Ultrastructural colocalization of the bioactive mediators 5-hydroxytryptamine and bombesin in endocrine cells of human fetal airways

Endocrine cells in the airway epithelium of human fetal lungs are known to contain an amine, 5-hydroxytryptamine (5HT), and a peptide, bombesin (BOM). These mediators may be involved in regulating smooth muscle and secretory activity in the airways as well as in development of the fetal lung. However, the exact endocrine cell type that contains 5HT and BOM has not been described at the ultrastructural level. This investigation provides immunocytochemical evidence that 5HT and BOM are stored in a single cell type, the P1 cell. Thin sections of airways from human fetal lungs were incubated either in anti-5HT antiserum (diluted 1:3000) or in anti-BOM antiserum (diluted 1:600) and then labeled with affinity purified goat anti-rabbit IgG coupled to 16 nm gold particles. For colocalization, thin sections were incubated on one side to demonstrate 5HT and on the other side to demonstrate BOM. Two different sizes of gold particles (10 and 30 nm) were coupled to IgGs and used for the labeled second antibodies. Controls consisted of absorbing of the primary antiserum with an excess of either 5HT or BOM. 5HT- and BOM -like immunoreactivities were observed in the dense-core vesicles (DCV) of P1 cells, and it was apparent from serial sections that 5HT and BOM labeling was sometimes present in the same P1 cells. Sections labeled for 5HT on one side with large gold particles and for BOM on the other side with small gold particles revealed that 5HT- and BOM-like immunoreactivities were located in the same DCV.