Which corneal layer is known for being extremely thin and innermost?

The endothelium is the correct answer as it is the innermost layer of the cornea and is indeed very thin, measuring only about 5 to 10 micrometers in thickness. This layer plays a crucial role in maintaining corneal transparency and metabolic exchange, as it regulates the fluid balance within the cornea. Its thinness is essential because it allows light to pass through without significant obstruction, which is vital for clear vision.

In contrast, the epithelium is the outermost layer, providing protection and is thicker compared to the endothelium. The stroma is the middle layer of the cornea and comprises a significant portion of its thickness, containing collagen fibers that provide strength and shape. Descemet's membrane, while also relatively thin, serves primarily as a supportive structure beneath the endothelium, but does not qualify as the innermost layer. Understanding the specific roles and characteristics of these layers is essential when studying corneal anatomy and function.