Immunoreactive bombesin and calcitonin paracrine cells of human fetal and newborn airways.

Lung tissue samples from 35 human fetuses (8 to 25 weeks gestation) and 18 newborn infants (25 to 41 weeks gestation who survived less than 28 days) were examined by light and transmission electron microscopy for the presence of Kulchitsky-like cells and for the peptide hormones IR bombesin and IR calcitonin. K-like cells were found in developing intrapulmonary conducting airways from bronchi through alveolar ducts, appearing both as single cells and in large clusters. IR bombesin was rarely found in bronchioles of fetuses in the first trimester, but by the second trimester K-like cells were readily identified in bronchioles, and a few were seen in alveolar ducts and bronchi. They were especially numerous in infants dying with post-respirator lung disease. IR calcitonin was not found until 20 weeks' gestation, and was also most common in post-respirator lung disease. The role of these hormones in the fetal and neonatal lung is at present unknown.