Fundamental importance of the cornea for the eye: The cornea is the eye's optical window that makes it possible for humans to see. The ophthalmologist is only able to discern structures in the interior of the eye because the cornea is transparent. At 43 diopters, the cornea is the most important refractive medium in the eye.
Shape and location: The cornea's curvature is greater than the sclera's curvature. It fits into the sclera like a watch-glass with a shallow sulcus (the limbus of the cornea) marking the junction of the two structures.
Embryology: The corneal tissue consists of five layers. The cornea and the sclera are formed during the second month of embryonic development. The epithelium develops from ectoderm, and the deeper corneal layers develop from mesenchyme.
Morphology and healing (Fig. 5.1):
❖ The surface of the cornea is formed by stratified nonkeratinized squamous epithelium that regenerates quickly when injured. Within a hour, epithelial defects are closed by cell migration and rapid cell division. However, this assumes that the limbus stem cells in the limbus of the cornea are undamaged. Regular corneal regeneration will no longer be possible when these cells are compromised. An intact epithelium protects against infection; a defect in the epithelium makes it easy for pathogens to enter the eye.
❖ A thin basement membrane anchors the basal cells of the stratified squamous epithelium to Bowman's layer. This layer is highly resistant but cannot regenerate. As a result, injuries to Bowman's layer usually produce corneal scarring.
❖ Beneath Bowman's layer, many lamellae of collagen fibrils form the corneal stroma. The stroma is a highly bradytrophic tissue. As avascular tissue, it only regenerates slowly. However, its avascularity makes it an immunologically privileged site for grafting. Routine corneal transplants may be performed without prior tissue typing. An increased risk of rejec-
tion need only be feared where the recipient's cornea is highly vascularized as may be the case following chemical injury or inflammation. Such cases require either a tissue-typed donor graft or immunosuppressive therapy with cyclosporin.
❖ Descemet's membrane and the corneal endothelium lie on the posterior surface of the corneal stroma adjacent to the anterior chamber. Descemet's membrane is a relatively strong membrane. It will continue to define the shape of the anterior chamber even where the corneal stroma has completely melted (see Descemetocele). Because it is a genuine basement membrane, lost tissue is regenerated by functional endothelial cells. The corneal endothelium is responsible for the transparency of the cornea (see also Transparency below). A high density of epithelial cells is necessary to achieve this. The corneal endothelium does not regenerate; defects in the endothelium are closed by cell enlargement and cell migration.
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