The circulation of fluid at the limbus (flow and diffusion at the limbus)

Diffusion of molecules, e.g. ions, proteins, etc. and flow of water takes place across the physiological limbus. This ‘structure’ is estimated to be a zone approximately 1 mm in width and to bridge the anatomical limbus. The source of most molecules that diffuse into the corneal stroma across the limbus under normal circumstances is the perilimbal vascular system with its rich capillary bed. Small ions are lost within 2–3 mm of movement into the stroma, whereas larger molecules may diffuse to the centre of the cornea. Diffusion or flow may be bidirectional; however, it is anticipated that the majority of the flow, albeit small, is inward toward the cornea. The sclera, compared to the corneal stroma, has been found to be less resistant to fluid flow, but more resistant to diffusion of ions and larger molecules. Under normal circumstances, there have been few substances identified of importance that diffuse or flow across the limbus in either direction. There are a number of substances that traverse the limbus that are of importance in disease states, e.g. Wilson’s disease, with the corneal Kaiser-Fleischer ring and angiogenic factors stimulating corneal neovascularisation. Under normal circumstances, the limbus would, therefore, seem to be more important as a zone that restricts flow and diffusion rather than an area of active molecular movement.