Corneal endothelial cells lie on the posterior surface of the cornea, composing a monolayer of closely interdigitated cells arranged in a mosaic pattern of mostly hexagonal shapes (see Fig 1-4). Human endothelial cells are thought to have limited proliferation in vivo, but they can divide in cell culture. If cell loss occurs, especially as a result of trauma or surgery, the defective area is covered via enlargement and spread of residual cells or perhaps peripheral stem cells. These cell findings can be observed by specular microscopy as polymegethism (variability in cell size) and polymorphism (variability in cell shape). Cell density varies over the endothelial surface; normally, the concentration is highest in the periphery. Central endothelial cell density decreases with age at an average rate of approximately 0.6%/year, diminishing from a count of about 3400 cells/mm2 at age 15 years to about 2300 cells/mm2 at age 85 years. The normal central endothelial cell count is between 2000 and 3000 cells/mm2. It has been observed that eyes with an endothelial cell count below 500 cells/mm2 may be at risk for development of corneal edema. The endothelium maintains corneal transparency by controlling corneal hydration and maintaining stromal deturgescence through its functions as a barrier to the aqueous humor and as a metabolic pump that moves ions, and draws water osmotically, from the stroma into the aqueous humor. The barrier and pump functions of the endothelium can be measured clinically by fluorophotometry and pachymetry. The endothelium must also be permeable to nutrients and other molecules from the aqueous humor. Increased permeability and insufficient pump sites occur with reduced endothelial cell density, although the cell density at which clinically evident edema occurs is not an absolute.
For more detailed information on the histology and physiology of the cornea, see BCSC Section 2, Fundamentals and Principles of Ophthalmology, Chapter 8.
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Nishida T, Saika S, Morishige N. Cornea and sclera: anatomy and physiology. In: Mannis MJ, Holland EJ, eds. Cornea. Vol 1. 4th ed. Philadelphia: Elsevier; 2017:1–22.
Whikehart DR, Parikh CH, Vaughn AV, Mishler K, Edelhauser HF. Evidence suggesting the existence of stem cells for the human corneal endothelium. Mol Vis. 2005;11:816–824.
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.