ONTOGENY OF NADPH-D EXPRESSION IN THE THYMIC MICROENVIRONMENT OF THE CHICK-EMBRYO

Nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histo chemistry was used to demonstrate the presence of nitric oxide in the developing chicken thymus. NADPH-d was first expressed in the epitheli al cells located at the corticomedullary junction of the thymic rudime nt on day 13 of incubation. The number of labelled cells gradually inc reased from day 13 to day 21. Ultrastructural evidence showed that the labelling was localized in a heterogeneous population of cells in the medulla near the corticomedullary junction, comprising the cystic, un differentiated, myoid, lymphoid and epithelial reticular cells. At thi s age, the vascular endothelium was NADPH-d positive. Labelling was al so detected in some macrophages. The reaction product primarily labell ed profiles of rough endoplasmic reticulum and to a lesser extent the outer membranes of mitochondria, portions of the nuclear envelope and the Golgi apparatus. By day 18/19, NADPH-d-labelled nerve fibres were occasionally observed in the interlobular connective tissue. By day 21 , these fibres formed perivascular plexuses. Labelled nerve fibres wer e occasionally observed in the medullary parenchyma. Possible function s of nitric oxide in the embryonic thymus are discussed.