Average, depends on the depth of damage
Pre-Descemet layer (Dua layer)	discussed, subtle	Biomechanics of the posterior region, surgical significance	Limited regeneration as a matrix
Descemet's membrane	10-12 µm in adults	Endothelial support, barrier	Partially restored as a matrix
Endothelium	monolayer of cells	Hydration control, transparency	Limited in adults

[20]

Transparency and nutrition: where do oxygen and glucose come from, and why doesn't the cornea swell?

Corneal transparency is determined by several factors simultaneously: the absence of vessels in the central zone, the ordered architecture of collagen fibrils in the stroma, and strictly controlled hydration. If water "overflows" the stromal matrix, the distances between the fibrils change, light scattering increases, and cloudiness appears. [21]

The cornea is nourished by diffusion. On the surface, oxygen and some dissolved substances come from the tear film, while on the inner surface, glucose and other metabolites come from the aqueous humor of the anterior chamber; additional contributions are made by the limbal vessels for peripheral zones. [22]

The "pump and leak" model explains the stability of hydration. A small "leak" of fluid and salts from the anterior chamber into the stroma is physiological, and the endothelium creates a directed ion transport that "pulls" water back into the aqueous humor, maintaining the stroma in a slightly dehydrated state necessary for transparency. [23]

Practical implication: any conditions damaging the endothelium or Descemet's membrane are highly likely to result in edema and "rainbow halos" around light sources. Superficial epithelial damage more often results in pain and photophobia, but with intact endothelium, transparency is usually restored more quickly. [24]

Table 4. Corneal nutrition sources and what they “bring”

Source	What substances are predominantly supplied?	When it is especially significant
Tear film	Oxygen, some electrolytes, protective factors	Open area of the cornea, condition of the eyelids and tear production
Aqueous humor of the anterior chamber	Glucose and metabolites, electrolytes	Posterior layers, endothelial support
Vessels of the limbus	Nutrition of the periphery, immune components	Peripheral cornea, healing at the limbus
Nerve fibers	Neurotrophins	Support of epithelium and sensitivity

[25]

Innervation and clinical consequences of the structure: sensitivity, healing, examination

The cornea is one of the most sensitive tissues in the body: it is innervated by long ciliary nerves from the ophthalmic branch of the trigeminal nerve, forming stromal, subepithelial, and epithelial neural networks. The high density of nociceptors explains why even a small erosion can be extremely painful. [26]

Nerve trophism is no less important than pain sensation. When sensitivity is reduced (for example, after herpetic keratitis, surgery, or neuropathies), neurotrophic keratopathy can develop: the epithelium heals poorly, forming persistent defects that threaten infection and scarring. [27]

Corneal layer specificity helps clinically "read" the problem by depth. Superficial processes more often cause pain, lacrimation, and a foreign body sensation; stromal processes more often cause clouding and astigmatism; endothelial insufficiency more often causes morning "foggy" vision and signs of edema. [28]

Modern corneal assessment relies on a combination of methods: pachymetry for thickness, keratometry and topography for curvature, optical coherence tomography (OCT) for layer-by-layer visualization, and endothelial microscopy for endothelial density and morphology. These measurements link anatomical facts to the risk of edema and to the choice of tactics in refractive and transplant surgery. [29]

Table 5. "If a layer suffers" - what changes most often

Affected area	What usually gets worse first?	Typical anatomical cause
Tear film and epithelium	Pain, photophobia, unstable vision	Loss of smoothness and barrier function
Bowman's membrane	Post-traumatic clouding	Tendency to scar when damaged
Stroma	Clouding, astigmatism, deformation	Disruption of lamellar structure and hydration
Descemet's membrane and endothelium	Edema, rainbow circles, persistent veil	Hydration control failure using the pump-and-leak model
Limbal stem cells	Chronic epithelial defects, vascularization	Loss of the source of epithelial renewal

Table 6. Corneal examination methods and what they show

Method	What does it measure or show?	Why is it used?
Pachymetry	Corneal thickness	Edema assessment, calculations before surgery
Keratometry and topography	Curvature and regularity of the surface	Diagnosis of keratoconus and astigmatism
Optical coherence tomography (OCT)	Layered structure	Control of layers, scars, swelling, after operations
Endothelial microscopy	Density and shape of endothelial cells	Assessment of the risk of decompensation and planning of interventions
Slit lamp examination	Epithelium, stroma, deposits, vessels	Basic clinical assessment and dynamics

[31]