| Abstract: | Intact, 14- to 21-day fetal rat lung pairs, neonatal lungs, and cultured 15- and 16-day lung explants were examined in 2-micron-thick glycol methacrylate sections stained by PAS-lead hematoxylin. Selected stages were also studied in histochemical preparations for aliesterase and formaldehyde-induced monoamine fluorescence, as well as by scanning and transmission electron microscopy. Neuroepithelial bodies (NEBs) first appear in pseudoglandular lungs at 15 days in vivo as pyramidal groups of basal, diffusely lead-hematoxylin-positive cells in glycogen-depleted epithelium of main and lobar bronchi. By day 16, primitive NEBs occur within three to four generations of the terminal buds, and older, proximal bodies are larger and more distinctive than at 15 days. Aliesterase activity is first detected in basally located, developing NEBs on day 16. During the canalicular and alveolar sac periods, NEBs appear and mature on a proximal-to-distal gradient along the airway, as they do in developing rabbit and human lungs. As earlier-formed airways elongate, additional NEBs appear and supplement the population already present. By days 20-21, NEBs occur at all airway levels down to the bronchiolo-alveolar junctions, and many of the cells have discrete PAS- and lead-hematoxylin-positive, infranuclear granules. Near term some NEBs exhibit serotonin fluorescence after incubation in 5-hydroxytryptophan and have abundant, ca. 100-nm, electron-dense granules. These are concentrated toward the cell base like the stained granules visualized by light microscopy. Similar results were obtained from lungs placed in organ culture. From 2 days in culture to a time equivalent to term, NEB formation parallels that in vivo, indicating that developmental requirements are met in in vitro. Taken altogether, morphologic and cytochemical evidence suggests that NEBs of rats are functional in late fetal life and that their development is relatively independent of extrapulmonary influences and of the intraepithelial ingrowth of sensory nerve endings. |
