Clara cell differentiation in the mouse: ultrastructural morphology and cytochemistry for surfactant protein A and Clara cell 10 kD protein.

The morphologic and functional differentiation of the nonciliated columnar (Clara) cell, one of two secretory cell types in distalmost bronchioles in mammals, was studied in the mouse. Lungs from embryos (16-19 days of developmental age, dDA; birth on day 19), postnatal animals (5-20 days postnatally dPN), and adult animals were investigated by transmission electron microscopy, using standard staining procedures and immunogold (GAR-Au10) labeling for SP-A and Clara cell 10 kD antigen (CCA). At 16 dDa, all the columnar epithelial cells lining prospective distalmost bronchioles lacked distinctive features. By 17 dDa, some cells displayed a few cilia or apical dense granules. At 18 dDa, many nonciliated columnar cells had apical protrusions, as are seen in adult Clara cells. Apical concentrations of glycogen observed in nonciliated columnar cells perinatally were absent by 5 dPN, whereas apical dense granules became more abundant. Profiles of smooth and rough endoplasmic reticulum (ER), which had been randomly distributed, exhibited a selective, adult distribution at 20 dPN (apical vs. basal cytoplasmic domains). Labeling for SP-A and CCA, which was almost absent between 17 and 19 dDa, reached adult levels at the same time. The two proteins differed in distribution. SP-A predominated in adluminal cytoplasmic areas, where it was found over dense granules, vesicles, and multivesicular bodies; it was also present in bronchiolar lumens and intercellular spaces but not in rough ER or Golgi apparatus. In contrast, CCA showed a more uniform distribution; it was present over the same structures as SP-A and in the synthetic organelles. Ciliated columnar cells were virtually devoid of SP-A and CCA. We conclude that mouse Clara cells acquire a mature phenotype by 20 dPN. They are likely to be involved in recycling and/or degradation of SP-A that is internalized from airway lumens through their apical or lateral cell borders; furthermore, they synthesize the Clara cell 10 kD protein. These two Clara cell functions (first detectable late prenatally) reach mature levels by 20 dPN.