The Corneal Structure of the Yellow-Legged Gull, Larus michahellis (Naumann, 1840).

The cornea is the transparent part of the eye's outer sheath and the primary refractive element in the optical system of all vertebrates allowing light to focus on the central part of the retina. Maintenance of its curvature and clarity is therefore essential, providing a smooth optical surface and a protective goggle to ensure a focused image on the retina. However, the corneas of birds have been largely overlooked and the structures and mechanisms controlling corneal shape and hence visual acuity remain unknown. In this work, the cornea of a seabird, that is, the yellow-legged gull, has been investigated using light and electron microscopy. Histological examination reveals that, as in other vertebrates, the cornea consists of five layers: outer epithelium, Bowman's layer, stroma, Descemet's membrane, and endothelium. The corneal epithelium is a nonkeratinized, stratified squamous epithelium approximately 3-4 cells thick that covers the front of the cornea. The surface of the cornea features two types of microprojections, microridges and microvilli. The acellular Bowman's layer is difficult to define because of its gradual transition into the more regularly arranged stroma, which constitute the bulk of the cornea, a collagen-rich central layer that comprises nearly 90% of the thickness of the cornea. The collagen fibrils are of uniform diameter and, within a given lamella, are all parallel to each other and run the entire breadth of the cornea. The lamellae are oriented at various angles with respect to each other. Between the lamellae, most of the keratocytes were concentrated in the central region of the corneal stroma. Desçemet's membrane is well-developed. The endothelium is a single cell-layer thick of approximately 3 µm in depth. The endothelial cells are polygonal and display irregular and interdigitating borders in basolateral plasma membranes. The results shown different diurnal lifestyle characteristics in the yellow-legged gull cornea.