Immunohistochemistry for active caspase 3 counterstained with methyl green. (a) Control rat lung, showing specific staining for caspase 3 in the alveolar septa (arrow). (b) SU5416-treated rat lung, showing an increase of positive cells in the alveolar septa (arrow). (c) SU5416 plus Z-Asp-CH₂-DCB–treated rat lung. (d) SU5416-treated rat lung. Positive cells (arrow) are observed in the endothelial cell layer of the pulmonary artery. Original magnifications, a–d: ×600.