Uveal compartmentalization in the hamster eye revealed by fine structural and tracer studies: implications for uveoscleral outflow.

The uveal tracts of hamster eyes have been examined using electron microscopy and electron opaque tracer infusion. A heretofore undescribed intrauveal "compact zone" was detected at the level of the ora serrata. Together with extensive, sheet-like fibroblastic layers within the lamina fusca and the well-known junctional barriers of the pigment epithelium, the compact zone appears to separate anterior and posterior uveal regions into defineable compartments. To test this possibility and study potential pathways of uveoscleral outflow with special reference to these morphologic features, horseradish peroxidase or anionic ferritin was infused into the anterior chamber at physiologic pressures, in some experiments coincident with vascular rinsing and/or fixation to prevent tracer redistribution. Tracer rapidly permeated outflow and connective tissue spaces of the anterior uvea and spread into the sclera and adventitia. However, it did not appear in the inner choroid unless vascular recirculation and redistribution of tracer was allowed. Potential structural boundaries that might prevent direct choroidal penetration include the compact zone, which never contained tracer, and tight-junctioned, sheet-like fibroblastic lamellae of the lamina fusca. The choriocapillaris and internal choroidal interstitium may thus be isolated to an appreciable degree as a "posterior uveal compartment." The position of this compartment is comparable developmentally to the subarachnoid space of the brain and optic nerve. The anterior margin of the compartment (the compact zone) limits the space available for uveoscleral outflow of aqueous humor in the hamster and may provide a focus for control of that process.