In vivo and in vitro observations on permeability and diffusion pathways of tracers in rat and frog lenses

Two tracers [horseradish peroxidase (HRP) and lanthanum] with different properties were injected separately into eyes of the rat and frog to determine the diffusion pathways of tracers in the lens. HRP can diffuse rapidly into lens tissue and can also be washed out, whereas lanthanum has opposite properties. In a short (2-5 min) in vivo intravitreal injection of HRP into the rat eye using an automated micro syringe, tracer reached the entire intercellular space between lens epithelium and fiber cells, and between epithelial cells primarily at the peripheral region; however, it reached the central zone in a longer period (10-20 min) of injection, as examined with light microscope and transmission electron microscope. HRP in epithelial intercellular spaces basal to the tight junctions was washed away upon 5-15 min washing with TC-199, but was retained in the spaces apical to the junctions and between epithelium and fiber cells. Lanthanum studies in both rat and frog lenses showed that within 3-4-hr incubation, tracer was seen in epithelial intercellular spaces only basal to the tight junctions at the central epithelium. At the peripheral zone, however, tracer was visualized in the entire intercellular spaces between epithelial cells, and between epithelium and fiber cells. These studies strongly suggest that both tracers seen in the intercellular space between epithelium and fiber cells are derived from the most equatorial region of the lens, and they may not leak out to the anterior lens surface through tight junctions between epithelial cells in the rat and frog. This leads us to conclude that the equatorial region of the lens is the major site of efflux and influx of substances for the anterior lens region.