Understanding the Cornea: Anatomy and Function

The layers of the cornea include epithelium, bowman’s layer, stroma, descemet’s membrane, and endothelium.

• Epithelium

The epithelium is the outermost layer of the cornea. The epithelium blocks the passage of foreign particles such as dust, water, and bacteria into the eye or other layers of the cornea. It also absorbs oxygen and cell nutrients from tears and then distributes these nutrients to the rest of the cornea. The epithelium is filled with thousands of tiny nerve endings, making the cornea extremely sensitive to pain when rubbed or scratched. The epithelium is a 5–6-layer structure with three types of cells: superficial cells, wing cells, and basal cells.

• Bowman’s layer

Bowman’s layer is the transparent sheet of tissue lying directly below the basement membrane of the epithelium. It is composed of strong layered protein fibres called collagen. Once injured, the bowman’s layer can form a scar as it heals. Some vision loss can occur if these scars are large and centrally located. 

• Stroma

Stroma lies beneath the Bowman’s layer. It comprises 78 percent of water and 16 percent of collagen. Collagen gives the cornea strength, elasticity, and form. The collagen’s unique shape, arrangement, and spacing are important to produce the cornea’s light-conducting transparency.

• Descemet’s Membrane

Descemet’s membrane is a thin, strong sheet of tissue that serves as a protective barrier under the stroma. It is a protective barrier against infection and injuries. Descemet’s membrane comprises collagen fibers of endothelial cells that lie below it. 

• Endothelium

The endothelium is extremely thin and the innermost layer of the cornea. Generally, there is a fluid inside the corneal layer and endothelium pumps this excess fluid out of the stroma. Without this pumping action, the stroma would swell with water and become hazy and opaque.  A healthy eye requires a perfect balance between the fluid moving into the cornea and fluid being pumped out of the cornea. Once endothelium cells are destroyed by disease or trauma, they are lost forever. If too many endothelial cells are destroyed, corneal edema and blindness ensue, with corneal transplantation the only available therapy.