The Clara cell: a comparative ultrastructural study in mammals and a study of secretory granules in rabbits
Clara cells in the terminal bronchioles of mouse, rat, rabbit, calf, human and monkey were compared by light microscopy, transmission and scanning electron microscopy, and species-differences were clearly present. Mouse Clara cells were most numerous and mouse and rabbit had large dense mitochondria. Rabbit and calf had glycogen in Clara cells and rat had the most variability in secretory granules, some of which had a crystalline structure. Calf Clara cells had deeply indented nuclei. Human Clara cells had the most prominent nucleoli and lacked smooth endoplasmic reticulum, which was a prominent feature of most other species. Monkey Clara cells had characteristics similar to human. No evidence of apical extrusion or apocrine secretion of Clara cell secretory granules was observed. Clara cell granules are PAS-negative.
In experiments to affect the Clara cell secretory response, neither corn oil, microorganisms nor histamine introduced by intubation into the lungs of rabbits caused any release of secretory granules, thus indicating that Clara cell secretions have no function in digestion of foreign organic substance or protection from invasion of bacteria, and no correlation can be made with an allergic response. Isoproterenol induced release of granules indicated adrenergic control but does not further clarify their nature.