Endothelial Glycocalyx Layer in the Aqueous Outflow Pathway of Bovine Eyes

Abstract

 

Purpose

 

The glycocalyx on the vascular endothelium is known to have an important role as a transport barrier and in the mechanotransduction of fluid shear stress. The glycocalyx in the aqueous humor outflow pathway has not yet been reported. The purpose of this study is to determine whether this layer exists in the aqueous outflow pathway and its distribution therein.

 

Methods

 

Enucleated bovine eyes were obtained within 6 hours post-mortem. The eyes were either perfusion fixed (N=2) or immersion fixed (N=2) with 1% glutaraldehyde and 4% paraformaldehyde in PBS containing 30 mmol/L MgCl2 and 0.05% (w/v) Alcian Blue 8GX. All eyes were cut into small tissue segments and post-fixed in 1% aqueous osmium tetroxide and 1% lanthanum nitrate followed by staining with uranyl acetate and processed for transmission electron microscopy. The distribution of glycocalyx was examined and the height of glycocalyx (in those regions where it was seen) was measured on the trabecular beams, and in the aqueous plexus (AP), and collector channels (CCs).

 

Results

 

The glycocalyx, which appears as a layer of hair-like brushes, was most prominent in the CCs but was also found on the endothelial surfaces of the AP and the trabecular beams. The glycocalyx was relatively uniform in the CCs (Fig. 1A) and had a mean height of 98±27 nm (Mean±SD; n=397 measurements of glycocalyx). In the AP (Fig. 1B) and trabecular beams (Fig. 1C), the glycocalyx was much more variable, with some regions showing little or no glycocalyx; in those regions with glycocalyx, its mean height was 99±37 nm (n=156) in the AP and 101±37 nm (n=154) on the trabecular beams.

 

Conclusions

 

A non-uniform glycocalyx was found coating the surface of the endothelium of the aqueous outflow pathway of bovine eyes. The uneven distribution in the trabecular meshwork and AP is perhaps due to the segmental flow of the aqueous humor through the outflow pathways. Further studies are needed to determine the function of glycocalyx in the aqueous outflow pathway and whether it contributes to the aqueous outflow resistance.

 

 

Figure 1: The glycocalyx (black arrows) was relatively uniformly distributed on the endothelium surfaces of the collector channel (CC) (A), and non-uniformly distributed on the endothelium surfaces of the aqueous plexus (AP) (B) and trabecular beam (C). ITS: intertrabecular space; Red arrows: area without glycocalyx.

 

Keywords: 633 outflow: trabecular meshwork • 448 cell membrane/membrane specializations • 597 microscopy: electron microscopy