Region-related and layer-specific permeability of the iris vasculature with morphological mechanism: A novel understanding of blood-aqueous barrier

The permeability of iris blood vessels has an important role in maintaining aqueous humor (AH) homeostasis, contributing to variation in iris volume and probably the pathogenesis of angle closure glaucoma. This study investigates the permeability of the iris microvasculature to plasma-derived protein and correspond it with the morphologic characteristics of vascular mural cells (MCs). Twenty-two enucleated porcine eyes were used in this study. 12 eyes were micro-perfused with vehicle alone as control or with FITC-albumin as a marker of protein leakage and histological sections subsequently made to examine for FITC-albumin presence. The other 10 eyes were immunolabeled via micro-perfusion for αSMA and VE-cadherin to investigate their topographic distribution in the porcine iris vasculature, and to cross correspond with the locations of FITC-albumin deposits. Distribution of FITC-signals exhibited a site-dependent pattern and time-dependent change in the iris. Fluorescence was initially detected around capillaries in the superficial and deep layer of the iris microvascular network. The pupillary region and the iris root retained more fluorescent signal than the iridal ciliary region. At low magnification, αSMA labelling displayed a regional variation which was inversely correlated with vascular permeability. At the cellular level, αSMA labeling corresponded with vascular MCs distribution in the iris vascular network. The correspondence between iris microvascular permeability to FITC-albumin and the pattern of αSMA distribution and MCs coverage adds to the understanding of the elements comprising the blood-aqueous barrier with implications for the bio-mechanics of iris volume change.