The mechanism of aqueous humour formation

Aqueous humour (AH) is an important intraocular fluid responsible for the supply of nutrients to and removal of metabolic wastes from the avascular tissues of the eye. It is also indispensable for the maintenance of the optical properties of the eye. The fluid dynamics of AH are frequently associated with the potentially blinding disease called glaucoma. Pharmacological treatment of glaucoma generally aims to lower the intraocular pressure by reducing AH formation. However, the mechanism underlying the formation of AH is still not well understood. Understanding the mechanism of AH formation and its regulation is paramount to develop rational and target specific drugs for the treatment of glaucoma. It is now generally believed that AH is formed mostly by active transport of ions and solutes across the ciliary epithelium. Many studies have been carried out in the past half a century to understand these transport processes. In the past several years, new information has emerged and a comprehensive review of these new developments is necessary. This review covers the ion transports in the ciliary epithelium, including the possible roles of sodium, chloride and bicarbonate ions as the driving forces. It also examines the current ionic models for AH formation and its regulation from a cellular transport perspective.