Expression of 5'-nucleotidase and alkaline phosphatase in human aqueous drainage channels

Lymphatic vessels and blood vessels can be distinguished histochemically by their expression of 5'-nucleotidase and alkaline phosphatase. The aim of this study was to compare the expression of these enzymes in human aqueous drainage channels with that seen in lymphatics and blood vessels.

Histological sections from the angular regions of human eyes were prepared both by enzyme histochemical and immunohistochemical methods to analyse 5'-nucleotidase and alkaline phosphatase expression. In some of these eyes, Indian ink-stained gelatin was injected into Schlemm's canal and the suprachoroidal space to facilitate the identification of aqueous drainage routes.

There was no expression of 5'-nucleotidase in the endothelium of aqueous drainage channels. Ocular blood vessels were characterized by strong expression of alkaline phosphatase, whereas the cellular lining of Schlemm's canal, the collector channels, the aqueous veins and a scleral channel from the suprachoroidal space showed significantly weaker expression or no expression at all.

The study failed to show a histochemical similarity between lymphatics and human aqueous drainage channels, as no expression of 5'-nucleotidase could be found in any part of the aqueous outflow pathway. The endothelium of aqueous drainage channels also differed from normal blood vessels by a much weaker expression of alkaline phosphatase. This makes a histochemical distinction between aqueous veins and scleral veins possible.