The Aqueous Humor Outflow Pathway of Zebrafish

Department of Cell Biology and Anatomy, Medical College of Wisconsin, Milwaukee, Wisconsin 53226, USA.
Investigative ophthalmology & visual science (Impact Factor: 3.4). 04/2009; 50(4):1515-21. DOI: 10.1167/iovs.08-3010
Source: PubMed


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.
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.
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.
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.

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Available from: Matthew P Gray, Jan 31, 2014
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    • "(shallow) to temporal (deep) manner (Yoshikawa et al., 2007). Although the overall tissue organization and presence of the annular ligament of the zebrafish iridocorneal angle is strikingly different from mammals , at the cellular and ultrastructural levels, the ciliary epithelium and cells of outflow pathway show a high-degree of conservation (Soules and Link, 2005; Gray et al., 2009). "
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    • "However , notable differences exist, for example in the development of the zebrafish lens, which delaminates from the overlying surface ectoderm as a solid mass of cells rather than invaginating to form a hollow vesicle (Soules and Link, 2005; Dahm et al., 2007; Greiling and Clark, 2009). Furthermore, zebrafish possess an annular ligament at the iridocorneal angle that is not observed in mammals, the iris and ciliary body are noncontractile and lack muscle cells (Soules and Link, 2005), and aqueous humor flow is vectorial (from dorsal to ventral) rather than circumferential (Gray et al., 2009). Nevertheless, the general conservation in the genetic pathways and morphogenetic events that shape the anterior segment between zebrafish and mammals, as well as the physiologically relevant phenotypes observed in zebrafish carrying mutations in anterior segment development genes, demonstrates that zebrafish are a useful model to study diseases of the anterior eye. "
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    • "Fig. S1, which is available online). Little is known about the morphogenesis of this region in zebrafish eyes, though it is thought to be homologous to Schlemm's canal in mammals (Gray et al., 2009). At $3 and 5 dpf, EGFP is expressed in amacrine cells of the INL, which, in contrast to $24 hpf, correlates with mab21l1 expression (Fig. 4E,F,H,I). "
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