Ultrastructure and organisation of the cornea, lens and iris in the pipefish, Corythoichthyes paxtoni (Syngnathidae, Teleostei)

Abstract

The corneas of nine pipefish, Corythoichthyes paxtoni (Syngnathidae, Teleostei), five freshly fixed and four museum specimens, were examined using light and electron microscopy. In transverse section, the surface of the corneal epithelium is covered by a complex series of ridges or microplicae which extends over the conjunctiva. The cornea is considerably thicker in the centre (80 microns) than in the periphery (40 microns) and can be separated into two distinct zones. The anterior dermal cornea (23 microns) consists of two layers of epithelial cells, a thick basement membrane (0.75 micron) and numerous lamellae of collagen fibrils with a few scattered keratocytes. This layer is continuous with the conjunctiva which also contains two layers of epithelial cells and lamellae of collagen fibrils. In the juvenile, separating the two zones, is a lens-shaped (concavo-convex) region approximately 6 microns thick in the centre and about 175 microns in diameter containing a fine granular material. In the adult, this region contains both granular material and fibres. It overlies the posterior zone which consists of an anterior iridescent layer (21 microns thick) possessing numerous cell processes parallel with the corneal surface and a few collagen fibrils. The scleral cornea contains 33 lamellae of collagen fibrils without cells and a single layer of cells with several cell processes, similar in appearance to the anterior iridescent layer, which may represent a second or posterior iridescent layer. There is a thick (2 microns) Desçemet's membrane and a thin (1.5 microns) corneal endothelium. There is a spherical lens close to the posterior corneal surface and the iris contains guanine crystals anteriorly and pigment granules posteriorly.