Serum albumin enters the posterior chamber of the eye permeating the blood-aqueous barrier.

In order to check the entrance site of serum albumin into the aqueous humor, rabbits were injected intravenously either with Evans blue (which reacts very quickly with albumin) or horseradish peroxidase. The Evans blue-albumin complex (Eb-a) was traced to the posterior chamber as early as 1 min after injection by examining frozen half eyes. The Eb-a was localized in frozen sections by fluorescence microscopy in the stroma of the ciliary and iridial processes, as well as in the lumen of all blood vessels from 1 to 60 min after injection even at doses as low as 3 mg/kg. The peroxidase activity was also localized on these same structures from 8 min to 4.5 h. Neither tracer was visualized in the iris stroma outside the lumen of blood vessels. This was also true for experiments with Eb (75 mg/kg) in which the blood-aqueous barrier was disrupted. The concentration (m/v) of Evans blue and the peroxidase activity in the aqueous humor of the anterior chamber were estimated by spectrophotometry. The morphological integrity of the blood-aqueous barrier was demonstrated by electron microscopy in all peroxidase-injected rabbits. Considering that (a) the Eb-a appeared first in the posterior chamber, (b) there was a high concentration of tracers in the stroma of the ciliary and iridial processes, (c) neither tracer was visualized in the iris stroma, (d) there was no evidence of disruption of the blood-aqueous barrier, and (e) the concentration of both tracers in the aqueous humor kept increasing up to 4 h after injection, it was assumed that serum macromolecules entered first the posterior chamber and subsequently migrated to the anterior chamber. Most likely they passed in between the cells of the inner layer of the ciliary epithelium, the site of the so-called blood-aqueous barrier. No evidence was found indicating migration of macromolecules from the stroma of the processes directly to the anterior chamber via the iris root.