Glial fibrillary acidic protein-like immunoreactivity in the iris: development, distribution, and reactive changes...

Using immunohistochemistry with antisera raised against the glial fibrillary acidic protein (GFA), we have studied the appearance and distribution of GFA-like immunoreactivity in whole mounts of rodent iris and in sectioned cat and cow iris. In the adult rat iris, a dense plexus of GFA-positive fibers was seen in both the dilator plate and the sphincter. The fluorescent fibers formed large meandering bundles and a dense irregular network of thinner fibers. In the sphincter, mainly thinner fibers were seen. Thin fibers were also seen winding around blood vessels in the dilator plate. In adult mouse iris, the GFA-positive fibers had a quite different distribution with a few radially oriented fiber bundles superimposed on a more regular network of thinner fibers. Adult guinea pig irides showed still another pattern of GFA-positive fibers with a low number of bundles and thinner fibers forming a sparse irregular network. In thicker fiber bundles of all three rodent species, as well as at branching sites of the thinner fibers, negative or weakly fluorescent swellings surrounded by GFA-like immunoreactivity were present. These structures probably represent the cellular origin of the GFA-positive fibers. Thick, strongly fluorescent fiber bundles, as well as numerous thinner fibers, were seen in sections of cat and cow iris. Prenatally, fibers were visualized at embryonic day 18 in the rat. In these irides as well as in irides from 21-day-old embryos and 1-day-old pups, most fibers were organized in a gradually increasing system of thin meandering fiber bundles that showed limited branching. At postnatal day 6, a more mature network of thinner fibers had developed between the now more numerous fiber bundles. No obvious increase or decrease in the amount of GFA-positive fibers was seen in irides grafted to the anterior eye chamber of adult rat recipients examined 1 and 6 days after grafting. However, in these irides, as well as in the host irides, strongly fluorescent spider-like cells with short branching processes and a negative nucleus were seen. These cells were more numerous and more strongly fluorescent in grafted irides as compared to recipient irides and in the 6-day iris grafts as compared to the 2-day grafts. In all probability, the GFA-positive fibers and cells forming a network in adult irides from different species and in embryonic and grafted rat irides represent Schwann cells and their processes. The cellular origin of the spider-like cells in the iris grafts is less clear.