Cell contact is important for normal maturation of chicken retinal Muller cells. In order to gain a better understanding as to how this occurs, we examined the ability of retinal cells with altered cell contacts to respond to an environmental stimulus. The response of Muller cells cultured under conditions which alter cell contacts was measured by activating intracellular signaling systems leading to induction of the earlyinducible gene c-fos. Chicken retinal cells were cultured as explants, reaggregates, and monolayers and exposed to extracellular stimuli in the form of the excitatory amino acids D, L-alpha aminoadipic acid (AAA) and N-methyl-D, L-aspartic acid (NMDA). Each culture was exposed to 1.25 mM AAA, 2.5 mM AAA,50 μM NMDA, or 100 μM NMDA. Toxicity was assessed histologically and by immunocytochemical labeling of Müller cells after 2 days of exposure. Activation of c-fos was determined by Western blot analysis for Fos protein after 30,60, and 120 minutes of exposure. Exposure to AAA led to a loss of Muller cells in explant and reaggregate cultures; however, Muller cells in monolayer culture; however, Muller cells in monolayer culture were not susceptible to AAA at either dose. NMDA was toxic to a specific population of neurons under all three culture conditions. Fos protein expression paralleled the histologic findings. Fos protein was significantly elevated after exposure to either dose of AAA in explant and reaggregate cultures but not in monolayer cultures. Monolayer and reaggregate cultures did not exhibit a significant increase in Fos levels in response to either dose of NMDA. Explant cultures exhibited an equivical response to only the 50 μM NMDA. These results suggest that the ability of Müller cells to respond to AAA by increasing levels of Fos protein requires that specific cell contacts occur during development. © 1993 Wiley-Liss, Inc.