Development of neuroepithelial bodies and solitary endocrine cells in fetal rabbit lungs. II. Nonspecific esterase as an indicator of early maturation.

Activity for nonspecific esterases has been demonstrated histochemically in fetal rabbit lungs using alpha-naphthyl acetate as substrate. Commencing with a homogeneous distribution in pseudoglandular lungs, activity gradually heightens in the epithelium, progresses centrifugally along the bronchial tree, and reaches full expression in postnatal lungs, when most cells in conducting airways are strongly reactive. Well before this general rise, small clusters become conspicuously reactive in the epithelium, first in the larger bronchi and later in the smaller bronchi. At 25 days' gestation most of these are neuroepithelial bodies, which are recognizable in frozen sections and occur selectively near bronchial bifurcations. Subsequently, while the bodies formed earlier continue active, other clusters and solitary cells become reactive in more recently formed airways, amounting in all to 1-2% of the epithelial cells present. Near term these and the more organized neuroepithelial bodies occur throughout the bronchial tree up to the primitive respiratory zone. The small clusters and single cells may include some mature elements; however, as they generally correspond in incidence and distribution to clear cells and clusters, identified as precursors of neuroepithelial bodies, many are incompletely differentiated. Esterase activity, a characteristic shared with APUD cells in other organs, evidently rises about the time pulmonary endocrine cells become functional. Using the histochemical method, a large population of these cells is therefore demonstrable in lungs until after birth, when the increasing activity of nonendocrine cells finally obscures it.