Embryologically, the corneal epithelium is derived from surface ectoderm between 5 and 6 weeks of gestation. The corneal epithelium is a stratified nonkeratinized squamous epithelium that is five to seven cell layer thick (40–50 μm). The superficial layer of the epithelium consists of flat polygonal cells, and deeper layers are mainly of cuboidal cells which are two to three cell layer thick. The posterior most layer is the basal layer which mainly consists of a single layer of columnar epithelium [6].

The epithelium overlies the basement membrane, which acts as a barrier between epithelial layers and the stroma. Epithelial-derived mucin layer provides protection against adhesion and entrance of antigens. The tear film also protects the corneal epithelial surface by the action of proteins in tears supplemented with the mechanical washing effects of the tear fluid and lid wiping. Thus, tear film and the corneal epithelium form symbiotic relationship anatomically as well as physiologically.

Epithelium derives its nutrition in the form of oxygen and glucose. It gets oxygen directly from the environment when the eyes are open, and when the eyelids are closed, the oxygen comes from the tear film through superficial conjunctival capillaries. The glucose mainly reaches the epithelium by diffusion from the aqueous humor.

Bowman’s layer is a specialized layer of collagen that does not regenerate after injury. It is acellular, except for the nerve axons that extend towards the epithelium.

Under electron microscopy, it appears as a feltlike composite of randomly oriented, striated collagen fibrils dispersed throughout an amorphous matrix. The anterior margin of Bowman’s layer is adjoining the lamina densa of the basement membrane of the epithelium and the posterior aspect of Bowman’s layer is contiguous with the striated collagen fibrils from the underlying stroma. Collagen fibers predominantly present in Bowman’s layer are types I and III [8].

The thickness of Bowman’s layer is 18.7 + 2.5 μm in normal eyes when measured in vivo at an A-scan rate of 100,000 scans/s. The thickness decreases from superior temporal to inferior nasal which is inversely related to that of corneal epithelium [9].

Bowman’s layer is typically hyperbolic which has the potential to influence the optical performance of the eye [10].

The corneal stroma is composed of cellular (keratocytes) and extracellular components. Keratocytes are differentiated mesenchymal fibroblasts that produce the extracellular matrix macromolecules as well as the enzymes responsible for their remodeling and degradation. New collagenase synthesis by stromal fibroblasts in and around the repair tissue is the first step in collagen degradation during long-term tissue remodeling. Extracellular matrix occupies a substantial part of the corneal stroma. It is composed of an organized meshwork of macromolecules.

The extracellular matrix is composed of fibrous proteins (collagen, laminin, and fibronectin) (the lists of collagens are shown in Table 3.1) and polysaccharide glycosaminoglycans (keratin sulfate, chondroitin sulfate, and dermatan sulfate).

Laminins are large multidomain glycoproteins located in the lamina lucida of the basal lamina. Each molecule is composed of three polypeptide chains which together form the characteristic cross-shaped laminin structure with three short arms and one long arm. They promote adhesion, growth, migration, and differentiation. Fibronectin contains two very similar polypeptide chains linked by disulfide bonds. Each chain has six domains with specific binding sites for integrins, proteoglycans, and collagen. In the unwounded cornea, fibronectin is found in the subepithelial region at the level of epithelial basement membrane and at the stromal side of Descemet’s membrane. Its primary role is to attach cells to extracellular matrix.

The cell surface proteoglycans are glycosylated proteins linked covalently to highly anionic glycosaminoglycans. The stromal matrix of human cornea contains keratan sulfate proteoglycan (KSPG) and chondroitin and dermatan sulfate proteoglycan (decorin), with KSPG being the major proteoglycan. The heparin sulfate proteoglycan (perlecan) is localized in the basement membrane. The biological functions of proteoglycans are derived from the physiochemical characteristics of the glycosaminoglycan component of the molecule from specific interactions with the ECM macromolecules through both their glycosaminoglycan and core protein components. Apart from their hydrodynamic functions, their involvement in many aspects of cell and tissue activities has been demonstrated. KSPG plays an important role in corneal transparency. It is absent or reduced in opaque corneal scars and reappears during restoration of transparency. Decorin regulates collagen fibrin formation and is a natural regulator of transforming growth factor-β (TGF-β) activity.

The extracellular matrix (ECM) plays an active and complex role in the regulation of cells, influencing their development, migration, proliferation, shape, and metabolic functions, in addition to providing a scaffolding to stabilize the physical structure of the tissue. Matrix molecules are constantly being remodeled, degraded, and resynthesized during development. During wound healing, also, there is degradation and resynthesis of matrix components. Regulating the balance of synthesis and degradation of ECM is crucial for normal embryogenesis and growth and for the repair and maintenance of proper tissue architecture [13].

The corneal stroma is made up of the lamellae of collagen bundles, which are arranged parallel to each other and also to the central corneal surface. The thickness of the stroma is about 500 μm [14]. These fibers run between the limbus and interweave with each other. The interweaving is greater in the anterior third of the stroma than the posterior [15].

It is the basement membrane of the corneal endothelium and is synthesized by the endothelium. It is composed of collagen type IV and VIII.

The corneal endothelium is embryologically derived from the neural crest. It is composed of a single layer of cells forming the posterior surface of the cornea. Anteriorly it is attached to the rest of the cornea by Descemet’s membrane, and posteriorly it is contiguous with the anterior chamber of the eye [16].