Tracheobronchial epithelium of the sheep: II. Ultrastructural and morphometric analysis of the epithelial secretory cell types

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Abstract
In a light microscopic study we have described the morphology and distribution of six distinct, granule‐containing cells in the tracheobronchial epithelium of sheep lung. We designed the present study to determine qualitatively and quantitatively whether these six cell types differ in ultrastructural morphology. Cell height varied from 30.6 μm for mucous cell M1 to 9.6 μm for Clara cells. Cell width varied from 21.2 μm for M1 to 9.3 μm for Clara cells. Nuclear dimensions ranged from 7.5 μm in M3 to 4.0 μm in M1 and M2. Mucous cell M1 had electron‐dense granules (1.5 μm in diameter); M2, electron‐lucent granules (1.6 μm); M3, nucleated electron‐lucent granules (0.51 μm); M4, cored granules (1.1 μm); serous (SC) and Clara cells (CC), electronopaque granules (0.58 μm and 0.37 μm). The volume fraction of the cell occupied by granules was 63% in M1 and M2, M4 39%, SC 23%, CC 5%, and M3 4.5%. Smooth endoplasmic reticulum was observed only in M3 (33.8%) and CC (49%). Granular endoplasmic reticulum (GER) was most abundant in SC (21%) and least plentiful in M4 (2.2%). We conclude that mucous cells M3 and M4 and serous and Clara cells differ from each other and from M1 and M2 cells. Mucous cells M1 and M2 differ from each other only in amount of GER and secretory granule appearance.