CORNEA Anatomy, Physiology and Pathology by Joseph Bacotti, MD, FACS After completion of this course the reader should be able to: 1. Describe the anatomy of the cornea and surrounding tissues 2. Describe each of the layers of the cornea 3. Describe the functions of the corneal tissues in relation to vision and a healthy eye 4. Describe the shape of the cornea and how this relates to vision 5. Discuss how the corneal shape relates to contact lens fitting and vision 6. Describe various disease states of the cornea ANATOMY of the CORNEA The cornea is the anterior most part mm less horizontally and vertically. The cornea topography is curved on of the eyeball. It comprises The thickness of the cornea varies the anterior and the posterior approximately 1/6 of the total from center to limbus, with the surfaces, ellipsoid or aspheric in eyeball. The cornea is the thinnest area centrally of shape on the anterior surface and transparent, optical portion of the approximately 550 microns thick more spherical on the posterior. eye. The word cornea is derived for and increasing to 660 microns of The anterior surface gradually Latin meaning horny. The cornea is thickness at the limbus. flattens as we move from the an avascular, transparent tissue geometric center of the cornea to the composed of an anterior convex The cornea has several centers; the edges at the limbus. The surface and a posterior concave optical center is where the cornea differences in the anterior and the surface that accounts for the optical curvature is greatest and usually posterior curvatures forms a lens properties it possesses. corresponds to the apex of the based on Snell’s law, the law of cornea or the area of greatest refraction, that the difference of the The main purpose of the cornea is curvature. The visual center is the 2 refractive indices of air and the optical to allow refraction of light actual area of the cornea that the corneal surface refractive index of entering the eye and the cornea patient uses for vision function with 1.3375 used for most common accounts for approximately 70% of the light passing thru to the fovea. calculations of eye/cornea refractive the refractive power of the eye as a The geometric center is the exact power. The anterior surface, whole (43.50D). Clearly there are middle where the point is elliptical and flattening from the other functions of the cornea; equidistant from all edges of the optical center to the limbal protection of the eye, maintaining cornea. The centers may not be and periphery can be considered as structural integrity of the eye globe, are usually not all located at the 45.00 diopters or 7.50 mm. The to keep the inner contents in place same point on the corneal surface. posterior radius of curvature for the and protected. The steepest area or apex (and cornea varies in individuals and can usually the thinnest area) is located range from 6.40mm to 6.72mm or slightly down and nasal from the about 52.00 diopters. The dimensions of the cornea are geometric or visual center. The calculated from the limbus area thickness of the cornea at various Changes in the flattening of the where the cornea joins the sclera areas is important when corneal surfaces along differing and conjunctiva. There are blood consideration is given for laser meridians from that of a sphere vessels in the limbus that extends vision correction, to assure that leads to a condition called 360 ° around the cornea, and the there is sufficient thickness and astigmatism. With the rule blood vessels terminate at the calculating glaucoma Goldman astigmatism is when the vertical limbus in loops returning to the applanation tonometry correction meridian of the cornea is steeper sclera in healthy eyes. The overall factor for intraocular pressure than the horizontal. A minus shape of the cornea anteriorly is cylinder lens in front of the eye elliptical. Anteriorly, the cornea The cornea is an optical system, place at the horizontal meridian measures vertically about 10.8mm refraction of light, which is needed compensates for with the rule (10-11) and is slightly larger in for sharp vision, the cornea would astigmatism; or a plus lens at the males about 11.5 mm. Horizontally need to be clear. No scars, good vertical meridian or axis. the cornea measures approximately clear transparency, avascular, Conversely, an against the rule 11.5 mm (11-12). The posterior of maintaining a deturgescent state of astigmatism would have the steeper the cornea is usually more spherical hydration, and have a high meridian on the horizontal corneal and measure 11.7 mm. In infants refractive power to allow focusing plan and would be corrected by a the corneal dimensions are about 1 of the light on the retina. 