The cornea is composed of four layers; the outer epithelium, stroma, Descemet’s membrane, and endothelium. The healthy cornea should be clear in order to refract sufficient quantity and quality of light into the eye. This light is then focused to form an image on the retina. Clarity is maintained by specific characteristics of each of the four layers. 

Epithelium 

The corneal epithelium is composed of non-keratinized stratified squamous cells. The epithelium is lipophilic, working as a barrier to the underlying stroma to maintain clarity. Absence of pigment, and vessels are also important for maintaining clarity and vision in the cornea. The entire epithelium has a turnover rate of approximately seven days and undergoes a constant cycle of proliferation and shedding in a healthy state. Healing of the entire corneal epithelium can occur within 72 hours in some species when damaged. Damage to this layer can result in focal or diffuse corneal edema from a decreased barrier to fluid. Chronic inflammation may result in vascularization or pigmentation of the cornea, thus impacting vision. 

Related: Get our Guide to Corneal Disease

Stroma 

The corneal stroma makes up approximately 90% of corneal thickness. This layer is hydrophilic and will swell with water uptake, reducing clarity. Blood vessels should not be present, though there is sensory innervation to the outer third of the cornea. This innervation is the cause of extreme pain for corneal ulceration. This layer is made of collagen fibrils arranged in a parallel fashion. Stromal replacement occurs after the epithelium works to cover defects. Over time, proliferation of collagen occurs to restore the corneal curvature. Disorganized arrangement upon healing may result in scarring and decreased vision and inflammation leading to vascularization of this layer may also impact vision. 

Descemet’s Membrane 

Descemet’s membrane is the basal lamina of the endothelial layer. This layer has mild elastic properties and does not retain fluorescein dye. In health, this layer becomes thicker throughout an animal’s life and during corneal healing, endothelial cells are able to produce new Descemet’s membrane after several weeks of healing for full thickness injuries.  

Endothelium 

The endothelium is the last layer consisting of a monolayer of hexagonal cells that do not undergo mitosis. Similarly to the epithelium, this layer is a barrier to the stroma to maintain corneal clarity which is aided by tight cellular junctions. Healing of this layer is accomplished via cell enlargement and migration or sliding. This results in decreased cell density of the endothelium after healing and with age. The endothelium contains Na+,K+ -ATPase ion pumps which work to remove fluid from the stroma to regulate its hydration. Thinning or damage of the endothelium can affect clarity as the hydration status of the stroma increases, resulting in corneal edema. Corneal endothelial dystrophy is an inherited condition leading to corneal stromal and epithelial edema. This can cause bullae, painful ulceration, and pigmentation in addition to the edema that reduces clarity and vision. Similar sequelae can be observed in older dogs that develop corneal epithelial degeneration as this corneal layer thins with age.  

References 

Gelatt, Kirk N. Veterinary Ophthalmology. Blackwell, 2013. 

Gelatt, Kirk N., and Janice P. Gelatt. Veterinary Ophthalmic Surgery. Saunders Ltd., 2011. 

Maggs, David J., et al. Slatter's Fundamentals of Veterinary Ophthalmology. Saunders Elsevier, 2008.