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Hyaluronan in the bovine ocular anterior segment, with emphasis on the outflow pathways
H Gong1, C B Underhill, T F Freddo
1Boston University School of Medicine, MA 02118.
Purpose:
It has been postulated that glycosaminoglycans play a role in the regulation of outflow resistance. The purpose of these studies was to localize the distribution of hyaluronan (HA) in the anterior segments of bovine eyes to understand better the possible role of HA in the outflow pathway.
Methods:
Eyes from four 2-week-old calves and four 1-year-old cows were examined using a biotinylated-hyaluronan binding protein to localize HA in tissue sections of the anterior segment of bovine eyes. Various fixations and microwave irradiation were compared. The vitreous body in each section served as a positive control. Sections treated with Streptomyces hyaluronidase were used to confirm specificity.
Results:
No significant difference in distribution of HA was found between various fixations and between calves and cows. HA was found in subconjunctival connective tissue and in intercellular spaces of limbal and conjunctival epithelium, but not in corneal epithelium. Staining was sometimes found on the surface of the corneal epithelium and endothelium, as well as on the conjunctival epithelium. No staining was found within the corneal stroma. There was HA in iris stroma, but not in the root of the iris or ciliary body stroma. HA was present in the anterior, nonfiltering part of the trabecular meshwork (TM) and surrounding collector channels and blood vessels in the sclera; to a lesser extent, it was present in the juxtacanalicular region of the TM. HA was detectable neither within trabecular beams nor filling the intertrabecular spaces. Strong staining was found, however, in the nerve bundles in the angle region. Staining for HA in vitreous was invariably positive, and in Streptomyces hyaluronidase-treated sections it was negative.
Conclusion:
HA was not uniformly distributed in the bovine TM. The distribution of HA in the flow pathway of the aqueous outflow system indicates that only a small fraction of the HA found in biochemical analyses of the bovine meshwork is located in the region where the flow resistance is thought to reside.