Which layer of the cornea is affected first by fluorescein staining?

Fluorescein staining primarily targets the corneal epithelium first due to its specific properties. The corneal epithelium is the outermost layer of the cornea and is composed of a thin layer of cells that can readily uptake the fluorescein dye when there's a disruption or damage. The dye binds to the epithelial cells and highlights any abrasions or lesions present, making it a vital tool for diagnosing corneal injuries or diseases.

When an epithelial defect occurs, fluorescein accumulates in the area where the epithelium is damaged, allowing for easy visualization under a blue light. In contrast, the corneal endothelium, Descemet's membrane, and anterior stroma do not take up fluorescein in the same manner. Endothelial cells are typically less accessible and have a different role in fluid regulation and nutrient transport, while Descemet's membrane serves as a basement membrane for the endothelium. The anterior stroma, though vital to corneal structure, typically does not exhibit direct staining unless there is a significant epithelial breach that allows the dye to penetrate deeper into the cornea. Thus, among the options provided, the corneal epithelium is always the first layer to