1 minus cylinder lens at the 90 axis or laser vision correction known as a plus at the 180. PRK or PhotoRefractive Keratectomy. Overlying the cornea is the precorneal tear layer. The tear layer serves several purposes and is The stroma is bordered on its actually composed of 3 layers that anterior side by Bowman's each serves a specific function for membrane, a band of randomly the eye and cornea. Where the arranged collagen that, under the epithelial cells abut against each light microscope, has no resolvable other there is a junction and structures. irregularity created. These irregularities are not optically Epithelium The lamina propria or stroma compatible with sharp vision. The constitutes 80% to 90% of the total tear layer fills in the spaces and Bowman’s membrane corneal thickness (about evens out the surface of the cells Stroma 500microns of the total 550 microns and actually becomes the new front Descement’s membrane of the cornea) and consists of about surface of the cornea. The outer 200 lamellae of collagen fibrils most layer of the tear layer is the Endothelium running parallel with the surface oily or lipid component and serves and superimposed upon one another to prevent evaporation of the watery at right angles. There is little or no component of the tear layer. The The anterior surface of the cornea is interweaving of these ribbon like lipids spread evenly across the tear covered by five or six layers of bands, accounting for the ease with film providing a smooth surface and epithelial cells that are continuous which the cornea can be surgically good optics. The meibomian glands with those of the conjunctiva, split into thin layers. Flattened openings are found along the lid except that there are no conjunctival stromal cells (keratocytes) lie margins and the glands are arranged goblet cells. The corneal cells between the collagen lamellae. The in the lids along more or less become more numerous from the keratocytes or fibroblasts are cells parallel rows. center of the cornea to the limbus, that remain more or less dormant where they are approximately 12-13 until an injury occurs then they aid The middle layer of the tear film is cell layers thick. The epithelium in repairing the tissue. The stroma aqueous or watery in nature and has stratified, squamous and non- blends into the sclera at the limbal contains salts and electrolytes and it keratinized cells. Basal cells are the area. The arrangement of the is the major part of the tear layer as cells that reform-regenerate the stromal collagen fibrils aids in the a whole. The tears help wash away epithelium when injured. clarity of vision. debris and contain an antibacterial Desmsomes form lateral adhesion agent to help ward off infections between the cells and the Zona Descement’s membrane is a called lysozymes enzymes. Tears Occludens form tight juncture at the regularly arranged in layers of fine protect the eyes from drying and surface cell layers in addition to the collagen filaments. lubricate the surface of the eye to Desmosomes. Healing of injuries The most posterior collagen lamella allow blinking. The tears provide occurs by mitosis and migration. of the stroma merges with the thick nutrition and oxygen to the Mitosis is by cell division that takes basement membrane (Descemet's avascular cornea as well. Tears are place near the limbus and moves membrane) of the endothelium. secreted mainly by the lacrimal centrally and migration the moving gland. of adjacent cells over the defect in The endothelial cells form a single the cornea. Superficial injuries to layer of cells separating the stroma The layer the closest to the the cornea, abrasions usually heal in and Descement’s membrane from epithelial corneal cells is the mucin 24 to 48 hours. the aqueous humor. The endothelial layer, which is secreted by the cells are more numerous at birth, conjunctival goblet cells, the mucin numbering approximately 6000 per helps to stabilize the tear layer. Bowman’s membrane is the square millimeter. The endothelial basement membrane for the cell count decreases rapidly to about epithelial basal cells and lies 4000 per square millimeter at age 5 between the epithelium and the years and gradually decreases with stroma. If an injury or disease age. Injury and disease can cause a process penetrates the Bowman reduction in endothelial cells, and membrane then usually a scar will the cells adapt by enlarging to cover form. There are no cells in the area of cells lost to disease or Bowman’s membrane. The injury. At around 400 to 500 cells epithelial cells can easily be scraped per square millimeter signs of or separated from Bowman’s. corneal edema are seen resulting in This is done for disease states and microcystic or Bullous keratopathy.