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The corneal endothelium is essential for maintaining hydration in the cornea due to its role in regulating fluid balance. This thin layer of cells, located on the innermost part of the cornea, actively pumps excess fluid out of the stroma and helps maintain a precise balance of moisture in the cornea. By controlling the amount of water within the cornea, the endothelium contributes to the overall transparency and clarity required for optimal vision.

Moreover, the cornea's unique structure relies on this hydration balance; if the endothelium becomes compromised or damaged, it can lead to corneal swelling, resulting in blurry vision. This function is critical as the cornea is avascular (lacking blood vessels), making it reliant on the endothelium for nourishment and maintaining its necessary shape and clarity.

The other layers do play important roles but do not primarily regulate hydration. The corneal epithelium serves as a barrier and protects against environmental damage; the stroma provides structural support and strength; and Descemet's membrane is a thin layer that offers additional support to the endothelium. Therefore, the endothelium’s active role in pumping fluid is what makes it the key layer for maintaining hydration in the cornea.