Cornea: anatomical and functional features, new methods of in vivo diagnostics of abnormalities

The cornea is a highly organized, transparent part of fibrous tunic of an eyeball. It acts as the primary infectious and structural barrier of the eye. The cornea is the major refractive element of an adult eye. It consists of epithelium, Bowman's membrane, stroma, Descemet's membrane and endothelium. Although the normal human cornea is avascular, it is supplied via perilimbal blood vessels, the aqueous humor (AqH) and tear film. Afferent innervation to the cornea is provided by long ciliary nerves, which form subepithelial and subbasal nerve plexus. Epithelium is a stratified, non-keratinizing squamous layer that consists of various cell types. Epithelial cells are connected to each other by zonula adherens, and to the basement membrane via hemidesmosomes. Bowman's membrane is composed of randomly-oriented type I and V collagen fibrils and anchoring type IV and VII collagen fibrils. The stroma consists of cells (principally keratocytes) and distinct lamella formed by collagen fibers, proteoglycans, elastin and glycoproteins. Descemet's membrane is a basal membrane, secreted by endothelial cells. It is a network organized by type VIII collagen molecules, which modulate the passage of growth factors, cytokines and nutrients from the AqH into the corneal stroma and backward. Corneal endothelium is a monolayer of hexagonal cells tightly adherent to one another. In vivo endothelial cells are arrested in G1-phase of cell cycle. The endothelium forms a physiological barrier between the nutrient-rich AqH and the corneal stroma. Tight and gap cell junctions and dynamic pump-leak system maintains corneal deturgescence and permit sufficient nutrient delivery into the stroma and epithelium. Disruption of the endothelial cells results in corneal edema. Modern non-contact real-time imaging of the cornea include specular microscopy, optical coherence tomography and in vivo confocal laser scanning microscopy. These methods can help to visualize corneal layers (during keratorefractive surgery, pre- and postoperative periods), detect localization and etiology of pathological changes.