Fig. 26. Diagram illustrating the relationship between central endothelial cell density, barrier function, and pump sites. Note that the number pump sites are not all maximally used in the normal states (5,000 to 2,000 cells/mm2). With increased leaking (2,000 to 750 cells/mm2), there is an adaptive phase in which the endothelial cells can form more pump sites to offset the leak up to a point. When the surface area of the lateral membranes of endothelial cells progressively becomes too small (750 to 0 cells/mm2), these adaptations max out and eventually decline. The point where endothelial cell pump-site adaptations cross permeability (500 cells/mm2) is typically when corneal decompensation occurs.