Schwalbe line cells in normal and glaucomatous dogs

Abstract

OBJECTIVE: The canine iridocorneal angle contains an operculum, which is similar to that in nonhuman primates and consists of a peripheral extension of the inner cornea that overlies the anterior-most portion of the corneoscleral trabecular meshwork. This region contains cells, the Schwalbe line's (SL) cells, that have been found to have secretory and epithelial characteristics. This region of the iridocorneal angle represents the nonfiltering portion and becomes altered early during spontaneous glaucoma in the Beagle. The present study describes the SL cell for the first time in the dog and changes associated with canine primary open angle glaucoma. PROCEDURES: The iridocorneal angles from 18 Beagles with inherited glaucoma (3 months-8 years old) and 17 normal, age-matched Beagles were placed in 10% buffered formalin for light microscopic evaluation, or 2.5% glutaraldehyde for ultrastructural evaluation. Using at least three fields from each region of the iridocorneal angle (opercular, corneoscleral, and uveal) at x 1000 magnification, trabecular cell nuclei were counted. RESULTS: The operculum in the canine iridocorneal angle consisted of the peripheral extension of the corneal endothelium and underlying anterior-most corneoscleral meshwork, having no direct contact with the angular aqueous plexus. The SL cells associated with operculum-retained epithelial morphology (polyhedral in shape with rER, Golgi apparatus, and secretory vesicles) in both normal, pre-and early glaucomatous dogs. In animals with moderate and advanced stages the SL cells often became less epithelial and secretory in appearance. The number of SL cells in normal dogs declined by approximately one-third by the end of their first year with gradual loss thereafter. In the glaucoma group the decline was more substantial and continuous through the first three years. CONCLUSIONS: The SL cell is morphologically a distinct cell type within the canine iridocorneal angle that is specifically associated with the nonfiltering portion of the corneoscleral trabecular meshwork. Changes in the SL cells of the glaucomatous dog occurred with regard to age and progression of the disease.