3. *Outermost layer of the
cornea
*Smooth corneal surface
*Cells regenerate (only layer
of the cornea that does this)
*
4. *Second layer of the cornea
*Is made up of a dense fibrous
sheet of collagen fibers
*A transition layer between the
epithelium and stroma
*Does not regenerate
*
5. *Accounts for 90% of total
thickness (largest layer)
*Fibers in this layer contribute
to corneal transparency
*
6. *Considered basement
membrane to the Endothelium
*Produces constantly, which
means it thickens over one’s
lifetime (will approximately
triple in thickness over the
course of your life)
*
7. *Innermost layer of cornea
*Composed of one single layer of
cells
*Cells do not divide or replicate
*Separates the cornea from the
anterior portion of the eye
*
The total refractive power of the eye is between 60-65 diopters. The cornea makes up between 43-48 diopters of this. The remainder is from the crystalline lens. Because of this, it is extremely important for the cornea to remain transparent, and intact. The cornea measures approximately 12mm vertically, and 11mm horizontally. The steepest part is in the center, and the flattens toward the periphery. Astigmatism basically means that the cornea is not always spherical like a basketball, rather it is steeper in one meridian, and flatter in another, like the end of a football. Five layers make up the cornea. We will review each layer in more detail, starting with the outermost layer (anterior to posterior).
The epithelium is VERY thin (5-7 cells thick). Important to know, it is the only layer of the cornea that can regenerate cells. The amount of time for the entire epithelium to regenerate is approximately 7days. Minor abrasions, or damage to the outermost layer of cells (called wing cells), heal within hours. The innermost layer of cells (called basal cells) are firmly attached to the next layer of the cornea (Bowman’s membrane). If this attachment is broken, it is difficult for these cells to reattach. This is called a RECURRENT EROSION. (We will talk more about this condition later).The epithelium also helps to control the amount of water in the stroma.The tear film is what creates a smooth optical surface. The smooth corneal surface depends of the replacement of surface cells that are constantly being shed into the tear film. The epithelium has a rich nerve supply. Because of this, it is extremely sensitive. (Good thing it heals so quickly)
Unsure whether Bowman’s Layer is even necessary to maintain corneal function. No long-term effects have been documented in patients where the Bowman’s layer was not intact (because of photo-refractive keratoplasty, PRK).
The stroma is composed of collagen fibers that form flat bundles called lamellae (pronounced la-mel-i). These bundles contain straight collagen fibrils arranged with regular spacing described as “latticework.” Regular arrangement of these fibrils allows light to pass through them without being absorbed. This accounts for the transparency of the cornea. If anything occurs to this regular pattern (injury), the cornea becomes cloudy, losing it’s transparency and light cannot enter the eye. *Remember the cornea makes up 43-48 D of refractive power of the eye.*The stroma contains 78% water (called partial hydration). It must maintain this amount to stay “normal.” If the water content increases, the stroma swells, and becomes cloudy. If it decreases, the cornea thins (called dehydration). How does the stroma stay at a water content of 78%? The epi and endo-thelium layers both play a part in keeping the water content intact. Stromal cells DO NOT regenerate.
The purpose of the endothelium is to allow oxygen to travel from the aqueous to the stroma, and to keep excessive aqueous out of the stroma. Cells in the Endothelium are polyhedral (5-7 sided). However, 70-80% are hexagonal (6 sided). The regular arrangement of these cells is described as the endothelial mosaic. Because cells in the Endothelium do not divide or replicate, if there is ever a defect or loss of cells, the cells migrate and spread out. This results in pleomorphism (cell loss) or polymegathism (change/variability in size).
