NADPH-diaphorase positive cells in the chick and rat thymus.

Nicotinamide-adenine dinucleotide phosphate-diaphorase (NADPH-d) positive cells were demonstrated in the medulla of the rat and chick thymus at light microscopic level. The NADPH-d positive cells were present as clusters and were predominantly localized near the corticomedullary boundary. These clusters were closely associated with the thymic cysts. Some cells were seen in close proximity to blood vessels. The presence of nitric oxide (NO) as indicated by the positive NADPH-d reaction in the cells forming the wall of the thymic cyst suggests a modulatory influence of NO over the activities of these cells. Other possible functions of NO are also discussed.