Microanatomy of the anterior uveoscleral outflow pathway in normal and primary open‐angle glaucomatous dogs

Abstract

To compare the morphology of the uveoscleral (US) outflow pathway in normal and glaucomatous canines. 10 normal beagles, 10 beagles with inherited primary open-angle glaucoma, 4 cocker spaniels with spontaneous glaucoma. Formalin-preserved globes were sectioned tangentially and sagittally and treated with H&E, Masson's trichrome, or elastin stains or analyzed by immunohistochemistry to visualize smooth muscle actin. Tissues associated with the US pathway were observed and compared using light microscopy. Tangentially oriented sections clearly revealed spaces for the transport of aqueous humor at the junction of the posterior iridocorneal angle (ICA) and anterior ciliary body musculature (CBM). Within the external anterior-most of the US pathway, the supraciliary space, distinct connective tissue cords and smooth muscle pegs fastened the ciliary body to the adjacent sclera. Compared to normal controls, glaucomatous eyes developed a robust scleral elastic sheath at the junction between the posterior ICA and the anterior CBM. In advanced glaucomatous beagles and cocker spaniels, a large amount of melanophores were seen in the US pathway and surrounding vasculature. Within the C8M of glaucomatous specimens, the smooth muscle bundles appeared fewer and separated by elastic-rich ECM. Structures of the US pathway changed little with age. The anterior portion of the canine US pathway is well defined and appears to be altered little with age. However with glaucoma, changes of the US pathway were associated with its the elastic components, as well as the accumulation of melanophores. Collectively, these changes may have an effect on US outflow and, subsequently, aqueous humor dynamics.