The corneal endothelium, located posterior to the Descemet membrane, is a monolayer of hexagonal cells with a diameter of 20 μm. In young adult eyes, the normal endothelial cell count is approximately 3000/mm2 centrally. The number of endothelial cells is higher in the periphery and decreases with age, with concomitant spreading and thinning of the remaining cells. The rate of physiologic corneal endothelial cell loss with normal aging has been reported to be 0.6% per year (Fig 8-6, Table 8-1).
Adjacent endothelial cells interdigitate in a complex way and form a variety of tight junctions, serving as a barrier to aqueous humor penetration, but desmosomes are never observed between normal cells. Approximately 20–30 short microvilli per cell extend from the apical plasma membrane into the aqueous humor. The endothelium functions both as a permeability barrier between the aqueous humor and the corneal stroma and as a pump to maintain the cornea in a dehydrated state by generating negative hydrostatic pressure, which also serves to hold free corneal flaps (eg, LASIK flaps) in place. The endothelium utilizes temperature-dependent Na+,K+-ATPase to maintain the hydration of the stroma at 78% and sustain corneal clarity. In vivo, the endothelium derives sufficient oxygen from the aqueous humor to maintain normal pump function.
If the endothelium is injured, healing occurs mainly via migration, rearrangement, and enlargement of the residual cells. Substantial cell loss or damage results in irreversible edema because human corneal endothelial cells have limited ability to divide after birth. Infiltration of polymorphonuclear leukocytes in response to severe corneal injury can induce endothelial cells to become fibroblastic and to synthesize a retrocorneal fibrous membrane (RCFM). RCFM forms between the Descemet membrane and the corneal endothelium and causes a significant decrease in visual acuity. Unlike normal corneal endothelial cells, which accumulate a limited amount of type I collagen protein, the fibroblastic cells isolated from the RCFM predominantly express type I collagen.
Figure 8-6 Corneal endothelium. Endothelial cells do not replicate. Over time, adjacent cells increase in size to accommodate for age-related endothelial cell loss. Left: Specular micrograph of the cornea of an 18-month-old infant. Right: Specular micrograph of the cornea of a healthy 74-year-old man.
(Modified with permission from Spalton DJ, Hitchings RA, Hunter PA, Tan JCH, Harry J. Atlas of Clinical Ophthalmology. 3rd ed. St Louis: Mosby; 2005:151.)
Table 8-1 Corneal Endothelial Cell Density
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.