Transmission electron microscopy. Ciliated and non-ciliated (Clara) cells in the peripheral lung of normal control mice (Untreated). Merocrine secretions of electron-dense secretory granules (arrows) are recognized in the Clara cell exhibiting many mitochondria and developed smooth endoplasmic reticulum. Hyperplastic Clara cells in the lungs of non sensitized mice exposed to EC-UFP, (NS/UFP). The arrow points at a ciliated cell. NS/UFP treated with NAC (NS/NAC/UFP). OVA sensitized and challenged mice showing goblet cell metaplasia (S/OVA). Strong ultrastructural damage and detachment of metaplastic cells (arrows) is shown in the peripheral lungs of sensitized mice exposed to EC-UFP 24 h prior to OVA challenge (S/UFP/OVA). Clara cells of sensitized mice exposed to EC-UFP 24 h prior to OVA challenge and treated with NAC show preserved ultrastructure with secretory granules (arrows), (S/NAC/UFP/OVA). NAC treatment was performed prior to and close to mid EC-UFP exposure. Scale bar, 2 μm.