Important functions of the cornea in the eye include protecting the structures inside the eye, contributing to the refractive power of the eye, and focusing light rays on the retina with minimum scatter and optical degradation. Considerable advances have taken place in understanding the organization of collagen in the corneal stroma and its clinical significance. In this review, the structure and function of various components of the cornea and ocular surface are presented.
Cornea and sclera constitute the outer covering or coat of the eyeball. The main purpose of this coat is to protect structures inside the eye. The cornea is a transparent avascular tissue that acts as a structural barrier and protects the eye against infections. Along with the tear film, it provides a proper anterior refractive surface for the eye. Cornea contributes to two-thirds of the refractive power of the eye.
The cornea is horizontally oval, measuring 11–12 mm horizontally and 9–11 mm vertically. The horizontal corneal diameter (white to white) using the ORBSCAN II system has revealed an average corneal diameter of 11.71 ± 0.42 mm. The average corneal diameter was 11.77 ± 0.37 in males compared to 11.64 ± 0.47 for females. The corneal diameter ranged from 11.04–12.50 in males and 10.7–12.58 in females.
The limbus is widest in the superior and inferior cornea. The cornea is convex and aspheric. The anterior curvature is 7.8 mm, and the posterior curvature is about 6.5 mm.
Cornea contributes to about 40–44 D of refractive power and accounts for approximately 70% of total refraction. The refractive index of the cornea is 1.376. There is a gradual increase in thickness from the central cornea to the periphery.
Alteration in tissue thickness is due to increase in the amount of collagen in the peripheral stroma. With different methods of evaluation, the central corneal thickness in normal eyes is found to range from 551 to 565 μ and the peripheral corneal thickness from 612 to 640 μ.
Thickness Of Cornea
The corneal thickness is found to decrease with age. Anterior corneal stromal rigidity appears to be particularly important in maintaining the corneal curvature. Anterior curvature resists changes to stromal hydration much more than posterior stroma.
The cornea is made up of cellular and acellular components. The cellular components include the epithelial cells, keratocytes, and endothelial cells. The acellular component includes collagen and glycosaminoglycans. The epithelial cells are derived from epidermal ectoderm. The keratocyte and endothelial cells are derived from neural crest. The corneal layers include epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium.
Recently, a layer of the cornea which is well defined, acellular in pre-Descemet's cornea is getting attention with the development of lamellar surgeries. Summarizes all layers of the cornea with their functions.
The minute surface irregularities of the corneal surface epithelium are masked by a smooth and regular overlying tear film. The tear film is the first layer of the cornea with which light comes into contact. In produces lubrication and hydration to the ocular surface. It is also a source of oxygen, immunoglobulins, lysozymes, lactoferrin, and α- and β-defensins. The tear film is traditionally told to be composed of three distinct layers.
Thoft and Friend first suggested that cornea, conjunctiva, lacrimal glands, and lids work as an integrated unit called ocular surface. The anatomical ocular surface is composed of the mucosa that lines the globe and palpebral surfaces, the corneoscleral limbus, the corneal epithelium, and the tear film.
Sclera, in contrast to the cornea, has collagen fibrils which are more haphazard and random. The bulk of stroma is mostly acellular, except for outer episcleral layer. Within the tarsus are numerous specialized sebaceous glands called the meibomian glands opening at the lid margin.
Their secretions are the main source of the superficial lipid layer of the tear film. Each eyelash follicle is also associated with sebaceous glands called the glands of Moll. In addition, there are associated apocrine glands called the glands of Zeis.