In situ actin distribution in excised retrolental membranes in retinopathy of prematurity.

Actin, an intracellular contractile protein, is associated with cell movements in both muscle and nonmuscle tissues. Using the fluorescent derivative of the mushroom toxin, phallacidin, which specifically binds to polymerized (filamentous) actin, we demonstrated the distribution of actin in retrolental membranes excised during open-sky vitrectomy from six infants with leukokoria due to retrolental fibroplasia. Actin was detected in ubiquitous linear bundles, which is consistent with the existence of contractile forces. Actin was also concentrated in the cortical cytoplasm of cells, thus outlining the plasma membranes. Transmission electron microscopy has identified many of these cells to be myofibroblastlike in appearance. Myofibroblasts are known to contract like smooth-muscle cells and have been reported as the putative causal factor in the contraction of scars and transvitreal membranes.