S-100 immunoreactivity in pulmonary neuroendocrine cells of newborn infants.

S-100 protein was demonstrated by immunohistochemical techniques in occasional solitary neuroendocrine cells of the lungs of newborn infants. The S-100 immunoreactivity was located in both the nucleus and the cytoplasm; its distribution in the cytoplasm was diffuse, there being no differences in staining intensity between the apical and basal poles. This homogeneus distribution of immunoreactivity argues against the S-100 being located in the dense-core granules, since these organelles are concentrated towards the basal pole. Double immunohistochemical techniques revealed that S-100 immunoreactive cells were also chromogranin A immunoreactive. Our results indicate that the S-100 positive population is a subset of the chromogranin A positive neuroendocrine cells. S-100 immunoreactivity was not detected in neuroepithelial bodies.