Understanding the "Endothelium"
The cornea is like a 5-layer sandwich. The innermost layer, the Endothelium, acts as a pump, keeping fluid out of the cornea to keep it clear.
When these cells fail (due to Fuchs' Dystrophy or post-cataract swelling), the cornea swells and vision becomes cloudy. Endothelial transplant replaces just this pump layer, not the whole cornea.