Fluorescein diffusion in the human optic disc. 
The characteristics of the transcapillary transfer of fluorescein dye in the optic disc of healthy individuals has been studied.  A diffusible fluorescein dye and a nondiffusible reference substance, indocyanine green (ICG), which was assumed to remain in the capillaries, were injected into the circulatory system.  The time courses of the concentrations of the two dyes in the optic disc were determined by simultaneously recording the fluorescence intensity of fluorescein and the infrared absorption by ICG with a fundus reflectometer.  The difference between the fluorescein concentration curve and the reference ICG curve is a measure of the accumulation of fluorescein in the disc tissue.  Our measurements indicate that fluorescein dye does not diffuse across the capillaries in the optic disc.  The accumulation of fluorescein in the disc only starts at about one minute after the injection and seems to be due to diffusion of the dye from the surrounding choroid.  The time constant of this diffusion process was found to be approximately one minute.