The aqueous humor outflow pathway of zebrafish.

PURPOSE The structures of the ocular anterior segment responsible for aqueous humor secretion and absorption have been well characterized in mammals. However, the underlying molecular and cellular mechanisms that regulate aqueous humor flow have remained elusive. Experimental analysis in Danio rerio, the zebrafish, is providing mechanistic insights into many cellular processes relevant to normal human physiology and disease. To facilitate studies on the molecular and cellular mechanisms of aqueous humor dynamics using this species, the authors have characterized the anatomy of aqueous secretion and outflow in adult zebrafish eyes. METHODS Analyses by light and transmission electron microscopy, coupled with molecular tracers of fluid flow, were used to identify and study the sites of aqueous humor secretion and absorption in adult zebrafish eyes. RESULTS Zebrafish eyes show aqueous humor secretion primarily from the dorsal ciliary region and outflow through a ventral canalicular network that connects with an aqueous plexus and veins of the choroidal rete. CONCLUSIONS Vectorial flow of zebrafish aqueous humor is in contrast to that in mammals in which secretion and absorption of aqueous humor are circumferential around and through the iridocorneal angle. However, local anatomy and ultrastructure of the tissues and cells specialized for aqueous humor dynamics in zebrafish show conservation with that of mammals. These observations suggest that zebrafish can serve as a useful genetic model to help understand the regulation and cellular basis of normal and abnormal aqueous humor dynamics in humans.