Summary and model. (A) Comparison of retinal glial and neuronal responses to p27 inactivation. Data values were standardized for cross comparison. (B) Model of p27 inactivation effect on glia and neuronal function. Conditional p27 deficiency results in cell cycle entry and soma displacement, as well as in enhanced intermediate filament expression (e.g., GFAP and VIM). Surprisingly, these reactive changes are compatible with homeostatic neuronal metabolism, electrophysiology and function. The benign impact of inducible proliferative reactive gliosis in p27L-/L- mice raise the hypothesis that the transition from supportive to deleterious is determined by external factors signaled from either neuronal cell stress, microglial reactivity, inflammatory response, blood barrier damage, or neuronal cell death.