Exposure to AgNPs compromises neuronal viability and perturbs neurite outgrowth. Phase contrast images of rat cortical neurons cultured either in the absence (A, control) or presence of various concentrations of AgNPs at 1 (B), 5 (C), and 10 (D) μg/ml for 3 days. (A) Shows the control group grown in the absence of AgNPs is characterized by healthy cell bodies and well-established neuronal processes (indicated by an arrow, also see insert). (B) Shows that AgNPs at the concentration of 1 μg/mL caused cell damage (indicated by an asterisk) and triggered fragmentation of developing neuronal networks (indicated by an arrow) (also see insert). (C) Indicates that cells cultured in AgNPs at 5 μg/mL have an increased number of deteriorating cells (indicated by an asterisk) and a markedly reduced degree of neurite outgrowth (indicated by an arrow). (D) In the presence of AgNPs at 10 μg/mL, the majority of cells underwent cell death and abnormal/limited neuronal sprouting and outgrowth. Arrows indicate neuronal processes and asterisks indicate neuronal cell bodies. Scale bar, 25 μm.